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Musings of an Energy Nerd

Heating a Tight, Well-Insulated House

For most superinsulated homes, a furnace is overkill

A cold-climate heat pump. This Samsung ductless minisplit unit is used to heat a superinsulated Passivhaus home in Urbana, Illinois. According to ASHRAE Fundamentals, Urbana has a winter design temperature of -3°F.
Image Credit: Martin Holladay

UPDATED on March 2, 2017 with information on the Dettson furnace rated at 15,000 Btu/h.

If you build a small, tight, well-insulated home — in other words, a green home — it won’t need much heat. Since typical residential furnaces and boilers are rated at 40,000 to 80,000 Btuh, they are seriously oversized for a superinsulated home, which may have a heating design load as low as 10,000 to 15,000 Btuh.

Builders have been struggling for decades with the question, “What’s the best way to heat a superinsulated home?” Your solution will depend in part on your answers to a couple of other questions:

Are you comfortable heating the house from a single point source? If you are, the best solution might be a wood stove, pellet stove, or a direct-vent space heater. These solutions work best in compact homes with open floor plans. Of course, the tighter the home’s envelope and the thicker the insulation, the more likely that indoor temperatures will remain fairly consistent from room to room.

Do you want an all-electric house? Green builders have diverging views on this question. Builders of net-zero-energy homes often avoid gas- and oil-fired appliances, preferring to balance energy loads with electricity produced on site by a photovoltaic (PV) array or a wind turbine.

Of course, most homes still depend on grid-powered electricity, and if your local electric utility generates power from fossil fuel, then it makes little environmental sense to heat with electricity. From a carbon-production standpoint, it’s usually better to burn fuels on site rather than in a remote power plant.

What not to install

Before moving on to right-sized solutions, it’s worth mentioning that it rarely makes sense to install radiant-floor heat in a superinsulated house.

As Alex Wilson has explained, for well-built homes, an in-floor radiant system is usually overkill.…

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  1. boone | | #1

    Radiant floor
    Radiant floors when installed in finished concrete floors and connected to a solar thermal system which makes it eligible for tax rebates (NC=65% fed and state) makes them a very affordable option. We have installed them in small efficient houses with commerical on-demand water heater that do both heating and hot water for 2 floors and about the same price as a mini split. Does not include AC though which is minimal in our area. No wall mounted unit with air blowing and really comfortable even heat.

  2. GBA Editor
    Martin Holladay | | #2

    OK, Boone -- how much did the system cost?
    Leaving aside the question of tax rebates, please provide the actual cost to install a solar thermal system, on-demand gas water heater, and in-floor radiant distribution system in a house you have worked on.

    Real numbers, please!

  3. Kyle | | #3

    mini split condenser adapted to a ducted system
    I was wondering if any one has attempted to take a condenser unit from a mini split and size a evaporator coil to it so as to make a small output centrally ducted heat pump system? Seems possible if a correctly sized small evaporator coil possibly from another application could be found. Of course it would involve some fabrication to adapt an air handler but not too bad. It would seem a desirable solution to get the right size central unit for a passive house in the mixed-humid climate zone.

    1. jaybickford | | #75

      Mitsubishi makes ducted minisplit systems.

      1. jaybickford | | #76

        Mostly intended for avoid lots of separate units in small rooms like bedrooms.

  4. GBA Editor
    Martin Holladay | | #4

    Passive houses don't need to deliver heat through ducts
    There's really no reason to deliver the heat through ducts. The heat can be delivered as intended by the ductless minisplit manufacturer -- through one or more fan-coil units.

  5. homedesign | | #5

    Daikin offers a ducted option.

  6. Riversong | | #6

    A Different Perspective
    Martin, I just posted the following rejoinder to Alex Wilson's critique of radiant in-floor heating in a tight house:

    Alex, you suggest radiant floor heating for a high heat-load, drafty house, but a floor at a maximum 85° surface temperature can deliver only 34 btu/hr·sf, which would not be nearly enough for most poorly-insulated (let alone drafty) homes.

    In a tight house, it's true that the low heat demand would require a lower floor surface temperature, and that might mitigate against the "warm toes" effect. But any surface that is above room air temperature increases the mean radiant temperature (MRT) of a space, and the MRT is at least as important as air temperature in determining human comfort. Human skin has an absorptivity and emissivity of 0.97 - higher than almost any know substance, including matte-black metal - so we are very sensitive to slight variations in MRT.

    A truly green house will not only be super-insulated but also small, and a small house (or any house,for that matter) benefits from the unobstructed wall area of a radiant floor heating system. While the initial installation costs will be higher for a central hydronic radiant system (compared to wall-mounted space heaters such as direct-vent or mini-splits), the uniformity of heat, the lack of room obstructions, and the higher MRT and human comfort can easily justify the additional expense, and some of that incremental cost difference can be made up in increased heating efficiency over the life of the system.

    And a high-efficiency (95%) direct-vent modulating boiler can also efficienlty provide unlimited hot water through an indirect tank, reducing the cost of the domestic hot water system and somewhat offsetting the heating system cost.

    As to your point that a superinsulated house will lose heat slowly during a power outage: the issue isn't how much heat (BTUs) it will lose (which, at design minimum temperature, will be maximum design heat load), but how quickly will it's indoor temperature drop. That's dependent not only on insulation levels but, more significantly, on the amount of thermal mass within the heated space. A house with a radiant concrete slab will maintain indoor temperatures much longer than a low-mass structure, because not only is there a great deal of thermal mass but the mass is warmer than room temperature and will continue to heat the space until it reaches thermal equilibrium.

    In addition to the higher MRT that a radiant heating system provides, it eliminates the air convection and turbulence (and noise) and temperature stratification that contribute to occupant discomfort.

  7. GBA Editor
    Martin Holladay | | #7

    Thanks, John
    John is right: Daikin offers an indoor unit designed for ducted systems. Their designation for the indoor ducted component is FDXS. Here's a brochure that describes both indoor unit options:

    Page 10 of the brochure describes the FDXS option.

  8. Kyle | | #8

    Thanks Martin and John for
    Thanks Martin and John for the responses!

  9. GBA Editor
    Martin Holladay | | #9

    No single answer on radiant heat
    The debate over radiant heat can't be resolved, since some people want radiant heat regardless of the cost. If you can afford the $15,000 (or more), and you want the system, then you might as well go ahead.

    If your house can be heated for a system that costs $5,000 or less, however, you might prefer to spend the extra $10,000 on better windows.

    You're right that a house with lots of thermal mass will cool off more slowly than one with little mass. So if you want mass, include mass. The radiant heat loop is optional. Books make great thermal mass. I've got a lot of books -- more every week. A concrete slab also works fine.

    As far as "air convection, turbulence, noise, and temperature stratification" are concerned, these are red herrings. My basic assumption is that we are talking about superinsulated houses with very low air leakage rates. These houses simply don't suffer from your catalog of horrors. They are easy to heat, have little temperature stratification, and don't suffer air convection or comfort problems as long as they are well designed.

  10. Marc Rosenbaum | | #10

    Minisplit heat pumps
    I have a serious interest in minisplits because the they are cheap, are a packaged system so don't require much engineering design, are easy to install, and are a good match for low load buildings with renewable power. But at this point I don't know whether i'm working with a COP of 2 or 3! There seems to me to be a pretty big gap between the AHRI ratings and the manufacturer's engineering data.

    What you should understand is that folks like Andy and I have no practical way to monitor COP in a system that delivers its energy via air (no simple BTU meter here) and is constantly varying the air flow and delta T of the indoor units (as well as varying stuff at the condenser). So the best we can do is an estimate of COP from how much energy our buildings use, which of course is subject to many factors of weather, setpoints, internal gains, blah blah. This is important IMO because the COP (HSPF really) in the AHRI ratings differ a lot from what the mfgrs claim.

    I have a simple glass dome meter on our Plainfield School prototype classroom so I'll know how much energy the Hyperheat uses. But I have confounding factors of adjoining spaces, insulated heat pipes running through the room, setpoints, etc. to contend with when I go to calc COP. We'll have meters on the 8 affordable houses we're doing, and on a small dorm. We've just turned a new house over to the owners with a ducted system but I haven't got meters there yet. We'll have meters on projects under construction this season.

  11. Gary K | | #11

    I was surprised that there was no mention of Geothermal Heating and Cooling for a super insulated house. My 105 year old brick home with refurbished windows, foam insulation in all wall cavities (around doors and windows), foam insulation in the attic, and foam walls and roof in a 350 ft kitchen and sun room addition, uses about 20% of the power of my former home of the same age, 300 ft smaller, same design and directly across the street from the super insulated one.

  12. GBA Editor
    Martin Holladay | | #12

    Ground-source heat pumps
    There is no doubt that ground-source heat pumps work, and that they can be efficient. The main drawback to systems like yours is their high capital cost. For a small, well-insulated house, a ductless minisplit (air-source) heat pump costs far less than a ground-source system. Since the heating load in a well insulated house is so low, it's hard to justify the cost of a ground-source heat pump.

  13. David Whte | | #13

    mini splits
    Most of my PH projects use mini splits, and mostly ducted (for the sake of concealing the unit and distributing heating and cooling to each room). Distribution sized to around 0.2" static pressure.

    Robert, the MRT and acoustical benefits of radiant are lost on superinsulated, super tight houses. In a house meeting Passive House standard, the floor will only be 2 F warmer than room temperature at peak load. And in these houses MRT is not of issue - even interior window surfaces are generally with ASHRAE 55 recommendations for asymmetry at design day.

    Same for acoustics and draft. A forced air system that only needs to deliver 3,000 to 10,000 btuh can do so silently and without draft.

    Marc, for monitoring mini splits, couldn't one get a good measure of flow rates (by garbage bag or flow grid) at installation? The fans are stepped speeds, so a logging voltmeter could indicate which speed is operating - that gives CFM at each time step. log supply temp, room temp, and outdoor temp. voila. or?

  14. homedesign | | #14

    Mini-splits and air distribution
    The mini-splits do seem to be very popular in Japan.
    I doubt that they locate a ductless unit in every room or that they are using the ducted versions.
    David White,
    How do you think they are distributing heating and coolong to every room?
    I had considered using a ducted minisplit system in North Texas ..but found the installed price to be extremely high.
    I also noted that the SEER ratings for the ducted versions are lower than the non-ducted.
    Any ideas for conditioned air distribution other than ducted or multiple units?

  15. jbmoyer | | #15

    Aesthetics of Minisplits
    I would think that most homeowners would not appreciate the look of wall mounted indoor blower in each room. I noticed in one of the pictures in the Daikin brochure referenced, a picture of what appears to be a supply grill above a closet. Is this a ducted system? Or was the wall mounted blower installed in the closet behind the grill. Would this be an appropriate installation? Can you hind the wall mounted blowers?

  16. GBA Editor
    Martin Holladay | | #16

    Wall-mounted blowers
    In Passivhaus homes, I have seen a single indoor blower unit — installed in a hallway or stairway — that provides heat for the entire house. In a superinsulated house, it isn't necessary to provide heat to every room.

    I have been living for 29 years in a 3-bedroom house with a single heat source (a wood stove in the living room). There is no need to provide heat distribution to every room.

  17. boone | | #17

    radiant floor costs
    My costs are in line with Michael's.
    I have installed a radiant floor system without solar on a 2 story 1300 sq ft house for $7000 and a one story 1100 sq ft with vac tube collectors and 120gal storage for $13,000. In 2 floors codes req at least one mini split on each floor =$6150
    So the mini splits provide AC and cost $900 less for the 2 story design. In a one story the cost is more like $3000 less but not able to connect to solar thermal which supplies the domestic hot water and improves the energy payback and does not connect to the coal fire plant as back up is natural gas.
    The mini splits do not heat water so that has to be added in to their costs.
    With a 65% tax credit the $13000 system, that includes solar water heating for domestic water, comes down to under $5000.
    One caveat is that we have 4 in finished concrete on the ground floor level and 2 inch concrete over wood frame on the second floor. Some people love the stained concrete and some don't like it but is an inexpensive ($2.50 sq ft.) durable, high thermal mass floor.

  18. Scott Heeschen | | #18

    Mini-split distribution
    I went through a similar thought process a few months ago in my remodel. I had originally specced Warmboard subfloor, but when the time came, I realized that the savings from using ordinary subflooring vs. the Warmboard would just about pay for a minisplit heatpump. Seemed like a no-brainer decision to me - get heating and air-conditioning for less. Yeah, no warm feet, but way cheaper in my situation. Several manufacturers have multi-zoned minisplits. I've seen units with up to 4 zones, maybe there are some with more. Of course the price goes up, but you don't need a compressor per zone.

  19. GBA Editor
    Martin Holladay | | #19

    Good to see real numbers
    Michael and Boone,
    Thanks for sharing some real numbers.

    My own comments:
    1. Keeping costs low on hydronic radiant systems is possible; it helps to have experience with these systems and to keep the number of trades involved down to a minimum.

    2. Many builders (and many owner-builders) have been quoted much higher figures. The more trades involved, the higher the numbers.

    3. Even using your numbers, ductless minisplit systems (which include air conditioning) are cheaper.

    Michael, I noticed that you recently estimated (in an answer to a Q&A question) that it costs $18,000 to install air conditioning in a tight, well-insulated 4,000-sf. North Carolina home.

  20. gas heater | | #20

    Hi, Congratulations to the
    Hi, Congratulations to the site owner for this marvelous work you’ve done. It has lots of useful and interesting data.

  21. dickrussell | | #21

    Capital cost of GSHP varies a lot
    In a couple of the above posts, there are references to the capital cost of a ground source heat pump as being high. This would seem to be a misleading generalization, as the cost is very situation-dependent. In one particular situation, a new and very low heat loss home in a rural area, and for which a water well would be drilled anyway, the capital cost vs. conventional gas or oil-fired system could be a wash. Water Energy Systems (NH) estimates $4.5 to 5K per ton of heat for "inside costs" for equipment and duct work, numbers provided to me at a presentation by them last April. The standing column well design needs typically 80 feet of water column per ton. This means that the well needed for adequate domestic use may easily be deeper than is needed for supplying heat to a GSHP. At most, only incremental drilled depth cost is incurred. The water-to-air system then also supplies A/C for the warmer times, although for a superinsulated house in a heating climate the load would be small and dominated by dehumidification of the fresh air brought in by the HRV. As with so many things, the answer to which system is best is: "it all depends."

  22. MICHAEL CHANDLER | | #22

    4000 sf number
    We won't build a house over 3,000 sf so I was just pulling a number out of the air on that.

    Generally on our 2,500 sf houses I'll be putting in a 15 SEER zoned-bypass system for dehumidification w/ MERV-11 filter for $12,000 to $14,000 in addition to the radiant floor and the solar hot water mentioned above. This is a luxury, not-so-big, aging-in-place clientele.

  23. Andrew Henry | | #23

    Geothermal (geoexchange) is expensive
    Hi Martin,

    When I factor in all the costs for the radiant / geoexchange heating system that I put in my home I come up with an embarrassingly large number. Which is why I wish I had known more about Passive House sooner before I set out on my home's addition and renovation.

    That said, the one thing about the Japanese heat pumps is that their level of design and engineering is much more advanced than that found in North American geoexchange heat pumps. I wonder what the COP would be if geoexchange water-to-water heat pumps were as sophisticated as the japanese and korean mini-splits. And what they'd cost.

    Also you point out one thing that I wasn't aware of when I put in my geoexchange system which is that system COP tends to be lot lower than the rated COP, the COP the manufacturer wants you to see. A friend of mine put in a water-to-air heat pump and noticed the system COP value in the heat pump manufacturer's software calculations that his contractor provided. When my friend asked his contractor why the system COP was lower than the rated COP, his contractor couldn't answer because he didn't know what the system cop value meant.

    Finally, I set up a Google alert on Passive House and noticed this in my latest alert. A cautionary tale when considering your next geothermal/geoexchange system!



  24. GBA Editor
    Martin Holladay | | #24

    System COP versus manufacturer's COP rating
    I agree that the COP ratings provided by ground-source heat pump (GSHP) manufacturers are always higher than actual system COP.

    I wrote an article on the issue that was published in the April 2008 issue of Energy Design Update. The article reported on monitoring data gathered by Rob Aldrich and Andy Shapiro. Aldrich measured a system COP of 3.5 on a GSHP with a manufacturer's rating of 5.0. Shapiro measured an average system COP of 2.75 for three GSHPs with an average manufacturer's rating of 4.0.

    The article said, in part:
    “Shapiro mentioned a few problems with the GSHP systems he monitored. One of the heat pumps had a bad valve, said Shapiro, and ‘it took a long time for the heat-pump rep to figure it out.’ Although the Econar specifications claimed that the heat-pump unit had a maximum output rating of 130°F — a useful temperature for domestic hot water — the unit installed in Vermont ‘barely makes 118°F or 119°F water,’ according to Shapiro.

    “On the plus side, since heat pumps are fueled with electricity, they are inherently compatible with on-site power generation (photovoltaic arrays or wind generators). While Shapiro expressed satisfaction that all three Vermont systems are working well, he told the Burlington audience that several issues ‘make me nervous.’ These include:

    • There remains a dearth of good monitoring data, allowing some GSHP dealers to make inflated claims.
    • The electronic components of GSHP systems may be vulnerable to damaging electrical surges from lightning.
    • Unlike most residential HVAC equipment, GSHP systems require engineering.
    • In most areas of the country, the GSHP industry is not mature; installations require interactions between several trade contractors who may be unfamiliar with GSHPs.
    • The complexities of GSHP systems provide many opportunities for mysterious glitches.
    • Some GSHPs are being installed in homes with high heating loads where insulation and air-sealing improvements would yield a better return on investment.
    • GSHPs increase utility peak loads during cold weather.

    Derating Manufacturers’ Listed COPs
    “The system COP measured by Aldrich amounts to 74% of the manufacturer’s listed unit COP. Similarly, Shapiro found that the average system COP for the three monitored GSHPs was 69% of the average unit COP in the manufacturers’ specifications.

    “The data suggest that, as a rough rule of thumb, builders should derate unit COPs by 26% to 30% to obtain heating-season COPs for GSHPs systems — at least until future data contradict that rule of thumb.”

  25. Andrew Henry | | #25

    Smaller geoexchange heat pumps
    Hi Martin,

    A couple of things I wanted to throw out there. The geoexchange contractor who put in my heat pump mentioned that he did the installation of some small 3/4 or one ton geoexchange heat pumps for one of the federal research labs here in Ottawa. So I would think there may be some small load geoexchange heat pumps coming down the line. Will these heat pumps be competitive with the mini-splits, probably not when you factor in the cost of the drilling, but you never know.

    Also, I wonder what the system COP starts to look like if geoexchnge systems are installed with good ECM circulators like the Wilo Stratos and Stratos ECO or the Grundfos Alpha? Also ITT bought Laing so presumably we'll see these pumps here soon. That said these ECM circulators aren't cheap compared to PSC circulators, but they are bound to come down in price as ECM circulators start to displace PSC circulators from the market.

    I got invited to a Wilo circulator demonstation (it's amazing what you can get invited to if you ask) where the presenter talked a bit about some of the stuff they're working on, and coming to market in Europe. Wilo has a really tiny circulator, the geniax that could really open up a lot of design possibilities for radiant heating. And I just noticed this one too!

    Anyway, it still comes down to the fact that superinsulated houses don't have much of a heat load so it may all be moot. There may not be a place for radiant floors in a super insulated house but I wouldn't count radiant out entirely given how little energy some of these new circulators use and that they can be internally and independently controlled.



    P.S. Martin, what are you doing up at 4:24 a.m. responding to comments?

  26. Andrew Henry | | #26

    Here's the link to the Laing circulators
    The comments need a preview option!.

    Here's the link to the Laing circulators...


  27. David Rouge | | #27

    Insulation and Heating
    Thanks for the comment on superinsulated homes.

    I am writing from New York City, which has an above-aeverage percentage of rental homes.
    Multifamily buildings in NYC reflect the national pattern of including heat in the rent, which is characteristic of no other country in the world (I realize that there are subtleties here - small and electric-heated units in the US where renters pay for all utilities -- apartments in formerly socialist countries where there is a flat add-on fee for heat. But in the main this is true.) The biggest challenge in the US is 1) To meter all energy use, and 2) to place or increase taxes on all energy sources to reflect true costs.
    Reducing heating costs for super-insulated homes is desirable, but not the big bang.

    David Rouge

  28. Doug | | #28

    GSHP can use domestic water well?
    All the closed-loop vertical-loop GSHPs I'm aware of use a dedicated well that's only for the GSHP, in fact it's grouted solid around the GSHP tubing. And, because these closed-loop GSHPs run their ground loop temps below freezing at times, even if you could fit the domestic water line & pump down the same well shaft, it would freeze solid at some point.
    An open loop GSHP is not an option in much of the country due to water quality (any but the purest water damages the exchanger in the GSHP). I've heard that the added pump energy required by an open-loop configuration means higher overall energy use as well.
    So, plan on a separate well in most scenarios, at full price.
    Overall GSHPs could be great, but because mini-splits offer such high efficiencies without the first cost, I question the merits of GSHPs using today's technology.

  29. Gayle | | #29

    Natural Gas Floor Furnace
    We have just purchased a nice smaller home in the Western part of Tennessee, brick and about 1400 sq feet. It is well insulated. It has a central split central unit using natural gas for heat. We are concerned about a not having a backup system should the electricity go off in the middle of the winter. We have had experience with a gas floor furnace installed in 1985 and were well pleased until when it was about 10 years old, we returned from a trip and smelled gas very stong in the house. We tried to have it repaired, but they never could get the leak stopped and we disconnected the unit and never used it again. We would like to purchase a floor furnace again for this house and try it again. Do you have any recommendations on what to install and have improvements been made in furnaces in the last 25-30 years?

  30. GBA Editor
    Martin Holladay | | #30

    Heat during power outages
    For heat during power outages, you have several options.

    1. The simplest and most common solution is a wood stove.

    2. Several models of gas-fired space heaters are available that do not require electricity. These are commonly installed in off-grid homes. I have a propane-fired Empire heater in my own home that keeps my house above freezing when I go away for the weekend. In the article above, I also mentioned the Robur TS2000, which requires no electricity.

  31. dickrussell | | #31

    GSHP can use domestic water well? by Doug
    I have to wonder if Doug is confusing closed horizontal loops or bentonite-packed vertical bores, with antifreeze in the circulating water, when he speaks of running the loop below freezing. Standing Column Well (SCW) GSHP designs are quite numerous in the northeast, where groundwater is plentiful and usually of good quality. A SCW running to the freezing point obviously has been poorly designed. The website I referred to earlier [I believe the correct name is Water Energy Distributors, URL is] speaks of having done over 11,000 installations in the northeast part of the country. Operating data is available for many of these.

    In terms of the carbon footprint of electricity-consuming GSHP vs. locally burned fossil fuel, even with an overall COP of only 3.0, with a delivered efficiency of the electrical power at only 33% of the fuel consumed at the power plant, the GSHP winds up at 99% overall efficiency, vs. low 90s for a high-efficiency NG or propane-fired boiler. If the electrical power comes from hydroelectric, the efficiency of the GSHP is quite large.

    Clearly, GSHP (or any heating source) isn't best for every situation. However, generalizing serves no useful purpose.

  32. Expert Member
    CARL SEVILLE | | #32

    Once again, warm climates are forgotten
    Martin- great piece, but with the exception of a very brief mention about mini splits being able to air condition, it would be nice to remember that much of the country lives in areas where air conditioning, and especially dehumidification is necessary for part of the year. The cold climate bias rears its ugly head (nothing personal) once again.

  33. GBA Editor
    Martin Holladay | | #33

    Forgetting cold climates?
    I'll accept the criticism, but only up to a point. An article about how to buy fresh fruit shouldn't be criticized because the author forgot to mention pork chops.

    No article can focus on every topic; this week's topic is heating systems. To the extent that heat pumps can provide both heating and cooling, that advantage was certainly highlighted. But I was focusing on methods of heating.

    The article noted that available heating systems are usually oversized for green homes. Certainly that point applies where you live, in Georgia, as well as in Texas and Florida. If anything, the topic I addressed — choosing an efficient heating system when the heating load is very low — is even more relevant in your climate than it is in Vermont.

    That said, I appreciate the reminder that hot-climate builders feel neglected. I'll try to return to topics that are more relevant to hot climates in upcoming blogs.

  34. furniturefarmer | | #34

    distribution not an issue?

    Very timely topic. In my experience the critical issue always becomes heat/cool distribution, and I have to wonder about your claims that distribution is not that important in a Passive/super-insulated House. I have not been able to sell my clients on point source heat (although that’s exactly what I would do if I were designing my own home). I would love to see more scrutiny ( i.e. data) on this issue because it affects the cost so directly. If the room with the heat source averages 72 degrees what does the back bedroom average? Don’t forget that the bedrooms are supplied with fresh air from the HRV that is typically cooler than the room air.

    You also mention that stratification is not an issue. Maybe in a one story building, but in the Passive House we just completed (basement plus two stories) we are already finding substantial temperature differences between the upstairs and downstairs.

    Also, I’m surprised you did not even mention the hot water heat coil made by Ultimate Air. It can be hooked up directly to the ERV supply ducts, can supply 8700 Btu/hr, and I believe it costs about $800. This could meet the heat load in many climates, and could be backed up with electric baseboard in many others.

  35. GBA Editor
    Martin Holladay | | #35

    I share many of your questions
    Like you, I'm eager to hear of other builders' experience with point-source heat. I'd like to know whether occupants are satisfied with such systems.

    When I recently visited Urbana, Illinois, I toured two Passivhaus buildings with ductless minisplits; the heat was introduced at single location, although the heat was supplemented by electric resistance heat distributed through the ventilation ducts.

    In my own house, which is heated with a single wood stove in the living room, two of the three bedrooms do get colder than the other rooms when the bedroom doors are shut. Whether or not this is a problem depends on the temperature one prefers for sleeping. If a bedroom door is open during the day, then the bedroom will be warm when one goes to bed. If the door is kept closed all night, the bedroom will be cool in the morning. The third bedroom is adjacent to the stone chimney, and stays toasty.

    I'm delighted you mentioned the hot water heat coil for Ultimate Air ERVs. I have heard of it, but haven't been able to locate information on the Web. I just phoned Ultimate Air, and the woman who answered the phone didn't know anything about it -- although she promised to get back to me once she made further inquiries. I'd love to hear from builders who have installed the unit. I'm especially interested to learn whether it comes with controls and a circulator, or whether it's up to the builder to design these parts of the system.

  36. user-723121 | | #36

    In my experience with superinsulation and airtight (1 ach50) or less housing I did not see any measurable stratification with an open floor plan. By open I mean 2 story living areas and open stairwells. I believe the open plan allowed the natural convective loop to occur and the indoor temperature stayed quite even from floor to floor.

  37. GBA Editor
    Martin Holladay | | #37

    More information on the ERV heat exchange coil
    I just got a phone call from Craig Kinzelman at Ultimate Air. He said the person who knows about the heat-exchange coil, Jason Morosko, is out of the office and won't be back until next week. I'll know more after I get a chance to speak with Jason.

    Craig said the option is brand new. He said, "We've sold a number of them but we haven't delivered any yet." In other words, nothing has been shipped.

    Craig said his best understanding is that the heat-exchange coil and the insulated cabinet to surround it cost about $325. That price does not include a circulator or controls, however.

    To get the unit to produce 8,777 Btuh — the maximum output — you need to crank up the water temperature to 160°F and operate it at 200 cfm.

    I'll post more information when I learn more.

  38. Jesse Thompson | | #38

    Here's a link to an ASHRAE paper by Duncan Prahl, Thomas Hartman, Bruce Coldham & Katrin Klingenberg from 2007. It has graphs of instrumented point source heated houses:

    More here from Coldham & Hartman's Rocky Hill Cohousing:

  39. user-626934 | | #39

    Air to Water Heat Pumps
    Coming soon is an air-to-water heat pump from Daikin, called Altherma. I'm told it's distribution is currently limited to he Pacific Northwest. Here's a link to what looks like the American brochure, thanks to the website of Nelson Mechanical Design -

    It can handle space heating, domestic hot water and (optional) space cooling. Distribution system for space heating and cooling is up to you (water-to-air coil in-line with ERV, radiators, in-floor radiant, etc.). COP for heating is 4.34 (at 45F outdoor temperature). Annual "COP" for domestic hot water should be really high (if COP is 4.34 at 45F, perhaps it's close to 8.0 or 9.0 at 95F?). Cooling EER is 12.17 at 95F outdoor. I'm waiting on more data for its performance/capacity at other temperatures, especially its capacity at low outdoor temperatures.

    Air to water heat pumps have been available in Europe and in Japan (where they're called "Eco-cutes") for the past several years. I would expect models from other manufacturers to follow soon.

  40. GBA Editor
    Martin Holladay | | #40

    Great links
    Thanks for the great links to papers and presentations that discuss this issue directly. It's great to see temperature measurements.

    I was struck by Bruce Coldham's conclusion, which exactly matches my experience living in my house for the last 29 years:

    "In order for the heat to distribute evenly through the upstairs, bedroom doors need to be left open or ajar throughout the bulk of the day. If they are, one would expect a temperature differential of no more than 5 degrees F degrees during the coldest days. If upper floor doors are closed, a 10 degree F differential is to be expected." [Coldham & Hartmann presentation]

    It's great to see researchers out there with their data loggers. But I can't help thinking what the reaction of most Vermonters when they read these papers: "Yup, they spent $20,000 of research money to show that you can heat your house with a wood stove. Oh, and they figured out that when you close the door, the bedroom is colder than the other rooms. I could have told them that."

  41. GBA Editor
    Martin Holladay | | #41

    Eco-Cute water heaters
    I wrote an article on Eco-Cute water heaters for the June 2007 issue of Energy Design Update. The article noted, in part:

    "Over the past few years, at least 12 Japanese manufacturers have entered the residential HPWH [heat-pump water heater] market: Chofu, Corona (Denso), Daikin, Hitachi, Kyocera, Matsushita, Mitsubishi Electric, Panasonic, Sanden, Sanyo, Shecco Technology, and Toshiba. As a marketing ploy, a coalition of electric utilities invited competing manufacturers to sell their HPWHs under a common brand — or, as several press releases call it, a 'pet name.' The utilities settled on 'Eco Cute' — a typical Japanese example of fake English dreamed up by the marketing department of Kansai Electric Power.

    "Most Eco Cute water heaters consist of two separate units: a heat-pump unit and an insulated water storage tank. Several manufacturers advertise Eco Cute appliances with a COP of 3.0; specifications note that the units can heat water to 90°C (194°F). A typical tank size is 370 liters (about 98 gallons); the relatively large tank size allows Japanese homeowners with time-of-use billing to take advantage of lower nighttime electric rates. ...

    "The fast-growing market for Eco Cute appliances in Japan is particularly surprising in light of their very high cost. The typical Eco Cute unit sells for 700,000 yen, equivalent to $5,800; the Japanese government hopes that improvements in manufacturing efficiency will lower the price to about $3,300 by 2010. A subsidy of $665 is offered to any homeowner who replaces an existing water heater with an Eco Cute appliance; for new home purchasers, the subsidy drops to $416.

    "So far, no Japanese manufacturer exports a Eco Cute unit to the US. ..."

  42. gaspr | | #42

    Natural gas vs electricity
    Interesting to note that the actual energy cost of gas is about one third the cost of electricity, at least in our market. But...because of the additional fixed monthly charges associated with natural gas, whether you use any gas or not, ($14.50/month here in Sask...over $30.00/month in Alta) it becomes much less of an advantage to use gas as the total energy use goes fact at some point, electricity becomes the cheaper alternative. The "greenness of electric depends of course on how it is generated...

  43. Kevin Dickson | | #43

    Significantly Cheaper than Minisplits: PTACs
    PTACs are all-in-one heat pump units, 12-24,000btuh that have an installed cost of $500-$800.
    Nobody makes one with a high a SEER and ECM motors yet, but who cares at that price? There are some other minor drawbacks, but in a small superinsulated house these are minimized because it's rarely on.

    Here's a link to a builder who used one at

    A little more theory at

    And here's a discussion of using a PTAC in an "air to air geothermal system":

    This thread is the near-term future of residential HVAC, but the average builder isn't dialed in. Some are (I believe) irresponsibly promoting Geoexchange for low energy homes such as:
    Why would you spend $17k on a geothermal system to save $87/yr?

  44. Travis Kettering | | #44

    HRV/ERV Use with Mini-Split
    I am interested in the use of a minisplit system. I was previously planning on using geothermal, but now I am wandering if it is worth the cost. If a minisplit system was used would you still use a HRV or ERV system? Wouldn't this help with only having one or two heat sources in the house?

    Also have you heard any further information on the Ultimate Air with heat exchange coil? Thanks.

  45. GBA Editor
    Martin Holladay | | #45

    Minisplits and HRVs
    A ductless minisplit system is used for heating and cooling. It has nothing to do with ventilation.

    If your house is tight, you probably want to provide mechanical ventilation. As with any house, you have several options, including exhaust-only ventilation, supply-only ventilation, and balanced ventilation using an HRV or ERV.

    UltimateAire sells the RecoupAerator ERV; as far as I know, it also offers a hydronic coil for insertion into the ventilation air stream. Once it is hooked up to a water heater or boiler, the hydronic coil can supply up to 8,700 Btuh. That's not much, but it might be enough for a very small, very well insulated house.

  46. Sean Armstrong | | #46

    living with my mini-split
    One new development the article didn't mention was the combination of a air-source heat pump heating a large tank of hot water that can be used for both radiant heat and domestic hot water. That system effectively combines the ideal heat source with the ideal heat distribution system, and it's already available on the market--Daiken makes them, Sanyo makes them, etc.

    I enjoyed the article in part because it affirms my own experience--I've foam insulated my 1100 sf, 100 year old farm house and installed a 9-12K BTU Fujitsu mini-split as the sole heating source, and all my guests comment on how warm and comfortable the house temperature is. I don't even use the ceiling fan with my 11' tall ceilings because the fan coil blows heat continuously, which seems to solve most of the heat stratification issues. So now I specify them for all my affordable housing projects, because going carbon neutral is important now, and we should stop pretending we have the luxury of proceeding with natural gas infrastructure as if global warming wasn't real.

  47. user-810821 | | #47

    Radiant on lower level...what on upper level
    I am working on the plans for a tight, well insulated home. The walk out basement is living space and the owner is thinking about radiant down there from a ground sourse heat pump. He is also looking into ducted air on the main level of this home. Does this make sense? Full south facing longitudinal axis of home so solar is of great potential.

  48. GBA Editor
    Martin Holladay | | #48

    I don't know your climate
    I don't know your climate, or the design of your house, so it's hard to comment. But as long as you asked your question — "Does this make sense?" — I'll answer: No.

    You're planning on investing tens of thousands of dollars to buy:
    1. A ground-source heat pump system. Let me guess -- about $20,000?
    2. Radiant floor tubing.
    3. Ductwork for the upstairs system, with heat provided by either a hydronic loop from the GSHP (I don't know if this would work, because GSHPs put out water with a maximum temperature of about 130°F) or a furnace.

    That's a lot of equipment to heat "a tight, well-insulated home." What if you invested $15,000 in better air sealing and thicker insulation? Then maybe you could get away with $5,000 for ductless minisplits.
    Any the homeowner would have much lower energy bills forever, because you would have upgraded the performance of the building's shell.

  49. Kevin Hanlon | | #49

    Please convert Daiken Altherma to HSPF
    I'm looking @John S statement that the Daiken Altherma has a COP of 4.34 (@ 45F). Where is this information available, outside of the product manufacturers brochure? Is there a 3rd party like AHRI or ESTAR who validates this, and label it as HSPF, the seasonal value. I'm asking as I have a large project in Nashua that wants to go with this Daiken ASHP, It exchanges to water, then drops heat to the DHW tank for potable, and drops hydronic heat to the air handler for ducted space heating. Thanks for any help.

  50. GBA Editor
    Martin Holladay | | #50

    Contact Daikin
    Your best bet is probably to contact Daikin's North American office:

    Daikin AC (Americas), Inc.
    1645 Wallace Drive, Suite 110
    Carrollton, TX 75006

  51. DoctorBeer | | #51

    System diagram for radiant system priced by Michael Chandler

    do you have a system diagram for the system you priced in your 11/17/2009 post?
    I'm building a passive solar house in Hyde Park, VT with R-40 walls and R-50 roof. It will be about 2100sf finished above grade. I'm leaning towards Inline triple pane windows.

    I'm having a hard time figuring out the heating system. I have been considering a radiant sub-floor system. I've had so many conflicting opinions about design for it.

    I'm leaning towards a closed loop if I go with radiant sub floor because the house will be a vacation home on well water and I am concerned about water sitting in the system for long periods at temps that favor bacterial growth. An open system where the DHW is shared with the radiant has that potential but one where the DHW is isolated shouldn't.

    I'm rethinking the whole radiant floor heating thing after reading this thread though. Especially because at least one plumbing supplier (out of Waitsfield) keeps steering me towards boilers that cost $3500 or more saying that On Demand HW heaters are not very efficient when used for low delta T uses such as heating water in a radiant loop. I've probably considered over a dozen different configurations for a heating system for this house.

    So I'm very curious as to what the system diagram for the one priced out looks like since it seems like a viable option that's much more affordable than anything I've considered so far.


  52. MICHAEL CHANDLER | | #52

    The system diagram I priced in that post
    Here is the system diagram I priced in that post. the key is to use a drain back type collector with a few adjustments to the Steca controller to adjust for summer over-heating. we kick the tank overheat lock-out up to 170 degrees since the collector area is way out-sized for the tank size and adjust the differential on-off to turn the pumps on when the top of the collector (T1) is only 12 degrees hotter than the bottom of the solar tank (T2) and to turn it off when the temp differential is only 5 degrees. if the bottom of the tank is 50 the thing starts collecting when the roof is 62. once the bottom of the tank is 170 the thing locks out until it drops 30 degrees (Hysteresis) to 140. even then it wont turn the system back on until the T1 over temp drops to 200 (200 on 212 off) to help minimize the system kicking back on with a raging hot panel which would make the solar fluid flash to steam and eventually boil off your drain back reservoir out the pressure relief valve.

    You have to match the re-charge pump to the flow curve of the demand water heater. (For the Quietside ODW 120A or 180A that's a Taco 008 or Wilo 32 for the Rinnai a Taco 009) If it runs too slow the DHW will modulate down to a low burn and run very in-efficiently (imagine of you drove everywhere at 15 miles an hour) you want it to take in water from the bottom of the tempering tank at about 120 and replace it with 140 degree water.

    You also need to control the tendency to "roll the tank" by using a perforated "solar" dip tube where the water comes back into the tempering tank to diffuse the flow. We've been replacing the electrical heating elements with 1"x 3" brass nipples and breaking the flow by dropping a 3/4"x 1" bushing into a 1" elbow on that nipple. It seems to work okay but probably not as well as a perforated dip tube. Removing the elements means you don't need a PT valve and frees up two or three ports on the tank. (you'll still have a pressure relief valve on the solar tank DHW and drain back tank.)

    Obviously the goal is to run the thing off of solar as much as possible at which point the water in the tempering tank may get as hot as 170 so you need a good tempering valve on the domestic hot and you'll need to bypass the red over-temp on the thermostat (assuming you use the tank t-stat to control the re-charge pump and the three-way valve.)

    Most of this stuff you can get from Graingers but I get my parts from Solar Hot USA in Apex NC. Their engineering support is really great and most of what we are doing has been worked out with friendly and patient advice from Dan Gretch over there along with Clay and Jeanette. I cannot say enough good about those folks.

  53. MICHAEL CHANDLER | | #53

    Cost of Radiant floor
    It was a great question to ask about the actual cost of Radiant floor installations. Here's how I figure the cost of my projects

    Radiant floor:
    1/2" PEX tubing $0.30 / lf @ 2,000 lf`/ 2,000 sf slab =$600
    flat plate heat exchanger from Solar Hot USA =$350
    two circulator pumps (Taco 009 bronze or eq) @ $160 = $320
    120 volt thermostat $15
    slab prep labor and parts not including foam under slab =$600
    Manifold and heat exchanger labor and parts $800
    Total for 2000 sf single zone radiant floor cost before profit and overhead $3,285

    Demand water heater w/ tempering tank for radiant floor heating
    quietside 120ODW condensing demand water heater 94% eff 120 kBTUh $1,100
    20 gallon electric water heater from Lowes for use as tempering tank $400
    high speed pump (Taco 009 bronze or eq) w/ iso-purge flanges $300
    Fittings and pump control wiring $400
    Labor to install $600
    Total heating system for endless 94% eff hot water and radiant floor heat before profit and overhead $2,900

    Solar drain back system three panel 120 gallon
    three 4' x8' solar panels $3,000
    Solar H2ot Solvelux drain back kit $900
    SS drain back tank $300
    120 gallon solar servant tank $1,200
    piping and roof labor $1,600
    total cost of a high output solar drain back system to drive floor and domestic hot water $7,000

    Obviously there are a lot of variables here but this is a good basic picture of the costs involved. with out an outdoor reset mixing valve which as I see it adds about $800 (and yes it does dramatically improve the system) $7,000 for the three panel 120 g drain back solar, $2,900 for the condensing water heater setup and $3,285 for a 2,900 sf radiant floor system = $13,185 total.


  54. GBA Editor
    Martin Holladay | | #54

    Response to Jay Hersh
    It's great that Michael shared the details of his system -- thanks, Michael -- but remember, Michael is in North Carolina. Where you are, in Hyde Park, Vermont, you aren't going to see much sun during the heating season, so it would be unrealistic to expect an active solar system to contribute much to a space heating system.

  55. DoctorBeer | | #55

    Thanks for the diagram, plus other heat & DHW questions
    Hi Michael,

    Thanks for posting the diagram.

    Martin, I'm aware the solar system won't contribute much to the heat during the cold winter months but thanks for making sure. I was up on my lot about 10 days before winter solstice and boy was the sun angle low :-). I've also picked up Daniel Chiras' book "The Solar House" and the map on page 12 showing that only NE Washington State and the Adirodacks have as little sun as northern VT was telling. I also went out to the Univ of Oregon's site and generated the azimuthal chart so I pretty much know the sun angle and direction for every day of the year now.

    Mostly I wanted to see Michael's diagram to understand his system better because I've gotten lots of conflicting opinions about the best way to heat the home. Ffor example some say on demand HW heaters for radiant flooring are inefficient since they're designed to heat a low flow of water across a larger delta T, others say getting a boiler is overkill and a waste of $.

    As a result I am still struggling with the design of a heating and DHW system. I'd really like to avoid over designing the system and wasting $ on equipment I won't need but I'm also concerned about under sizing it and being cold.

    The house will be well sealed and insulated, ACH under 1.0 (hopefully even below 0.35) and R-40 walls, R-50 roof. The rooms facing south will have VT Green Slate for the floors installed over mortar on Schluter Ditra so they should absorb and hold a bit of passive heat. Windows will most likely be Inline 325 Triple Pane Hard Coat casements.

    I did have some thermal modeling done with REM/Report on the prospective design, but it needs to be redone with the correct U and SHGC values for the windows I plan to use as the ones used to do the modeling were chosen as "typical" and are not as efficient as those I plan to use. The modeling was also done with a less optimal orientation as the short side (30ft) was facing south and we've now changed the orientation so the long side (48ft) will face south thereby increasing passive solar gain.

    This thermal modeling showed a 32KBTU/hr heat loss at -6F which is likely to be an upper bound given that the changes made since then will improve the home's performance.

    Also I currently have 3 ten tube banks of solar thermal tubes I plan to use. The home will be a vacation home for at least the next 10 years (probably 20) so the DHW needs are basically zero when we're not there. Since the sun does sometimes shine even in VT in winter (average of 5 days in January according to the NWS) it seemed like a waste to collect the heat and have it just sit there waiting for us to show up and take a shower, or to not even collect it at all, if it could make at least a small contribution to the heating needs. Maybe I'm still being too optimistic.

    I was originally leaning towards radiant sub floor heating because that seems like the only type of heating system that would allow whatever solar heat is collected by the tubes to be put to use. Radiant floor heat is fairly pricey to install even if I could settle on a design. After reading this article and Alex Wilson's one on Radiant Floor heating I'm reconsidering that because it seems like it's actually a somewhat inefficient way to heat a well insulated passive house. I'm back to considering electric (baseboard or boiler) and ductless mini split heat pump as options.

    Propane costs in VT seem to run $3 if you buy over 500 gal but more if you don't. Maybe as much as $4.85/gal from what a friend tells me. Electricity is $0.175 per KwH over the 1st 100KwH in Hyde Park. Unfortunately I don't have a feel for how much electricity it takes to create a BTU of heat on electric baseboard or boiler. If the home's heating needs are indeed low enough then I'm not sure I'd use 500 gal of propane in a season (again maybe too optimistic) which is 45MBTU and so could actually make heating a well insulated home like this costlier (on a per gallon basis) with propane.

    First question: How do I determine where the trade-off point is at which having a system which uses only a small amount of propane actually becomes more expensive to heat a well insulated home than using electric heating assuming I know both the price of a gallon of propane and a KwH of electricity????

    The drawback I see with electric baseboard heating is that at those times when I might actually get some BTUs out of the solar thermal system (occasional sunny winter days and shoulder months) such a system wouldn't be able to take advantage of it. As far as I can tell only some type of low temperature hydronic system can do that.

    This brings up the 2nd question: Is there any way besides a radiant sub-floor to utilize heat collected by a solar thermal system, keeping in mind that we're talking about VT?

    If I give up on trying to capture any heat from the solar thermal to use for heating the home and use some other heating approach then the solar thermal system would be dedicated for heating DHW. For at least some of the year it may only produce enough heat to bring well water (likely 40-50F) up part of the way (say to 70-90F) for DHW use and some backup will be needed. Again I've been told that on demand propane HW heaters have poor efficiency using higher temperature input water and that I should consider a different approach. Propane boilers are pricey and seem like overkill though.

    This brings up my 3rd & final question: What is the best way to bring partially heated water (say at 70-90F) intended for DHW use the rest of the way to about 105-110F?

    1) electric mini tank
    2) electric boiler (like the Electro)
    3) propane on demand HW heater
    4) propane boiler

    #4 is probably the most expensive option as far as I can tell.
    #1 might be the cheapest especially since I might even be able to use 120V on these.

    Thanks in advance for your opinions and wisdom....


  56. GBA Editor
    Martin Holladay | | #56

    More info for Jay
    First, a conversion equation for kwh:
    1 kWh = 3,412.9 Btu

    Electric resistance heat is 100% efficient, and a ductless minisplit might be about 200% efficient.

    I've lived in Vermont for many years, and I'm not a fan of propane. It's expensive. If you need fossil fuel, go for fuel oil. Otherwise, try to design an all-electric house, and balance it with PV.

    I think you should use your solar thermal collectors for domestic hot water, and forget about installing an active solar thermal space heating system. The BTUs you get will during the winter from your solar collectors never be enough to justify your huge investment in hardware.

    I have two flat-plate solar collectors, each 4'x8', on a 12-in-12 slope south-facing roof, and they are now covered with snow. They will produce zilch, nada, zip, even when the sun is shining, until late March. If you can rig up a system to remove the snow from your collectors, that's great. But you can't do it on a vacation home when you aren't there.

  57. DoctorBeer | | #57

    Electric in some form is sounding better
    Thanks for the conversion factor Martin. When this thing gets built I'll have to have you by for a homebrew or two....

    To answer you about the siting of the tubes, I've tried to think this out. The open meadow on the lot where the house will be is large enough, and the house's foundation will be cut into a small rise, so that we'll be able to position the solar thermal collectors at what is ground level on one side of the house but still about 6-8ft above the basement floor and they'll pretty much get a whole day of sun even in winter. I was going to pitch them at or close to +15 degrees over latitude (i.e 60 degrees) to maximize late fall, winter and early spring performance while simultaneously lowering summer production to avoid over heating. I was also going to build a roofed 3 sided shed around them which will allow lower angle light to get at them but block higher angle sun with the idea that this would also reduce winter snow build up and summer over heating. So in a perfect world they'll stay clear of snow most of the time...

    We do hope to add a moderate sized wind turbine and/or solar PV with partial battery backup at some point. That's real $ though :-). But it is one reason why I'm leaning to electric. I doubt I would put the heating system on the battery backup, as it would draw it down too fast in a power outage.

    One option now looking promising is going with an electric boiler like the Electro Industries EB-WO-13 coupled with hydronic baseboards. I know how to pipe solder so it would be easy enough for me install the pipes and baseboard (which are fairly inexpensive), thereby reducing costs somewhat. This boiler appears to have an outdoor reset controller and also allows for adjusting the water's high end temp. In addition it stages the KwH usage in increments of 4.5KwH up to the full 13KwH to allow for greater efficiency by only using as much electricity as needed instead of overheating as a single power level might.

    I also did a little research on Slant-Fin baseboard heat output. They rate them all the way down to 110F at which temp they put out about 150 BTU/HR/LF. For a house that needs 32,000 BTU/HR that comes out to about 213 feet. At 120F it goes up to 200 BTU/HR/LF which comes down to 160LF. I'll have to review the plans but if the house loses 32,000 BTU or less I might be able to put that much in.

    In case you're wondering, here's why I'd want to do that. I'm thinking that if I can use baseboard radiators with low temperature water heated by an electric boiler it might still be possible to use the solar thermal to give the heating system a boost, at least for a few months per year on the shoulders, i.e. Oct/Nov and Apr/May, when there is more sun but we're not yet completely out of heating season.

    If designed right at the outset I'd hold off on doing that until after evaluating the solar thermal system's performance working with the DHW system. Then if it does look like I can get enough out of it to give the heating system a boost I'll just modify the system later on. It wouldn't cost much or be much effort since I can just run the hydronic system's return through a heat exchanger in the solar thermal system's combined drainback-thermal store tank. As long as the water running in the hydronic baseboards was low enough and that of the solar thermal tank high enough there will be sufficient delta T present for heat to transfer into the hydronic system.

    Does this sound like a reasonable approach or do you think I'm still barking at the moon so to speak?


  58. GBA Editor
    Martin Holladay | | #58

    Electric boiler?
    It's your house, and you should design it the way you want to.

    For the reasons explained in my article, I think the best way to use electricity for space heat is to install a ductless minisplit system.

    If you want to use electric resistance heat, why not just install electric resistance baseboard units? They are much cheaper to install than a hydronic system.

    My guess is that by the time you add up the cost of an electric boiler and a hydronic distribution system, your equipment will cost just as much as a system with a Mitsubishi Mr. Slim -- and your fuel costs will be twice as much as with the minisplit system.

    On the back of an envelope, calculate the BTUs that you think your solar collectors will produce from November to March, and then calculate how much that fuel is worth. It isn't much -- maybe $20 to $100 worth of heat. So why go out of your way to design a complicated, expensive system just to make use of such a small number of BTUs?

    Is it because you think the BTUs are free? Trust me, they aren't free.

  59. DoctorBeer | | #59

    Shoulder season heat load
    Hi Martin,

    I do appreciate your advice. I definitely will also look into the ductless mini-split systems which I only just learned of. My slight hesitation at those is more aesthetic than anything else since I'm not sure how to mount the fan units in the various rooms without it having a visual impact.

    WRT whether or not to try to squeeze anything out of the solar thermal for heating needs perhaps I'm not understanding how much passive solar can do to heat the home. And I admit I haven't yet run a cost benefit calculation, especially against the mini-split systems.

    I'm definitely following your advice regarding putting $ into getting good windows. I don't disagree that from Mid Nov to the end of March the heat contribution from solar thermal would be minimal. It's the shoulder season where I had thought (perhaps naively) that there might be some potential since the heating load is a lower then and the sun more plentiful.

    That thought comes from experiences in my MA home, where late Sep to mid Nov and April to mid May (what I call shoulder season) accounts for about 18-20% of my total heating usage of around 110MBTU. Admittedly that house is not well insulated, nor does it have windows ideally suited for passive solar gain but it does have good southern exposure and gets noticeable heating in shoulder season.

    My (perhaps invalid) assumption has been that the VT house will also use about 15-20% of its heating needs in shoulder season. Even with the better insulation initial REM/Design modeling seems to indicate a bit over 55M BTU/YR which is about 1/2 my MA home's usage. That's not bad for VT (which is noticeably colder) but at $3 gal of propane (91,600 BTU/gal) that's still $1800/year and at $3 gal of oil (139,500 BTU/gal) it's around $1200/year. If solar thermal can produce enough in the shoulder season to cover heating needs amounting to 15% then even for the lower cost of oil it's contribution would be around $180/year.

    My initial inclination to try to design a system that might still utilize whatever solar thermal (however small) is available to contribute to heating stems from the fact that if I'm using solar thermal (I already have 30 tubes and a tank) for DHW needs I'll still need a backup heat source for it. If I understand correctly this is true even if I use a mini-split for heating. That's why I was thinking it might be worth considering designs which could use a single heat source for both heat and DHW needs while also incorporating the solar thermal hardware that will already be there, and which I think could be added as easily and cheaply as simply routing the heating system return through one extra heat exchanger.

    You've definitely convinced me it's worth looking into the mini-split ductless systems and running the costs for it to compare against the other types of heating systems I was considering. There's nothing like running the numbers oneself to convince yourself of the cost benefit tradeoffs. I'll also see if the equipment for it can be placed with minimal enough visual impact that the wife is OK with it.

    I really do want to say once again how useful this forum is and how much I appreciate those contributing to it. Thanks.

  60. GBA Editor
    Martin Holladay | | #60

    Shoulder seasons
    1. The thicker a home's insulation, and the lower the air leakage rate, the shorter the shoulder seasons. In a Passivhaus building, the heating season begins much later than it does for conventional buildings. It also ends earlier.

    2. Just because 15% to 20% of a home's heating load occurs during the shoulder seasons, doesn't mean that your solar collectors will be able to supply 15% to 20% of your home's heating load.

    3. During the shoulder seasons, much of a home's heating load can be met by passive solar features without resorting to an active solar system.

  61. DoctorBeer | | #61

    passive solar house available to rent in VT, floor thermal mass
    Those are very good points Martin which I hadn't considered having no real experience with living in a solar home. We're hoping to do some winter sports this year in the area where we'll be building in the summer. If there was an opportunity for us to rent a passive solar house up that way it might be good to do that and get a feel for how they perform. If you know of anything please email me, I think you have my email address.

    I've also read that one of the mistakes you can make with passive solar is not having sufficient thermal mass to absorb the heat and re-radiate it back at night.

    If you have a rough idea of how many sf of floor will be lit by the sun is there an easy way to determine how much mass (by weight) the floor should have?

    For the rooms constituting 3/4 the length of the south facing side of the house I'm planning on using Vermont green slate tiles. Not sure if they'll be floor tiles or flagstone. The floor tile is quite cost effective though at under $3/sf. Also I think it will look good in a post & beam house with T&G pine walls. I loved the look of Vermont Verde Serpentine (see the floor of The Whip at the Green Mountain Inn in Stowe) but I think it's too costly to use on a large expanse of floor like ours. I hope the medium to dark green slates will help to better absorb the heat. I was probably going to install it by mortaring them over Schluter's Ditra (whether or not I use radiant floor heating) to allow for the thermal expansion and contraction likely to occur.

    Does this sound like a reasonable approach to creating a suitable amount of thermal mass to absorb and re-radiate whatever passive solar heat the house picks up?

    BTW I am enrolling in Efficiency VT since I think/hope that what we're building will

  62. DoctorBeer | | #62

    Is there a specific mni-split ductless system you recommend?
    Hi Martin,

    I went and looked at the mini-split heat pump offerings from Mitsubishi. I'm not sure which one, if any, could work for me.

    REM/Design indicates that for the house as designed the worst case heat loss at -6F would run around 23.5K BTU/HR. That's for design spec:
    R-40 Polyiso SIP Walls
    R-50 Polyiso SIP Roof
    0.5ACH enclosure (which hopefully would be higher than actual)
    R-20 ICF foundation with R-10 under slab
    Inline 325 Triple Pane Low E Hard Coat windows

    Mitsubishi quotes the heat capacity ratings at 17F. The largest of the multi-zone outdoor units (model MXZ-8B48NA) rates for max 48K BTU/HR and 34,700BTU/HR at 17F. That one looks like it could meet the -6F needs calculated by REM/Design if the output doesn't fall off non-linearly as the temperature gets lower.

    However that is the only individual multi-zone unit that could meet my home's needs. The price seems to run about $3800 for just the unit itself not including the inside blower-heat exchangers. The specs on the website didn't really give much of an idea of what power consumption was at the particular heat BTU outputs.

    The smaller units don't put out enough heat based on the 17F spec to meet the worst case (-6F) loss indicated by the thermal modeling of the home.

    I could possibly use a combination of smaller multi-zone units or single zone units but the smaller multi zone units aren't really much cheaper, maybe a couple of hundred $ less so using multiple smaller units runs the price up almost linearly with the # of units used.

    Do you know of any source of data on the power consumption per BTU output on these units at various temperatures?

    Without that data it's hard to tell if these units have a sufficiently higher efficiency than other approaches such that over time the operational savings would cover the extra up front costs compared to some other potential solutions.



  63. GBA Editor
    Martin Holladay | | #63

    Minisplit heat pumps
    I'm no expert on all of the available models of heat pumps, but I'm impressed by the specs for the Mitsubishi Mr. Slim Hyper-Heat unit (model PUZHA36NHA). As I noted in the blog, this unit has a nominal heat output rating of 38,000 Btuh. According to the manufacturer, at an outdoor temperature of -13°F, its heat output drops only 21%, to 30,000 Btuh.

    I think that -13°F is a more realistic design temperature for your location than -6°F. The model PUZHA36NHA has enough capacity to meet your needs, since it can put out 30,000 Btuh even at below-zero temperatures.

    The Mitsubishi Web site includes spec sheets on all their models. My Internet service provider is having a slow-down today, so I'm unable to do the Web surfing necessary to provide a link to the spec sheet. But you should be able to find it with a little bit of trial-and-error clicking.

  64. DoctorBeer | | #64

    Thanks for that model #
    I found the details at in case anyone else is interested.

    I used -6F because that's what REM/Design specs for Montpelier, which is the closest VT location in its database. It doesn't have Morrisville or Hyde Park. Curiously it doesn't let you change that but it does let you change the number of degree days. I decided to use 8839HDD which is the amount for Morrisville from the NOAA monthly normals 1971-2009 rather than the lower 8245HDD for Montpelier. That's what generated the 23.5KBTU/HR spec.

    I decided to see if I could find someplace else in the country that REM/Design would run for -13F. I found that St. Cloud, MN was listed for -15F so I ran that one and it came up with 26.4KBTU/HR

    I was figuring on getting something with more capacity than the spec no matter what, so the 30KBTU/HR on that device should work.

    I'll take a closer look at it. Thanks again.

  65. DoctorBeer | | #65

    Comparison of recommended heat pump against other approaches
    Hi Martin et. al.,

    This post is for those interested in the cost calculations associated with the mini-ductless heat pump. Hopefully at least some of you will find the results of my exercise here interesting. Please take into account that I have not calculated the installation cost of a complete heating and hot water system based around the various approaches listed so I'm really only comparing operational costs using assumptions for my home's design and location (VT just north of Stowe or should it be spelled $towe :-).

    After tracking down the spec for the Mitsubishi Mr. Slim Hyper-Heat unit model PUZHA36NHA (which appears to be the proper size unit for my home's design) I compared it with some other approaches I've been considering on both up front HW costs and annual operating costs. I also considered other Mitsubishi units like MXZ series but they don't appear to perform well at temperatures below 5F so I'm not sure they'd be able to keep up on a really cold day even with my home's relatively small projected 24KBTU/HR at -6F loss rating.

    REM/Design (which is proving to be a really good and easy to use tool for thermal modeling) calculates an annual heating usage of 38 Million BTU/YR for my planned design. Based on energy costs of $3.45/gal for propane and $0.0935/KwH for the 1st 100 and $0.1725/KwH thereafter for electric the projected annual costs for the heat sources (without HW included in the calculations) I considered were as follows:

    $1898 - Electro EB-MO-10 10KwH 34KBTU/hr electric boiler w/ outdoor setback controller
    $1389 - Prestige PS-60-LP 10K-60K BTU boiler w/ outdoor setback controller
    $1179 - Mitsubishi Mr. Slim Hyper-Heat PUZHA36NHA (does about 38K BTU to 5F drops off by 25% to -13F)

    The spec chart for the heat pump shows total input ratings at 5F of 5.86KwH of 38KBTU for example so the output would appear to be 6484BTU/KwH. At least on paper this is 90% more than the output of the electric boiler which produces 3400BTU/KwH. If the rated performance in Mitsubishi's product spec were linear I would have expected that the electric boiler would use 90% more electricity to produce the same amount of heat and would therefore have an operating cost 90% higher.

    Surprisingly, as calculated by REM/Design, the annual operating cost of the electric boiler was only 60% ($715/yr) higher. REM/Design specifically requires input of the HSPF, 17F capacity and 45F capacity for the heat pump specification, so presumably it knows how to accurately model the power usage of a heat pump.

    I haven't yet priced out the complete system so I can't say for sure but it appears that the for a multi zone system requiring multiple inside units the heat pump will cost more than the electric boiler, but it certainly seems to have the potential to pay for the extra up front equipment costs via it's significantly lower operational costs.

    Comparing the heat pump against the propane boiler was trickier. Based on the REM/Design modeling the propane boiler costs $208 more per year to operate at current propane and electric costs. However with the Heat Pump I'd need to use some kind of hot water heater. Probably something like an electric on demand system such as Bosch's PowerStar AE125. With the propane I'm assuming I'd use a tank for an indirect DHW system.

    Working with those assumptions according to REM/Design heating water with the propane boiler costs about $109/year less at current energy rates. When you subtract off the $109/yr difference in extra DHW costs for the heat pump + on demand DHW approach over just the propane boiler the operating cost differential is more like $100/yr. I'm pretty sure that REM/Design assumes continuous occupation of the home for its DHW cost calculations. In my case it's a vacation home so the DHW use will be lower and the cost difference less, but for now I decided to stick with the numbers as REM/Design calculates them rather than guessing at an adjustment.

    Also I don't think REM/Design takes into account the setback controllers on the electric and propane boilers which improve their efficiency by modulating their energy use downward when there is a lower heat load. I believe that it assumes operation at full capacity. This would have the tendency to over estimate the electric and propane boilers energy usage if it is the case.

    I think it's hard to say without completely spec'ing a system and pricing out installaton costs for both approaches whether or not the heat pump would cost more than a propane system. Not knowing that it's hard to say if it were more expensive if it would pay for the extra up front costs through the slightly lower operational cost. If it were cheaper to purchase and install then it would be a clear win.

    There are, of course, other considerations involved.

    I ran calculations on the per room heating losses at -6F. The large open area with cathedral ceiling looks to lose 17.5KBTU/HR, the smaller rooms such as bedrooms, bathrooms and TV room look to lose anywhere between 1000-3600 BTU/HR.

    The trick with the heat pumps is that they have indoor units associated with them and you have to size them correctly. There are two types of indoor units, those that go on the wall right in a room and those that sit in the overhead and distribute heat via ducts to multiple rooms. These appear to have capacities starting at 6000BTU/HR. For the MXZ series there is lots of info about the possible sizing combinations you can use. It looks like some of them can handle up to 8 wall units. I couldn't find the same type of info for the P series.

    The smallest indoor unit still distributes far too much too heat for them to be used in the smaller rooms in my home which have projected heat losses as low as 1000BTU/hr. For the large cathedral area I'm probably better off using two smaller units to distribute the heat to both ends of it. This makes me think that unless your home is one big open space you'd probably want to use several of the ducted units strategically located to allow the plumbing runs and duct lengths to be as short as possible.

    There are other intangible considerations. The ducted units mount up on a wall and have aesthetic impact. The ducted units are lower profile because they hide in the wall or overhead and have only a duct in the ceiling. Of course certain types of construction (such as post and beam with exposed beams like I'm considering) don't always lend themselves to the easy placement of ducts. Whether you use the ducted or ductless inside units to distribute the heat both utilize fans and you also have to consider the noise difference between this approach and hydronic. Judging from the spec these systems are not particularly noisy, around 55 decibel, which for comparison purposes is in comparable to normal conversational voice levels in a quiet setting. If you're a light sleeper like me you may want to hear one in operation before deciding.

    So Martin is definitely correct that the Mr. Slim Hyper-Heat PUZHA36NHA is much more efficient than other electric heating options. It's also more cost efficient than propane based on the current costs of electricity and propane in VT.

    Of course there is one other unknown in running these calculation. That is the effect that Vermont's shutting down Yankee Rowe (which appears likely) will have on electricity prices. If that does happen VT electric prices could go up and that could completely change the economics of these cost comparisons.

    I have requests in to some area HVAC contractors to try to get some more info on how to spec a heat pump system and what it would cost. I can say however that running the above (admittedly incomplete) comparison was definitely not an exercise for the faint of heart :-)­

  66. user-939142 | | #66

    mr slim 36
    the one issue with that mr slim model is its in their 'pro' line which costs significantly more than the regular tier models

    it is around $6k for that version, where you could buy 4 individual 9,000btu mr slims for the same price

    the other issue is mits offers no multi unit mr slim models yet, so those of us looking to exploit the new tech have to buy multiple units to get any sort of zoning which seems wasteful; the 36,000 but model can be 'twinned' by having two inside output units split, but not independently controlled

    the 36,000 btu model doesn't offer a ducted unit, which is a downside compared to other heating options you listed. you'll have to place it centrally and hope the single location blast distributes evenly. it's meant more for a large retal space, etc

    i've seen charts which figure the relative HSPF for your area, converting the manufacturers claimed HSPF at 45 degrees to your local area.
    in my area, central IL, the mr slim 10.0 HSPF drops to 6.4 HSPF taking local real temps into account

    mits does publish some COP numbers for the 9,10,12K models. IIRC they near a COP of 2 at 17 degrees

  67. GBA Editor
    Martin Holladay | | #67

    Response to Bob Coleman
    According to Marc Rosenbaum, the ducted minisplits all have lower COPs than the ductless ones -- at least at this point. So it behooves designers to find ways to use the ductless models.

    Since I've lived in a cold-climate home with a single point source for space heating -- a wood stove in the living room -- for decades, without noticeable problems with bedroom temperatures, I think that the cold-bedroom problem is overstated. Build a good envelope -- fairly airtight, with good insulation -- and don't worry about heat distribution.

    But if you have whiny clients ready to take you to court for a bedroom that is 5°F colder than the living room, I guess you have to satisfy them.

  68. user-939142 | | #68

    i'm wondering if those COPs would be that much different if the ducted models are placed inside the interior envelope, after adjusting a bit for possibly larger fan energy consumption. a few i compared had similar motor specs

    the numbers drastically change even tough its the same outside unit, and that is the brunt of the system. i guess the way the numbers are measured in ducted systems really penalizes you

    the real issue for many though is that none of the multi-unit models feature the low temp or higher effeciency mr slim featuers yet, and your limited to a ton without paying a lot more for the three ton pro.

    i think it would be tough to get the average client to buy into a single unit model. it would seem like a downgrade. going from central air, back to a single like window model. would take a lot of education

  69. user-942328 | | #69

    Mini Split vs Radiant In Floor in Warm Climate-No cooling needs
    Well I have found this thread very informative - the question I have is what is the best option for a warm climate with no cooling needs.

    My current total home worst case heat loss is aprox 1200 btu/h which is pretty low. 2750 sq/ft home with NG available.

    Current radiant heat installation quote using a mod-con and indirect tank for DHW with pex in slab downstairs and pex in Warmboard upstairs is over $25,000.00 - which seems very high. This doesn't include the Warmboard cost.

    I was looking at options like a mini-split which would work great for the upstairs as it is basically one large room, however the downstairs has bedrooms in a L shape that are somewhat far apart with a courtyard between them - so I would need more than one heat source downstairs.

    What is a better and less costly option, including cost to operate and installation cost for a home with low 1200 btu/h on the coldest day???


  70. GBA Editor
    Martin Holladay | | #70

    Response to Monty
    Are you sure you have your numbers right? If your heating load is really 1,200 Btuh, that's only 350 watts. You could heat your house easily with a refrigerator, a TV, and a computer -- not to mention a few lightbulbs.

    My guess is your number is wrong.

  71. user-942328 | | #71

    Are you sure you have your numbers right?
    No I am not sure, but I have the numbers from an energy calculations run on the designed home.

    The upstairs total hourly heat loss is listed at 20,000, the downstairs rooms combined is at 14,000 - So 35k btu/hr - this is on a pretty liberal 27deg coldest day - which is lower than we have seen in many years...We are somewhere around 1800-2200 HDD

    I mistakenly read the latent heat gain...

  72. GBA Editor
    Martin Holladay | | #72

    Second response to Monty
    Q. "What is a better and less costly option, including cost to operate and installation cost for a home with low 1200 btu/h [I guess you mean 35,000 Btu/h] on the coldest day?"

    A. The entire topic of my blog was a listing of the best space heating options for small homes. If the list of options I have suggested isn't enough of a range to help you decide, I don't know what to say. It's your choice.

    By the way, if you come up with another option that I didn't list, please tell me about it.

  73. user-1005777 | | #73

    Mitsu 12,000 btu in a minihome
    We are into our 4th winter with a mitsu heat pump with the indoor unit in the living room. This is located about half way point in the trailer (mini home). Walls are 6 inch fiberglass . Windows are double glazed from when the unit was built in 1992.. The house was heated by electric baseboard before the mitsu was installed. The heat pump can handle the load down to about 0 F. The bedrooms are at each end and are normally about 3 F below the living room. We set the baseboard thermostats to maintain the lowest temp we want in each area. As an example: the living room programmable baseboard is set at 61 F most of the day, but also set to 68F between 8 am and 11 am. The heat pump is set at 72F all the time. The bedroom thermostats are set to come on at 61 F, but rarely do these rooms drop below 65 F.
    If we had the funds I would now bring all the supply air from the HRV to the heat pump wall unit and exhaust air from kitchen, bathrooms and bedrooms. I think that would help the efficiency of the system.
    Cost of air sealing, heat pump, and HRV was about $7,000, with a rebate of $2,500, so $4,500. Dropped the power bill from $170 to $110 a month. With the increases since, we are at $120 versus $185. Payback is just under 7 years.
    The air sealing really cut down on drafts in the home and makes it way more comfortable.
    We have about 9,000 DHD a year and the lowest temperature has been around -28 F.

  74. mscannell | | #74

    This is a PTAC that I
    This is a PTAC that I actually learned about on GBA. I
    Considering it for a 500sf dry-van house I'm building. The heat load calc is 6000BTU, so I think this unit would be great. It's not as efficient as a DHP, but it's less than 1/3 the initial cost. If anyone has feed back would be appreciated.
    GE Zoneline AZ85E09DAC | Vertical PTAC Air Conditioner W/ Universal Electric Heat | 9,000 BTU/HR Cooling | 208/230 Volt

  75. user-1072251 | | #77

    I've installed one geothermal system and close to two dozen ASHP systems over the last twelve years. Tight, well insulated houses and minisplits; simple & the best systems I've ever used. My question is why, especially here, in GREEN Building Advisor, is anyone talking about using fossil fuels? Are these Climate Change deniers? Simply put - don't use that stuff! Second a memorable comment made by Marc Rosenbaum at a seminar - about how stupid it is to either install a large tank of toxic, flammable liquid or a pipe bringing flammable gas into a house we live in. Eliminate burning anything inside!

    1. GBA Editor
      Martin Holladay | | #78

      I agree with you: an all-electric house is the way to go. (To read a recent article that includes that advice, see "The Best Way to Build a House.")

      That said, this article was written in 2009, when the nation's electricity grid was considerably less green than it is today.

      Even in 2021, those who live in states where electricity is produced from coal-burning generating plants -- states like West Virginia -- can reasonably make an argument in favor of space-heating equipment that burns natural gas.

      Whatever your stand on this issue, we can all agree that reducing space heating needs by choosing a small house that is tight and well-insulated is a climate-friendly approach.

  76. user-1072251 | | #79

    Yes, we are getting there with all-electric houses, but here in New Hampshire, our PUC has voted to eliminate the utility incentives that promote our Energy Star system and building upgrades, claiming it's too expensive. Still 1989 here with a governor slow walking renewables and the majority of builders using fossils.

    1. nickdefabrizio | | #80

      Well, New Jersey was way out in front....Then recently our otherwise progressive governor did the same thing. At about the same time they rolled out some of the most aggressive climate/C02 reduction goals of any state they punted the HVAC rebates back to the utilities for "efficiency" sake. They tried to sell this as a great improvement but the result (as anyone could have predicted) is that the $1,000/2,000 rebates for high efficiency cold temp mini splits and heat pumps are now down to $400 or so. Meanwhile, HVAC contractors in N J had jacked their mini split prices to the moon (possibly spurred on by the idea that homeowners were getting some of it back). I do not see any evidence these prices are coming down. Six months ago I got an estimate for $18,000 for a 2.5 ton and 3 ton Mitsubishi units (not hyper heat) with five heads total....and I hear prices are rising.

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