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Musings of an Energy Nerd

Choosing HVAC Equipment for an Energy-Efficient Home

If you’re building a new green home, you’ll find that most available furnaces and air conditioners are oversized

Combo systems use a water heater to provide space heating as well as domestic hot water. Most combo systems use ducts to distribute space heat. This type of system — one with a hot water coil in an air handler — is often called a hydro-air system.
Image Credit: First Company

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

Why are the smallest available American furnaces rated at about 40,000 Btuh? Back in the 1960s, a house in a cold climate may have needed such a powerful furnace — or even one rated at 60,000 or 80,000 Btuh. But these days, many new homes have design heating loads that are much smaller — as low as 10,000 to 20,000 Btuh. Over the past 30 years, building envelopes have become tighter and better insulated, but U.S. furnace manufacturers haven’t kept up with the times. For mysterious reasons, they don’t offer furnaces that are small enough for today’s energy-conscious builders.

I wrote about this frustrating problem in a 2009 article, “Heating a Tight, Well-Insulated House.” In the four years since the article was written, furnace manufacturers haven’t budged; their smallest models are still twice as big as most energy-efficient builders need.

John Straube outlines the problem

John Straube is a professor of building envelope science at the University of Waterloo in Canada, as well as a principal at the Building Science Corporation in Massachusetts. Straube has been wrestling for years with the question of how to heat small, energy-efficient homes. On February 7, 2013, Straube gave a presentation on the topic (“HVAC for Low-Load Buildings”) at the Better Buildings by Design conference in Burlington, Vermont.

Straube defines a low-load house as one with a design heat load of less than 25,000 Btuh or a design cooling load of less than 1.5 tons. Homes like this are increasingly common. “Peak demand for superinsulated houses of 2,000 square feet is often 20,000 btuh or less, and townhouses can be under 12,000 btuh,” Straube noted in Burlington.

Finding the right equipment is hard

If you install conventional (oversized) equipment in a low-load building, says Straube, “You get a choice of freezing or…

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  1. user-1050854 | | #1

    Demand for Right Size Furnaces
    It is the cat chasing its own tail. The manufacturers will not make right sized equipment, so dealers can't install it. If they can't install it, they can't order it. If the manufacturer doesn't get an order, they will not make it.

    I have specified Mini splits on 4 residential projects. All 1500 sf or smaller. The lowest price was 12,000 installed. Nice to see John's comments on mini-split pricing.

  2. user-2310254 | | #2

    Cooling Climate
    Hi Martin:

    The article is great but seems to emphasize heating over cooling. Are the issues the same for a climate zone where there is an equal or greater need for cooling? Also, if comfort is more important than cost savings, would the best option be to installed dedicated split minis in most rooms of a well insulated house?

  3. Expert Member
    ARMANDO COBO | | #3

    Great article
    Another problem with heating small heating load houses with a water heater is that when the warranty from the original installers (hopefully and HVAC/Plumber company) is over, home owners have difficult servicing issues with “regular” HVAC or plumbing companies that do not know how to work on their hydronic systems. Too bad because it is really a great solution otherwise.

  4. GBA Editor
    Martin Holladay | | #4

    Response to Steven Knapp
    In most locations in the U.S., even areas with a hot climate, you still need space heating during the winter. The problem of finding low-load heating equipment is even more acute in Georgia than it is in Minnesota, because heating loads in Georgia are lower than up north.

    When you are specifying heating and cooling equipment, the first step is always a load calculation. If you need 2 tons of cooling, you have plenty of equipment to choose from. If you only need 3/4 ton of cooling, however, you're in the same boat as a Passivhaus builder looking for a furnace: all of the available equipment is oversized.

    Ductless minisplits or ducted minisplits are probably the best solution. Only you can determine your comfort needs -- and whether your budget allows you to install a minisplit in every room, or just one unit to serve the whole house.

  5. user-543035 | | #5

    I agree with John's evaluation of heating systems, we have been installing low load systems on all of the homes we work on and typically use very well installed air source heat pumps, (sometimes with two stage equipment where the second stage is used in place of resistance heat when the temps are below 20 degrees. And frequently a combined hydronic air handler on a condensing tank with a simple open loop system. I have one on my house that I monitored this past winter and based on our instillation it is putting out a very even 3500 BTU/hr. at a flow rate of .25 gallons per minute. If you add in waste heat from the fan and pump running at 89W the overall load is just about right. The system runs 20hrs on design days and provides uniform surface and air temperatures through out the house. We achieve the low flow rate with any manufactures smallest equipment with reduced pipe sizes. The inclination might be to use a valve but when you have confidence in your design, the simpler you keep plumping systems the more robust they are. So to archive the 3500 BTU/hr rate I used 1/4 inch mini-split refrigerant tubbing. Unconventional and very effective. Incidentally you will always be off the low end of the charts for temp and flow that the manufactures give, because even the hydronic air handler manufactures think we need 60,000BTU/hr.

    We moved away from gas furnaces in California because of their incompatibility with our Air Conditioning air flows. In a climate where dehudification is not necessary, we need to move 500-600CFM per ton for efficient air conditioning. The high air conditioning air flows with a two stage furnace locked in low risks condensation in the primary heat exchanger.

    Lower air temperatures are critical to success. Most engineer will design their systems to be at 120 to 140F, with registers that blow air on people. We lower our air temperatures to 90 F or less. If your moving air fast at those temperatures you don't want to blow on people. So grills should throw air out of the occupied path. That makes our heat pump or combined hydronic strategies work well.

    The assumption is that only supper insulated homes will have low loads, but in much of mild California the poorly done houses have loads under 20,000BTU/hrs this is where it gets trick to use point source systems like mini splits in choped up houses. The room walls and window don't maintain as well and so the temperature drift in those spaces can be very large. Finally great duct design is critical keep the static pressure and watt draws of your fan low. It would be silly to a have a system where your air handler was using 500-1000 watts (typical condensing gas furnace install) to move the air when it could be using a 100 or less if you had paid attention to ducting.

  6. GBA Editor
    Martin Holladay | | #6

    Response to Armando Cobo
    I agree with you: most plumbers don't know how to maintain or repair a combo heating system. That's why I wrote, "It’s hard to find the right combination of contractors to install them, and the systems are often quirky to troubleshoot and maintain."

  7. Expert Member
    Dana Dorsett | | #7

    Low demand is probably due to the replacement market
    I suspect that most furnace sales in the US are the replacement market, not the new-construction market. And even though those 60-100KBTU/hr+ behemoths installed decades ago were already way oversized for the original house on day-1, home owners are reluctant do downsize, an contractors have little incentive to educate them (even if the contractors understand the problem, which is not a safe assumption.) It's often quite difficult to convince people that their heat loads are as low as they really are, even when you can prove it with 3 minutes of pencil-scratch-on-napkin analysis on fuel use history.

    In CA under Title 24 it's much harder to get away with 3,4, 5x oversizing factors on heating equipment, (even as replacements), but that's not the case for most of the US & Canada. With low-mass hot air furnaces there isn't much of an efficiency penalty to for gross oversizing, but the comfort factors are very real.

    I remain unconvinced that mini-split cooling & heating in CA in better-than-code or even code-min houses is fraught with efficiency or comfort problems, so long as west-side solar gains can be well managed. This is of course easier to deal with on new construction than existing homes, but the low latent loads make temporarily overheated rooms more tolerable than it would be in more humid climates. If anything it's EASIER to be comfortable with point-source cooling & heating in CA than in the torrid southeast or New England coolth, and as fully modulating systems they have a part-load efficiency advantage over a typical 2-stage, even if not fully optimized for sensible-cooling-only.

    Straube is mistaken in his criticism of Marc Rosenbaum's penchant for all-electric, at least at Rosenbaum's grid-mix:

    “I think Marc is right but, as usual, he is ahead of his time,” Straube responded. “In the time that the typical furnace will take to wear out, we won’t even be halfway to decarbonizing the grid.

    In Rosenbaum's MA coal-fired power is already dead, having been displaced primarily with combined-cycle natural gas, at more than 50% better thermal efficiency, and something between 25-35% of the carbon output per kwh of coal fired generation. MA is also ahead of schedule on meeting its Renewable Portfolio (RPS) target of 15% by 2020, and ~30% fraction of the local grid is from legacy nukes with low/no marginal carbon per kwh. At the current grid mix, resistance electric heating has comparable carbon footprint to mid-efficiency gas (though far more expensive) even today, and ANY heat pump technology is significantly lower-carb than condensing gas.

    That's significantly MORE than "...halfway to decarbonizing the grid..." at least on Marc's region grid. Clearly the numbers will differ in high-carb grids such as UT/WY, but there are many local grids that come in lower carb than Massachusetts.

    But gas v.s. electricity is the LEAST of it at New England's space heating mix, which has an a high legacy fraction of oil-fired space heating, much of which could be displaced economically with mini-splits, as outlined in this recently published policy piece from Rocky Mountain Institute:

    Not only can the outputs be made more right-sized for the loads (those oil-burners are typically 3x the load), even with 15 cent electricity it's dramatically cheaper than the heating-oil at the 5 and 10 year recent averages. At the 3 year historical average it's self-funding- the mini-splits will pay for themselves on heating cost savings alone.

  8. GBA Editor
    Martin Holladay | | #8

    Response to Dana Dorsett
    You hypothesize that "Low demand is probably due to the replacement market."

    You may be right. But it's really hard to gauge demand for a product that is not yet on the market.

  9. Expert Member
    Dana Dorsett | | #9

    Response to Martin
    True, but if they were getting 100 calls a day for a 25KBTU/hr 2-stage you can bet they would at least think about putting something out there. At the moment code-min homes in New England are still mostly-OK comfort wise with a 40KBTU/hr furnace, even if it's 2x oversized for many of them. Tract builders can pick a model that can handle the biggest and leakiest house in the development with some margin and use that for ALL of the houses, independent of size & shape, site orientation, etc. as long as local codes don't penalize them for oversizing.

    CA Title 24 code limits on oversizing factors may eventually provide a big enough market to get the gas-burner manufacturers to make a move, but since most homes in CA are built with air conditioning an heat pumps can handle the heat loads, heating-only appliances like gas furnaces are at something of a disadvantage there.

    According to local utility company analytics I was reading last year, the average design condition heat load for existing single family homes in MA is about 13-14kilowatts, or about 44-48000BTU/hr. Even though ~2500' code min new housing comes in at about half that, the turnover in housing stock is slow- those older homes will still be around for decades (centuries) to come. Even with ongoing efforts at retrofitting higher performance, it'll be very long time indeed before the old housing stock has improved to the point that a 50K 2-stage would be so oversized as to have a comfort issue. The market for 40-60K 2-stagers has legs, even if they were oversized for ALL new housing (which they're not.)

    On better-than code new housing and smaller code-min houses, there's a void, but it's pretty tiny and low-profit compared to the market as a whole, but it's also a market that can economically be met with ductless heat pumps more often than not. If the paradigm ends up shifting from ducted output fossil-burners + split-system-AC coils to Marc Rosenbaum's mini-split nirvana, that's not necessarily a bad thing, though it would be nice if the current R410A refrigerant paradigm also shifted to R744 (CO2), at a miniscule fraction of the global warming hit from refrigerant leaks. (There are already some R744 cold climate heat pump options available in Europe & Asia, not so much in N. Amer.)

  10. jinmtvt | | #10

    mini-splits ...
    How many bed rooms are there in average US home nowadays ??

    Fujitsu has a 9K BTU model ( 9RLS2... ) which is a little bit cheaper than the 12K model
    and eve has higher efficiency ( its the 27 SEER one .. )
    Why not have one installed in each bedroom if temperatures in the bedrooms are so important ??

    Judging by unit prices , installing 4 single zone 9RLS units should have the approximate same
    total cost a using multi head units , and still provide much higher efficiency in both cooling and heating .

    The cost difference going from central large units, or fuel guzzler heating devices that still requires some AC to be installed Vs mini splits must be a real joke.

    And then u get double or even triple the efficiency on energy usage.

    For now, it looks like a no brainer on 95% of situations, and so many different models
    and options from the 3-5 top brands that it should work in any setup.

    And still get rid of the damn fuels and smokes !!

  11. Marc Rosenbaum | | #11

    Ducted mini splits
    Fujitsu is introducing a line of single zone ducted minisplits with high efficiency and output at low temperatures (ratings go down to -5F). The smallest unit is a 9,000 BTU/hour cooling rating but is rated at about 15,000 BTU/hour heating at 0F (I'm not at a place where I have the spec at hand). I'm planning to use one in the small house I am renovating. My recollection is that the HSPF of the small unit is 12.2.

  12. GBA Editor
    Martin Holladay | | #12

    Response to Marc Rosenbaum
    Is this the unit you are talking about?

    One option for this Fujitsu minisplit line is the "Slim duct indoor unit" option.

  13. GBA Editor
    Martin Holladay | | #13

    Fujitsu capacity tables
    Here is the link to the Fujitsu capacity tables:

    As an example, if you pair the Fujitsu AOU18RLXFZ outdoor unit with the 9,000 Btuh indoor unit, you get a heating output of 10,270 Btuh at an outdoor temperature of 5 degrees F dry bulb.

  14. jinmtvt | | #14

    I had a discussion with a
    I had a discussion with a Fujitsu rep about a week ago,
    and i was told that there is a new model for zone 6-7-8 dubbed 2H ..example 9RLS2H
    which is tuned down lower even, and will work down to -15f at the same efficiency than the other model @ -5F .. will get the numbers straight if i can ..but out of my head this is what i t was about.
    Same pricing .

    ah found it at

    i believe it is a new model that just got out a few weeks ago and websites aren't all updated yet

    the 9K ( 3/4 ton ) model reports a Heating Range (BTU/h): 3,100-22,000
    22K ? really? wow

  15. Redjeepjamie | | #15

    Variable speed compressor heat pumps
    Both Carrier and WaterFurnace have released variable speed compressor heat pumps. I just installed a 4 ton Waterfurnace geo that can deliver as little as 10,000 BTUs and it only takes about 900 watts to do it , the 3 ton system can go even lower. I see these systems as the future of HVAC especially as we increase the use of PV with a goal of net zero. Geo systems do cost more up front but the have twice the life span of air source heat pumps and more than twice the efficiency so they are well worth the extra cost. I know mini-splits are popular in the energy nerd world but the truth is my customers hate the way they look . People want attractive comfortable homes and mini-spits are lacking in both areas. Also You might want to ask a few HVAC contractors about your mini-split pricing, as a business I can't put one in for less than $4,000 and make any profit. Are you going to be at ACI ? I'd love the chance to speak with you personally.

  16. GBA Editor
    Martin Holladay | | #16

    Response to Jamie Clark
    You wrote, "You might want to ask a few HVAC contractors about your mini-split pricing, as a business I can't put one in for less than $4,000 and make any profit."

    I've heard that before. I also know that Carter Scott's contractor charges him between $5,500 and $6,000 per house to install two ductless minisplit units, and John Straube's contractor charges about $2,300 to install a Fujitsu ductless minisplit.

    Prices vary. But as these units become more common, it looks like the installed cost is going to drop.

  17. jinmtvt | | #17

    You can see the pricing on
    You can see the pricing on you nits dot com ...
    how does one charge 4000$ for a ~2000$ worth of parts mini-splits?
    If you accept the price, you are to blame.

    I've been quoted locally at around 3000$ installed for 12RLS2 ( 4 at a atime though .. )
    and normally prices for this kind of stuff are higher here than down in the states.

    The fact that you need/want to make profits doesn't have anything to do with the efficiency of the products, the way that minisplits are currently filling up for the job and have a real payback time.

    What is the total cost of the geo systems you are referring to ?

    I'd like to see a geo/HP system without any failures and maintenance in a 15 years period.

    Because quality minis tend to do that ( take Mitsu and Daikin for example .. )
    Alot of 20+years old daikin systems are still working around the world.

  18. Redjeepjamie | | #18

    Martin, as a professional
    Martin, as a professional HVAC contractor I fight the one man in a van or other low ball contractors everyday . Those prices are well below what contractors should be charging. As a leader in the building science world I think you have a responsibility to quote accurate "market rate" pricing, you are doing my industry a disservice when you use the price some poor uneducated contractor is "willing" to slap it in for. Builders read a blog like yours and then expect good HVAC companies to install at prices that are unrealistic. GBA might want to start a column on how to run a successful business. There are a lot of contractors out there that are failing because they do not know the business side of their business. I enjoy your blog I just disagree with you on this issue.

  19. Redjeepjamie | | #19

    Response to Jin
    Jin, it's not about profits it's about cost of doing business . With labor, workers comp, insurance, cost of trucks, maintenance , fuel , office overhead ; even an efficiently ran HVAC company needs to sell at a 40% GP just to break even, that works out to about cost times 2.
    After tax credits a 3 ton geo on new construction comes out at about $13,000-$15,000 depending on specifics ( in Lexington KY and the tri-state area )
    According to ACCA geo systems average 24 years and air source is 12-15. I've not seen any life span data on minis. I go back to the fact that they are unattractive to the average consumer and they do not move enough air to equally condition larger areas. I'm not anti mini on a room addition or sun poarch, I just don't see the average consumer wanting a houseful of them on a new build.

  20. jinmtvt | | #20

    JAmie: if you can pull out 3
    JAmie: if you can pull out 3 tons geos at that price, good. i don't see that around here though.

    But maybe your business is not a good one ??

    Attractiveness has little to do on this forum, i do understand your point, but aren't we looking for efficiency here ?? I don't like much better to see 20 air traps all around the place
    and fugly ducts through the basement.

    Cheap minis don't and wont last .. always and electrical board failure somewhere, preventing the hole unit to work even though everything else is good, and since the no brands do not give a long warranty and there is no replacement parts..they are replaced.
    But the high quality ones ( aka Daikin, Mitus and Fujitsu ..some other are up there now also )
    Last a very long time, and parts are available to fix the small problems for a 15-20 years period.
    ( i believe mitsubishi was boosting about having parts for models of 25 years ago. .or something like that .. . i know that MEAU has for all cnc controls ..same company )

    What are the prices before the taxe incentives on this kind of setup ?
    and who is paying the incentives ?

    Keep in mind one thing though,
    it will be much harder to justify a 12-14 000$ setup in the near future,
    when the loads will be much lower in high efficiency houses
    where a ~3000$ split with 2-3 baseboards backup and a very simple mixed air strategy with the HRV setup will be more than what is required for the loads.

  21. GBA Editor
    Martin Holladay | | #21

    Response to Jamie Clark
    When it comes to prices, I am a reporter. You have shared valuable information: in Lexington, Kentucky, a reputable HVAC contractor charges about $4,000 to install a ductless minisplit. That is one data point, and it is valuable. Thanks for sharing it.

    You make a valid point about the cost of doing business, and the fact that many young contractors who own little more than a pickup truck don't understand how to run a business. As a former builder, I can validate your observation. Like most contractors, I used to be the stupid guy with a pickup truck when I started out. Like most contractors, I started out not charging enough.

    A reputable contractor has insurance, pays for workman's comp for employees instead of pretending that his employees are all "independent contractors," shows up in clean clothes to discuss new jobs, cleans up the work area after every day on the job site, answers phone calls promptly, and takes care of problems during the warranty period without delay or fuss. Such a contractor also need to charge enough to afford the occasional vacation, and to build up enough capital to help the business grow.

    GBA has published many articles on business issues for contractors. Here are some links:

    Trade Contractor Management (Part 1 of a 5-part series)

    The Business Side of the Green-Built Environment

    Doing Business in the Green-Built World

    The Business of Building a ‘Building Business’

    How to Position Yourself as an Expert Eco-Builder

    My 1st Commandment: Make Detailed Estimates

    My 2nd Commandment: Don’t Give Anything Away!

    My 3rd Commandment: Change Orders Are Nothing But Trouble

    My 6th Commandment: Don’t Try to Be Something You’re Not

    My 9th Commandment: Weak Plans & Specs = Weak Project

    The Business Case for ‘Smaller Is Better’

    Do It Now: Review Your Company’s Health Insurance Policy

    * * * *

    That's not all -- there are many more business articles on GBA. For a complete list, see this page (and the subsequent pages): Business Advisor blogs.

  22. GBA Editor
    Martin Holladay | | #22

    Heat-pump longevity
    Jamie and Jin,
    Concerning the longevity of heat pumps: I think it's fair to say that no one has much data on the longevity of residential ground-source heat pump systems or ductless minisplits.

    I tend to be a skeptic when I hear ground-source heat pump installers brag that their systems will run for 20 or 25 years without problems. I have heard a great many stories about ground-source heat pump systems with problems: failed parts, improper commissioning, poor performance issues that cause multiple visits with factory reps, and lightning strikes.

  23. DennisDipswitch | | #23

    Combo System Updates?
    Martin,your article links to a BSC paper that is 11 years old on Hot water/Space Heating combo systems.Since that time,have any conclusions,updates or improvements been made?

    It seems that one of the conclusions is that the system itself must be designed properly,and isn't as simple as the average Plumber or HVAC tech is used to installing or servicing.So,is there a proven,trusted path of design for these systems?

    I get it,one point being that the usual separation of tasks for the plumbing and HVAC contractor collide on this type of system,but if you take that out of the equation,is there a reliable way to make this work,and efficiently? Am I wrong to believe that many have just thrown in the towel,and have just chosen to stay away from it?

  24. GBA Editor
    Martin Holladay | | #24

    Response to Dennis Dipswitch
    Researchers, engineers, and builders continue to install, study, and monitor combo systems.

    One of those researchers is Armin Rudd of the Building Science Corporation (BSC). The last time I spoke to him on this topic, he said, "BSC is not yet ready to recommend this approach to builders, because these systems have glitches that can cause major headaches.

    "Beginning in the mid-1990s, we began using water heaters for space heating. We had failed check valves in Taco pumps. We had scale. You get thermosyphon flow that gets past the check valve. On some systems, we got hot water flow to the coil during cooling. You get hot water heating systems and cooling systems fighting each other. To cut that short, most of those builders went to a different system because of the problems.

    "Then we started using tankless water heaters. In 2006, decided to take another look at this. We had the low flow problem. The units wouldn’t turn on unless you needed 0.6 gpm. Then we had the frequent on/off flow problem -- a lot of that. You end up with slugs of hot and cold water in the system.

    "Since 2006, manufacturers have responded to these issues. They are now down to the 0.4 gpm threshold. ... We're working with NYSERDA on a pilot project up in Utica. We are now monitoring two systems, with and without a small storage tank. We're getting plugging of the inlet filter of the tankless heater. It happens a lot. The aluminum anode rod was corroding enough to plug the filter. It's a recirculation problem. If you have any recirculation, any decay of the anode rod, it will end up plugging your system. So we addressed that problem with a unit with a stainless-steel strainer. Now, it becomes an annual maintenance thing. Before that they plugged up in a matter of weeks. If you have to clean the filter once a month, that’s no good.

    “We have builders asking if we are ready to recommend this type of system yet, and we are saying, 'not really.' We are still worried about the high frequency of the cleaning of the inlet filters. Building America doesn’t want to recommend a system that will give us a black eye, and we want to be sure we get the savings that we are predicting. Should we be adding a buffer tank or not? If you don’t have that, what are the problems? We're not sure yet."

    In spite of Rudd's list of possible problems, plenty of builders are successfully installing these systems. The ones who are doing a good job are generally the ones who learned the hard way -- by first making four or five different design or installation errors.

  25. DennisDipswitch | | #25

    Are you out there? "Properly designed and installed Combo System
    There you go again...the mysterious "Properly designed and installed Combo System".The more I search for one,the more queasy I get.

    I think at one point I read a lot of Armin's research and read a lot of Building America documents and saw videos of experimental equipment combos being done in Minnesota.That whole Armin Rudd/Nyserda project was especially discouraging,with all the scale and clogging problems they had.It seemed to me the fundamental flaw was using an open system with domestic water being heated and circulated through a steel buffer tank with anode rods. The scale that quickly was produced just seems to be a failure,no matter how you try to filter it out. And I guess small cheap stainless holding tanks are just not out there?(He was trying to use a 10 gallon tank or something).

    Is there any reason that you just can't keep the space heating water separate from the drinking water? I just didn't see the point in not having a large holding tank (stainles(superstor style) and a closed system heat coil/air handler.Yes,I understand that pretty much negates the reason for using a tankless heater,but there could be many reasons you may want to have your water heater(tankless),your heat coil/air handler and your storage tank all in different locations.

  26. jinmtvt | | #26

    That also brings a + on the mini's side.
    How many variables need to be working all together to get the system right on a GEO/HP system ?
    Then how many different problems or parts could eventually hinder the system performance or prevent it to work ??

    Can we fairly assume that many workers in the HAVC trade ( many as in a large % ) are either uninterested, uninformed or just plain not smart enough to perfectly install complexe GEO/HP systems ??

    When using a factory tuned/assembled system, such as mini splits in this case,
    where the installer only has to install the units, without much room for errors because he ain't doing much more than connecting it and mounting it; then as no touch of the design or balance or tuning of the system ... makes it a lot more reliable performance and reliability wise.

  27. GBA Editor
    Martin Holladay | | #27

    Response to Dennis Dipswitch
    Your comment confuses me. "Are you out there?" Are you talking to me, to Armin, or to an HVAC system? And does "out there" mean "listening" or "out to lunch" -- or simply "a real rather than imaginary thing"?

    I don't know where you found the quote about a "properly designed and installed combo system." I don't think I used that phrase on this page.

    Q. "Is there any reason that you just can't keep the space heating water separate from the drinking water?"

    A. No. This can be done with a flat-plate heat exchanger, or with a condensing water heater with an integral coil.

  28. DennisDipswitch | | #28

    Martin,thanks for the morning laugh.
    I will choose the answer "a real rather than imaginary thing"?

    I don't know if anyone has used the phrase "a properly designed and installed combo system",because I can't find anyone who will give the definition of one. It's kind of weird that folks have been experimenting and installing a combo system for at least 12 years,but no one will endorse a proper and reliable way of implementing it.

    It seems that the workarounds to fix the shortcomings,kind of negate the benefits of the system itself,like in the addition of the buffer tank,it seems to bring on a whole new set of problems.

    It's one of those "oh so close,yet oh so far" scenarios.The idea behind the combo system really makes sense.If you have some examples of contractors having great success and trouble free combo systems,it would be great if they would share the knowledge.

  29. GBA Editor
    Martin Holladay | | #29

    Response to Dennis Dipswitch
    The problem with these installations is that many inexpensive installations aren't supported by water heater manufacturers. If you choose a method that is supported by the water heater manufacturer, you need to invest in a very expensive water heater like the Polaris -- and your savings go out the window.

    Here is a link to a Polaris document verifying that Polaris supports the use of their water heater for space heating: Polaris Residential Gas Water Heater Installation Instructions.

    The Polaris water heater sells for $4,000 to $5,200 -- and that price doesn't include the cost of the air handler, pumps, etc. At that price, an inexpensive furnace starts to look like a bargain.

    I'll attach an illustration from the Polaris installation manual. The illustration notes, "Massachusetts code does not allow this type of installation."

    [Click on the image to enlarge it.]


  30. Redjeepjamie | | #30

    I think you have a

    I think you have a misconception about minis as they are heat pumps ( usually) just like a conventional or a geothermal, they have the same basic refrigeration cycle, the same type of compressor, reversing valves, and most other internal parts. So it would be inaccurate to claim that they are less complicated and easier to install. Yes you do not have to deal with duct work issues ( unless you are using a ducted split of course) which does simplify that part of the install, but you still have to field charge the refrigeration which is where so many systems run into problems. By that logic a geothermal package unit would be the easiest HVAC system to install because there is little to no field refrigeration work being done. In my experience minis are just as temperamental as any other refrigerant based HVAC system and needs a professional installation to make it work properly( FYI don't ever install one for a smoker!!). Yes we as an industry have issues with a workforce that for the most part needs to raise their game.
    Back to pricing variables: It is not a fair to compare price differences in geo to that of minis because minis have a very fixed amount of cost, I can get the same price on my equipment here as you can anywhere in the country. With geo the equipment cost is fixed but the drilling/trenching cost will vary wildly depending on your geology. Kentucky and most of our surrounding states are fairly low cost to drill because we have soft limestone and sandstone to work with, out west where the rock is harder it is significantly more expensive. It is also a matter of supply and demand, Texas has more geo than any other state largely because there are a whole lot of drilling companies in Texas and that keeps costs low.
    As far as life span goes if a geo and a mini both have the same basic components and the mini sits outside baking in the sun and freezing in the snow while the geo sits in a comfy climate controlled basement, I'll put my money on the geo outliving the mini any day of the week.

  31. GBA Editor
    Martin Holladay | | #31

    Another response to Dennis Dipswitch
    Another water heater manufacturer that supports the use of their water heaters for space heating is Rinnai.

    Details can be found in the installation manual for the Rinnai air handler.

    I'm attaching an illustration of the recommended piping arrangement for a combo system using a Rinnai tankless water heater.


  32. user-945061 | | #32

    Modulating equipment
    Another great article, Martin!

    There's a lot of mainstream equipment on the market that has outputs in the 20,000-25,000 Btuh range. Some of it may require additional programming, but not really difficult stuff. Not perfect, but worth exploring.

    I'm somewhat familiar with York's equipment. Their Affinity furnace series starts at 60,000 Btuh but modulates down to 35%. Raising both the stage 'deadband' and delay would probably equal 2 hours of continuous running at the lowest output.

    Tstat instructions used by York, pg 29:

    As far as I know, most of Trane's air-flow settings are also at the Tstat. Historically their high efficiency equipment (XV95) has been 2 stage, starting at 60,000 Btu max, modulating down to 38,000. Fairly certain that there's some control over 1st-2nd stage in the Tstat - they definitely aren't in the dipswitches - so installers should be able to avoid cutting. Not positive, but Trane's XC95 is probably even better, with much the same functionality as the York above.

    I have no direct experience with Carrier's equipment. According to their website the infinity 98 starts at 60,000 Btuh and modulates down to 40% for ~25,000 Btuh. Again the airflow can be set up in (you guessed it) the thermostat.

  33. DennisDipswitch | | #33

    Martin,thanks for providing
    Martin,thanks for providing the links and illustrations.So,there seems to be some direction given by water heater manufacturers for a combo installation.

    So,let's cut to the chase,it all comes down to cost.That's the attraction of this system,and the attraction of tankless heaters as the hot water source in this system.

    But it seems that a lot of effort is given to improve the thing that the tankless heater was first intended for: domestic hot water.

    Armin Rudd is using the buffer tank to mostly to eliminate "cold slugs" and to improve the "low flow" response and short cycling..But the buffer tank is also causing most of the scaling and blockage problems.

    So,what kind of issue would a manufacturer of a tankless heater have with using a tankless for space heat?

  34. jinmtvt | | #34

    Dennis Dipswitch: what would
    Dennis Dipswitch: what would be the point in using a tankless for space heating ??

  35. jinmtvt | | #35

    Let's end up the battle already ...
    Unless you've got access to a lake or very large pond, there is currently nothing that will beat air HP
    efficiency because of the initial cost and the very low energy demand.

    If you need more than what is currently offered with mini-splits , look into investing on insulation, because your house doesn't quality as "energy efficient home "

    Now what we need are cheaper water heating airHP from the main vendors...

  36. GBA Editor
    Martin Holladay | | #36

    Response to Jesse Smith
    Thanks for explaining that some furnaces can be programmed at the thermostat. If I understand you correctly, you're telling us that the installer can lock the furnace into low range and prevent the furnace from using high range, simply by programming performed at the thermostat. I wasn't aware of that, so thanks for explaining it.

    However, it still rankles that builders have to buy furnaces that are much too big, and then have to program them so that they only use 1/3 of their capacity -- just because the furnace industry won't make right-sized appliances.

  37. GBA Editor
    Martin Holladay | | #37

    Response to Dennis Dipswitch
    You wrote, "Armin Rudd is using the buffer tank to mostly to eliminate 'cold slugs' and to improve the 'low flow' response and short cycling. But the buffer tank is also causing most of the scaling and blockage problems. So, what kind of issue would a manufacturer of a tankless heater have with using a tankless for space heat?"

    You have properly described the current situation. Rinnai is promoting these systems, so they apparently work. Armin Rudd says that these systems have problems and benefit from the addition of a small tank, which introduces new problems.

    We need feedback from builders who have installed these systems, so that we know how often these systems develop performance and maintenance problems.

  38. DennisDipswitch | | #38

    Response to Jin Kazama
    Jin,the reasons for considering a tankless water heater for space heating,for me,would be:

    1) Cost
    You can find a decent one for $800,more efficient ones at not too much more.

    Tankless heaters are small and light and can be mounted nearly anywhere.

    2A)Elevated Mounting
    You can get these units high off the floor,if you are concerned about flooding.

    3)Sealed combustion/direct vent
    Since they are small and easy to locate,they can be easily positioned for direct vent use.

    4)Ability to locate different components of a heating system in their most advantageous locations

    You may want to have the tankless near the garage,let's say,because it is easiest to direct vent there,you may want a heating coil/air handler in your attic,because that is where your ductwork is located,and you may want an insulated holding tank centrally located in your house,because your kitchen and bathroom plumbing is near each other and hot water could be instantaneous.

    5)You need domestic hot water anyway,why not use it for hydronic heating also?

    I think that "deconstructing" the boiler room and moving the boiler room components around the home holds a lot of potential.

    Yes,you can do all of this with a Buderus,modulating condensing boiler($4000),see #1.

  39. DennisDipswitch | | #39

    Armin Rudd Buffer Tank
    Martin,wouldn't the scaling/clogging problems that Armin Rudd was experiencing have been eliminated if his buffer tank was stainless steel?

  40. user-945061 | | #40

    Programming furnaces
    Exactly right Martin! Most major furnace manufacturers now have modulating equipment that can be set to run in the 20k-30kBtu/hr range, and programming the thermostats (which should be part of every furnace installation anyway) will increase the run time on the lowest stage. Of course many good houses won't need to increase the 1st stage run time - their thermostats will satisfied within ~10 minutes.

    Equipment manufacturers are gradually abandoning dipswitches in favor of thermostat controls. Having evaluated dozens of systems most dipswitches aren't set correctly. Not a trivial matter as most manufacturers send blowers from the factory set for the highest possible air conditioner. It's more common to see 5 tons of airflow blowing across 3 ton equipment than it is to see properly matched airflow. I think John Straube is a brilliant guy, but partly I wonder if some of the issues above stem from installer error/confusion.

    There's plenty more to rankle you about heating and cooling equipment :)

  41. GBA Editor
    Martin Holladay | | #41

    Respose to Dennis Dipswitch
    Q. "Wouldn't the scaling/clogging problems that Armin Rudd was experiencing have been eliminated if his buffer tank was stainless steel?"

    A. Yes. As you pointed out, it all comes down to price. It's easy to buy stainless-steel tanks that are rated for residential water pressure. Here is one supplier. The only problem is the cost.

    An 8-gallon tank costs $416.

    A 12-gallon tank costs $473.

  42. GBA Editor
    Martin Holladay | | #42

    Response to Jesse Smith
    Thanks for the further information. You wrote, "Most major furnace manufacturers now have modulating equipment that can be set to run in the 20k-30k Btu/hr range, and programming the thermostats (which should be part of every furnace installation anyway) will increase the run time on the lowest stage."

    That's a slight improvement over a 40,000 Btuh furnace, I suppose. But if my energy-efficient house has a design heat loss of 15,000 Btuh -- and I only need that much heat for a few hours a year, on the coldest days of January and February -- a 20,000 to 30,000 Btuh furnace still sounds oversized. But it's better than a 60,000 Btuh furnace, for sure.

  43. user-1118336 | | #43

    Mini-split -Plus system
    The answer for the central space mini-split option is electric radiant ( in the remote bedrooms (floor heat in the bathrooms; cove heater or ceiling panel in bedrooms). Cheap, easy to install, closest to "right-sizing", zonal control; no extra fan energy in the remote rooms.
    The mini-split ht pump keep everyone happy during milder weather at high COP; bedroom Delta will be small; mini-split does the heavy lifting during cold snaps; the radiant keeps surfaces warm and tops-off the extra needs only when needed in remote rooms. Smallest 500 Watt radiant units may still be oversized, but you're a lot closer to right-sizing equipment and get the bonus of sleeping spaces having zonal control to the "likes" of the occupant.

    During coldest weather, the COP of the mini-split is dropping off anyway. Not much of a hit on efficiency to turn on 1.0 COP heat for a small portion of the overall heating.

    As for carbon, should anyone be designing a house to last 100 years based on the current estimates of carbon emissions (actually estimates of last decade's carbon figures if you're relying on eGRID). I'm lucky that many local utilities get 90% of their power from hydro and the last two coal plants in OR and WA will be off-line by the end of the decade. If you're truly worried about carbon, don't design tomorrow's buildings based upon yesterday's energy mix. Distributed generation, cogen and renewables are coming on-line as fast as coal is going off-line.

  44. GBA Editor
    Martin Holladay | | #44

    Response to Mark Heizer
    Others agree with your approach. In his article, Living With Point-Source Heat, Marc Rosenbaum discusses the same approach (a ductless minisplit in the living room and electric-resistance radiant ceiling panels in the bedrooms).

    Marc Rosenbaum published a report of the energy used for space heating by 8 similar homes at a development in Massachusetts. Here is a link to the report:

    The homes had a ductless minisplit in the living room and electric resistance radiant ceiling panels in the bedrooms. Here is the key finding: "Heat pump energy is relatively even. The electric radiant panel energy, however, varies by a factor of fourteen to one." In other words, some families used a LOT of electricity to keep their bedrooms warm. Others didn't. These systems can be very efficient -- or rather wasteful -- depending on how the occupants use the equipment.

  45. Perry525 | | #45

    Heating a home.
    There are many areas that have soft water, that doesn't produce scale and therefore do not have this problem. Where there is hard water, this can be chemically treated to make it soft.
    Water is much better than air for delivering heat, about 4,000 times, it makes sense to use water for space heating, it takes up little space and is quiet in operation. It is particularly useful for UFH, where it frees up the walls for other things.
    Running a hydronic system where the home heating part is a closed circuit that includes a heat exchanger coil inside a hot water tank works perfectly. With insulated pipes, once the hot water is run in the morning - there are no cold slugs.
    With these systems the size of boiler is relatively unimportant as the system is divided into zones each of which has its own thermostat driving a motorized valve, including one on the hot water tank.
    As each part of the home becomes cold the valve opens the boiler fires and only that part of the system becomes hot. Meaning that only hot water is heated, or a bedroom, or lounge or all at once.

  46. user-1118336 | | #46

    Response to Martin on radiant
    Great point on the occupant training when you have radiant: the average homeowner is used the instant gratification of warm air systems. Radiant can seem "glacial" when you're in-the-moment and cold. The average customer thinks: "the higher you set the thermostat, the faster the room gets warm" and these systems spin the electric meter pretty fast. Be direct with the owner about what/how radiant reacts...and politely remind the owner: Step away from the thermostat, It's not an accelorator.

    Control solutions are coming that will allow greater owner satisfaction. Already, a creative contractor could piece together a relatively low cost control system that locks out the radiant and creates a "smart" radiant tstat. Current radiant floor sensors might be used: have good PID programming for this slow reacting system (and with remote sensors, can keep the controller out of sight and 'temptation' to adjust). New z-wave (or other protocol) outlets, relays, controllers and sensors could be used to develop creative lock-outs & custom programming for less than $$ than going hydronic or adding mini-split zone to all rooms. Added bonus to integrate with other systems (lighting).

  47. user-1005777 | | #47

    Minisplit and baseboards
    My minihome had baseboard heat with mostly programmable thermostats throughout. Heating cost was about $800 a year. We added a Mitsubishi minisplit in the living room that is in the center of the house. I set the thermostats about 5 F less than the mini. It has worked beutifully over the 2 winters we have had the system. When the Grandchildren visit, I raise their rooms to a comfortable level, but the baseboards rarely come on. We live in an 9000 DHD (F) area and our heating cost is around $400 a winter. All electric at 9.85 cents /Kwhr. I am thinking of installing 6 inch ducts from the living room to the end rooms and moving 30 CFM in each direction. I can do this with a total of 3.5 watts of energy using high efficiency DC fans.

  48. Expert Member
    KOHTA UENO | | #48

    BSC's Combi System Expert Meeting
    Martin--nice column as always, and great discussion.

    If anyone is interested, the presentations and writeup from BSC's Building America meeting on combi systems is available online:

    Recommendations for Applying Water Heaters in Combination Space and Domestic Water Heating Systems Workshop Westford, MA - July 31, 2011
    [scroll through using green slider to see individual presentations]

    Report on the Expert Meeting for Recommendations for Applying Water Heaters in Combination Space and Domestic Water Heating Systems
    Armin Rudd, Kohta Ueno, Daniel Bergey, Rosie Osser

    Also, in re that stainless steel tank discussion--you could also get rid of the anode rod by using a non-metallic (polybutylene) tank like a Rheem Marathon 15 or 20 gallon point of use heater. However... the cost... ~$450. Ah well... back to square one.

  49. Jpmorgan5150 | | #49

    A slightly different perspective...
    i've been meaning to write this for a while. I'm a typical American homeowner. I am almost done building a house. I'm a tightwad. I built my house as energy efficiently as possible without blowing a ton of money. My motivation was getting my bills as low as possible, I'm not really what you would call an environmentalist. I like in Western Kentucky, with some snow in the winters and brutally hot and humid summers.

    I used cellulose insulation and some foam (between the floors). I built a basement and a main level, 3,550 square feet. I caulked every seam on the wall construction (top/bottom of studs, floor plate, top plate, etc.). I kept the ducts inside the envelope. I used can lights that were ICAT certified and put cellulose on top of them thick enough to stop air movement. I used radiant barrier roof decking (though it probably didn't help much with the ducts in the envelope). I did central heat and air with a ground source heat pump. I didn't do ventilation other than the bathrooms.

    I didn't spend a ton of money to super insulate. I barely spent up (windows, caulk, roof decking, insulation) compared to a normal home. I spent a ton of money on the ground source heat pump and bought a Marathon storage water heater.

    I have not done a blower door test, but the loads for my house are around 30,000 BTU for both cooling and heating. I put in two units, both Daikon McQuay. They are the new Inverter models (variable speed compressor and fan) that scale up to 4 ton and down to 1 ton based on load. They are impressive!

    Why am I posting this? I have done a ton of research on green building. There seem to be two schools of thought on green building - the Green Building Advisor's super insulate (and spend money there) to lower the house load. There are very real comfort issues in this approach that are summed up in the article above. VERY REAL COMFORT ISSUES. I understand this is the only acceptable option for those who are concerned about the environment. Many typical consumers are not willing to spend to lower the loads if it leads to comfort issues (like me).

    The other school of thought is to save money and be comfortable. Build and spend money where it makes sense to lower the loads in the house (which means minimizing air leakage). Don't spend so much it moves the house outside of normal market value. I would say Doug Rye represents this school of thought well (even though he is not always right).

    My break even on the additional cost for ground source heat pump is 5 years and three months. The savings per month on utilities exceeds the additional cost per month to buy the geothermal. It makes financial sense. My house has return and supply air in every bedroom and main living room. Comfort is prioritized and the cost per month to heat and cool is guaranteed not to exceed $77/month.

    I built my house for less than $110/square foot. I did some of the finishing work.

    Two questions:
    1. Are you all willing to recognize that the approach I took is a great middle ground that is acceptable to much of the market, while super insulating is probably never going mainstream due to the costs? I understand super insulating is the only option for many people who are sincere environmentalists (and I'm OK with that).

    2. I don't understand why cellulose doesn't get more respect from this site! Foam is king, but cellulose and tons of caulk can, from many places I've read, offer similar performance in air leakage in real world settings. Blower door tests prove this. Foam has issues, per the other email. Why don't you guys love cellulose as much as you love foam?

    I'm sure I'm going to get a sharp reply. Before you give it, I have thoroughly read your site and openly considered my options. I have a limited budget, and I did what made sense on my budget. I think what you recommend makes a ton of sense to the sincere environmentalists who are building a house. There is a different path that is not well represented that uses many of the recommendations you all have but costs less and is less environmentally friendly.

    However, the route I chose will probably much more palatable than the route you recommend.

  50. GBA Editor
    Martin Holladay | | #50

    Response to Jared Morgan
    You wrote, "I don't understand why cellulose doesn't get more respect from this site! Foam is king."

    There may be some lovers of foam who write for GBA, but I'll speak for myself. Here are some quotes:

    In 2010, I wrote, "If you are paying for the labor, it's hard to beat double-stud walls filled with cellulose insulation for a low-cost high-R wall."

    In the same year, I also wrote, "When it comes to high-R walls, the most cost-effective option in most areas is a double 2x4 wall with a total thickness of about 12 inches, insulated with dense-packed cellulose."

    In 2012, I wrote, "As long as you have enough room in your attic for deep insulation, you'll get more R-value per dollar invested with cellulose on the attic floor than with rigid foam on your ceiling."

    Just two weeks ago, I wrote, "Among the most popular insulation materials specified by green builders are cellulose insulation and mineral wool insulation. For certain applications, it’s hard to beat rigid foam or spray foam, although some green builders try to minimize the use of foam insulation products."

  51. jinmtvt | | #51

    Jared ...
    I agree with Martin , many here also seem to agree that cellulose can have the best "bang for buck" when going for thick framed wall . Impressive R values can be reached with much simplicity and not a very large expense. The only downside i can see on cellulose is the " degradable" factor and sensitivity to water damage ??

    Then, why would a super insulated building suffer from comfort issues ??
    comfort is more often a problem related to cooling/heating methods and distribution ,
    and a super insulated building will always be more comfortable compared to a less one , using the same HVAC setup .

    Lastly, heating/cooling mechanics will probably evolve and change much faster
    than the construction/insulation methods, mechanical installations will never outlast
    insulation , thus up to a point, green insulation will always be much better investment in the life of a building than its HVAC equipment will, as the impact shall be 24/24 over a much longer period of time.

    calculations = determine best cost compromise VS payback VS HVAC size.
    If ur HVAC needs to be larger than a few mini-splits = get back to adding more insulation.

  52. GBA Editor
    Martin Holladay | | #52

    Another response to Jared Morgan
    Congratulations on building a house for $110 a square foot, and for keeping your heating and cooling costs below $77 per month. It sounds like you researched your options and built the house you wanted.

    If I understand correctly, your 3,550-square-foot house is equipped with two 4-ton heat pumps that cost (in your words) “a ton of money.” That sounds like a lot of equipment; I’m wondering whether one heat pump might have sufficed.

    I don’t doubt that your house is comfortable. You’re happy, and your electric bill is affordable. So that’s good.

    You have correctly outlined two choices. One approach is to use code-minimum insulation levels and to install HVAC equipment that costs a ton of money. That approach worked for you. Other people prefer to spend more on insulation and less on HVAC equipment. But this is a free country; either approach can work.

  53. dnardoza | | #53

    What about Radiators ????
    I've been reading a lot on this site, about ways to heat & cool a house.
    I live in a 144 year old house with hot water radiators, and an indirect water heater.
    The downstairs has central air (they were able to run the ductwork in the basement), but there is no AC upstairs. At the moment I have a few window units I use upstairs but in the next couple of years I was going to go with a ductless unit to cool the upstairs.

    When I bought the house all the radiators were on 1 big loop, and I used to rent out the upstairs. I split the upstairs and downstairs with 2 furnaces, so they could control there own temp and I wouldn't have to pay the bill. I lived downstairs and had them section off the 2 bedrooms, the living room and then the rest off the downstairs into zones giving me 4 zones. The plumber told me it was roughly about 500 bucks to create a zone. But I am now able to heat just 1 room if I want to.
    The other thing I did was when I bought the new furnaces I went from oil to gas. In the cold months I used to spend about $1000 a month to heat the house. After splitting everything up (and only having to heat the downstairs) my bills went to less then $200 a month. This year I took over the whole house ( I got married and now also have a daughter) so I will see what it will cost to heat the whole house.
    But it's very clear to me that zoning off the rooms and only heating where you need it makes a huge difference. I'm hoping using a couple of ductless systems will work basically the same way, cooling the areas I am using and not having to cool the whole house and saving money.

    I know hot water or steam radiators are expensive to install and I'm guessing a lot of people don't like the looks of them, but I love them and once you heat 1 up they stay warm for a very long time, they seem to be very efficient when done right but maybe I"m wrong. It seems strange to me that (based on what I've read so far on this site) no one talks about radiators as a way to heat a house.

  54. GBA Editor
    Martin Holladay | | #54

    Response to Dan Nardoza
    Radiators aren't a source of heat; they are part of a heat distribution system (like ductwork or a diffuser).

    Your home isn't heated by radiators. It is heated by a boiler. The boiler makes hot water. The heat distribution system consists of pipes or tubing connected to radiators. (Most modern residential hydronic systems now use baseboard fin-tube radiators instead of old cast-iron radiators; their purpose, however, is exactly the same.)

    Although you have told us that your house has two furnaces, I am doubtful that it does. My guess is that your house has two boilers. (Furnaces are used for forced-air systems; boilers are used for hydronic systems.)

    It isn't very surprising to hear that your decision to heat one floor instead of two, and to switch from fuel oil to natural gas, resulted in substantial savings.

  55. dnardoza | | #55

    Thanks for the correction and the responce
    boilers , furnaces , kind of the same to me, but yes you are correct, and I do understand the difference, I'm obviously not a contractor :)

    I guess what I was getting at is, are those old cast iron radiators more efficient then the baseboard ?
    Yes they are a delivery system and not the heat production, but I'm wondering, shouldn't that matter? Is one more efficient then the other?

    I was also wondering if that and adding the ductless air conditioning upstairs was a good way to go as far as using less energy?

    I realize upgrading an old house is very different then building a new one, but I have to assume most people don't have new houses build, they work with what they've got.
    I was also wondering if those big old radiators was something that they got right 100 years ago?

    I'm just always looking for ways to improve on the house, I've been looking for ways to tighten up the house since I've bought it, and also make it more efficient.

  56. GBA Editor
    Martin Holladay | | #56

    Response to Dan Nardoza
    Q. "Are those old cast iron radiators more efficient then the baseboard?"

    A. No. The efficiency of a hydronic heating system is governed chiefly by the efficiency of the boiler, and to a very small extent by the efficiency of the circulators.

    Q. "Is adding the ductless air conditioning upstairs was a good way to go as far as using less energy?"

    A. Yes. Ductless minisplits are likely to be more efficient than most alternatives.

    Q. "I was also wondering if those big old radiators was something that they got right 100 years ago?"

    A. There is nothing wrong with old cast-iron radiators. If your radiators are in good shape, and you like them, leave them in place. But if you want to stick with a hydronic heating system, and if you are want to improve that system's efficiency, you need to focus on the efficiency of your boiler rather than the details of your distribution system.

  57. user-984364 | | #57

    Electric resistance floor heat
    We're doing a renovation, including 3 bathrooms, which need some kind of heat source even in these small rooms. The builder really wanted to put in electric floor heat, which I'm sure feels great, but I balk at using electric resistance for heat at all. We chose Dianorm towel rack / radiators instead, at a fairly significant cost I think. But they're not in yet. ;)

    The thing that scares me about in-floor electric resistance heat is that I have no idea how much energy they might use over a season. Are there any rules of thumb or even order-of-magnitude guesses?

  58. GBA Editor
    Martin Holladay | | #58

    Response to Eric Sandeen
    It's hard to make an estimate, because the answer depends on (a) the size of your bathrooms, (b) the amount of insulation in your walls and ceiling, (c) your climate, (d) the watt rating of the electric floor mat in each bathroom, (e) your local electricity rates, (f) the temperature setting of your wall thermostats, and, most importantly (g) whether you leave the heat on all day or control the electric heat mats with a timer.

    According to the DOE, "Because of the relatively high cost of electricity, electric radiant floors are usually only cost-effective if they include a significant thermal mass such as a thick concrete floor and your electric utility company offers time-of-use rates. Time-of-use rates allow you to 'charge' the concrete floor with heat during off-peak hours (approximately 9 p.m. to 6 a.m.). "

    That said, some people are very happy with their electric radiant floors. The way to keep your electric bill low is to use the floors as little as possible -- by putting them on a timer and running them (for example) from 6:30 a.m. to 7:30 a.m. during the months of Dec., Jan., and Feb., so that the bathrooms are warm for your morning shower.

  59. user-984364 | | #59

    Response to Martin
    Yes, I guess I knew about a) through g) - although I still don't have a sense of whether it might be 10kWh or 100kWh or 500kWh per month, but I guess it's so situation dependent it's impossible to say.

    And time of use is great for cost, but inconsequential for environmental impact, unless I can convince myself the wind blows more at night so it's all ok.

    Well, thanks for the input. Warm towels sound nice too, we'll probably stick with that.

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