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Best heat source for this house?

jiggyj | Posted in General Questions on

We recently finished building a super-insulated house in a very cold place. We had to scrape and pinch budgets to get it built, and some important points didn’t get the attention they deserved. Live and learn.

One of those points was, we just didn’t know how much additional heating the structure would need. None? Some? Lots? We installed just a tiny bit and now it seems like we need more.

The house has an ERV in the sealed crawlspace. The ERV has 3 little inline electric heaters, and is activated by a standard thermostat (as well as the particle detector). Those inline heaters are enough to maintain temps on some cold days, but not all, and they have to work really hard.
* If we turn the thermostat down to e.g. 60 overnight, bringing the house back up to 68 might take 12-24 hrs.
* If we leave it at 68, the ERV is blowing nonstop, and it can’t quite keep it there on the really cold days.

And, of course, when the thermostat tells the ERV to start working, that means it’s doing extra ventilating: more cold air in and hot air out.

The house also has 2 heated bathroom floors, which the builder thought might work as “make up heat”. We haven’t tried cranking those way up; maybe that would work, but it doesn’t seem like a terribly safe or efficient way to heat for extended periods. Thoughts?

I am wondering whether it would make sense to install a ducted heat pump in the crawlspace? Maybe it can be integrated with the ERV somehow?

Or I could install some baseboard heat, but since the house has ducts, using them seems smart.

I’m curious if anyone with real experience suggests any other creative ideas to consider?

Thanks.

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Replies

  1. walta100 | | #1

    "Best heat source for this house" is dependent on a lot of factors. The first question is how much heat do you need and the best way to know is to have a manual J calculation done for your home. Generally these calculations are worth what you put onto them. If someone is doing this work for little or no cost the numbers are unlikely to be accurate and of little value to you. It is best to pay someone to do the calculation that is not going to supply or install the equipment.

    "Best heat source for this house" The word “best” can mean different things to different people who have different goals. Some would argue electricity as fuel is the greenest option. Other could say given my utility rates natural gas gives me the greatest number of BTUs per dollar. Another group would say heating with a wood stove using wood harvested on their property is the most sustainable way. Each is best to their way of thinking and each is wrong in the eyes of others.

    I like heat pumps a lot of people have had good luck down to negative 15 or 20°F

    Walta

  2. user-4053553 | | #2

    I agree with you must get a proper load calculation. When you have concrete numbers to work with you can decide on a comprehensive solution that will work properly instead of adding things piecemeal until your comfortable which may cost more in the end and in perpetuity.
    You can tell us about your house, where it is, what the square footage is, how much and what type of insulation it has from bottom to top and how efficient are the windows and air leakage rate if available.
    But you need a proper load test to make informed decisions, And as stated they need to be accurate, something that can be hard to find if you have only bad auditors in your area.
    What you might be able to do is get the house modeled in software since you know how it was built (and presumably have the engineering drawings).
    The air change rate can be done by any auditor who does blower door tests.

  3. Expert Member
    MALCOLM TAYLOR | | #3

    J,

    It sounds like you are experiencing the classic problem with heat delivered through ventilation systems. The volume of air necessary to circulate the heat is much higher than that necessary to provide fresh air. Because of that I would suggest staying away from any solution that further integrates heat sources and your ERV.

  4. ROBERT OPALUCH | | #4

    You don't describe your climate, levels of insulation, airtightness, utility rates, etc so its difficult to imagine why you need more supplementary heating than you anticipated.

    For little or no additional investment in heating equipment, you may be able to provide some supplementary heat for those coldest nights.
    -- Do you have an electric oven or electric stove?
    -- Would you be willing to operate one or more small portable electric heater(s) during those coldest periods? Or wall-hung plug in units?
    -- Would you consider using electric blankets to pre-heat your bed(s) or keep them warm overnight?
    These are low-investment solutions.

    Why do you believe the bathroom floor heating would not be safe?

    Have you considered window treatments that would reduce overnight heat loss? Insulated shutters, cellular shades or insulated curtains with side-tracks (reasonably airtight versions)

  5. Jon_R | | #5

    I expect that a small non-ducted cold climate mini-split head installed in your sealed crawlspace with no ducts at all would add enough heat. A low cost way to find out - put a couple of plug in heaters down there. The numbers work better than using ventilation airflow. edit: ignore if you don't have the typical sealed/conditioned crawlspace that has the wood floor above it uninsulated.

  6. jiggyj | | #6

    I'm not fluent enough to know what a load calculation looks like -- would it be in a document called a PHPP? The fabricator of the panels we used provided a PHPP. I'm attaching it. Does this help with a recommendation?

    The home is in climate zone 7, according to this map: https://basc.pnnl.gov/images/iecc-climate-zone-map.

    1. I'm starting to surmise that trying to use the ERV for heat is actually wasting a ton of energy (because it's using the ventilator's accelerated speed to bring in outdoor air, modestly heating it, and exhausting already ambient indoor air). Since the ERV is attached to a thermostat, it seems like the best thing to do would be to set that at a *lower temp* than whatever we can achieve through other means (passive, baseboard, bathroom floors etc.). Like maybe 58 or 60. Does this seem like a good guess?

    2. Are those heated bathroom floors (all electric, not water) appx. as efficient as a baseboard heater? Is there any reason we couldn't set them at like 80 degrees and leave it there? Maybe with the ERV not blowing so hard, that would keep the house comfortable?

    3. If we wanted to get more heat, reading suggests a mini-split would be the best way. True? How long can the refrigerant run be? Could the refrigerant run through the crawlspace (~55 degrees F) for 20' without too much waste? Note: The floor is fairly airtight, OSB top and bottom with loose insulation 10" thick inside. (Note: I've read that putting the condenser part of the mini-split in the crawlspace is not great; I would be happy to do it, if folks think it would be OK: https://www.energyvanguard.com/blog/58396/Does-a-Heat-Pump-or-Air-Conditioner-Condenser-Need-to-Go-Outdoors)

    4. I have a quote from an electrician to get a couple 5' baseboard heaters, centrally located, for $1k. Just another data point.

    Thanks for sharing your knowledge with a newbie.

  7. jiggyj | | #7

    Replying to specific questions:

    -- Do you have an electric oven or electric stove?

    Yes. The house is not occupied full time. When occupied, we can run the (all electric) oven, lights, stove, ventless dryer, etc, and all of those help. It still sometimes gets down to 62 at night, and then it takes all day to get back up to 68. Never seen it at 70 in the winter.

    -- Would you be willing to operate one or more small portable electric heater(s) during those coldest periods? Or wall-hung plug in units?

    Could do, sure. I'm trying to figure out the best solution to...
    a) warm (at least some of it) up quickly when we arrive
    b) keep all of it (house, sealed crawlspace) at a pipe-safe temperature when it's vacant

    So a space heater can solve a), but I'm not sure how to solve b) efficiently yet.

    -- Would you consider using electric blankets to pre-heat your bed(s) or keep them warm overnight?

    Night time is totally OK so far; comforters and body heat work. It's really about a) and b) above.

  8. ROBERT OPALUCH | | #8

    A minisplit centrally located would be very helpful and is 2-3 times as efficient as resistance heaters (e.g., portable plug-in heater, baseboard heater, bathroom floor heating). However if you don't need much more supplementary heat, a minisplit may not be worth the additional investment. You need to know the heating load, or the space heating shortfall (since your home is completed). The heating load should be done before building, and its better to do room by room rather than just total for the house.

    Since your home is complete except for this space heating shortfall problem, why not buy a small portable electric heater (cheap), and see how many hours you need to run it to estimate the BTUs needed to meet the shortfall you are experiencing? Consider the cost of utility bills vs. the cost savings and cost to install a minisplit.

    You mention "superinsulated" which is a vague term. You should know the R-value of insulation in your attic and walls, the U-value of your windows, etc and the areas of each. If you can share those numbers, and the average outdoor January temp and design temp, one of us can give you a quick rough estimate of the heating load. We can compare this to the output of your existing space heating appliances. Still wouldn't know the infiltration heat loss but at least we can begin to get a quantitative estimate.

    The ERV's function is to deliver fresh air and expell stale air, not primarily a heating system. ERVs should not overventilate the building, or you are wasting energy as you noted. I'm just guessing here, but if your ERV inline heating elements aren't working because you have to run the ERV almost continuously, your home may be too leaky or isn't anywhere near as "superinsulated" as a passive house, or both.

    Is your crawl space inside the thermal envelope of the home and approximately the same temperature as the rooms in the home??

    Why don't you trust the "safety" of your bathroom floor heating?? IF your building shell is superinsulated and airtight, indoor heat should move amongst rooms more than moving through your building envelope to the cold outdoors.

  9. Expert Member
    Peter Engle | | #9

    J has given us some hints that we're not considering:
    -His PHPP shows space heating load of 5.37 Btu/hr.sf. If he gives us his total square feet, we will know his heating loads (if the PHPP is correct.)
    -J might also be able to find the output of his resistance in-line heaters. It sounds like they *almost* keep up with the heating load on a cold day. That would provide us with a comparable data point.
    -The home is unoccupied part-time. So, he needs a solution that provides cheap, low, reliable steady-state heat for maintenance, enough oomph for a boost to occupied temperatures, and then cheap cruising heat for maintenance of the occupied comfort levels. Sounds like a combination of a heat pump to meet just the design demand load, and some electric resistance to meet the spike during warmup.

    J: if you do add alternate heat sources, you should set the HRV to run rarely during unoccupied times. The air exchange is just wasting energy when you don't need it for air quality purposes.

    You should investigate web-enable thermostats for the place. It takes lots less capacity to raise the temperature slowly than all at once. If you can raise the thermostat several hours before you arrive, you can get by with less auxiliary heat. Maybe none at all if it's not too cold out.

  10. walta100 | | #10

    From the look of this PHPP it looks like someone is drinking the Passive house cool aid and is trying to design your house to fit that formula. The way I understand it the house is designed to use X number of BTUs per year per square foot regardless of where on earth the house is located regardless of how much it cost to meet this arbitrary goal and get your plaque.

    Everyone has their own reasons and goals I just never could get my head around this one. To me it does not make sense to spend more money on insulation than it will ever save in fuel in today’s dollars.

    I have no doubt you have paid a ton of money have this place designed and built. If the house is not meeting your expiations it seems you should be asking the designer what went wrong and let the finger pointing begin.

    An ERV is not normally considered a heating devise. Most do not have a heater even when they do the idea normally is to warm the incoming air enough to keep the heat exchanger from becoming a frozen ice block. Most EVRs supply air to the home that is cooler than the air in the house. It seems unlikely that your home does not have a heat pump. Your heat pump should have a thermostat to set and control the temperature and you HRV should have a control panel to set and control the relative humidity of the air.

    Assuming you have a heat pump it should produce heat for ½-1/3 the cost of using the resistance heat in the bathrooms floors or new base board heaters both will give you 3412 BTU per kWh.

    Before you decide you have a problem try leaving your thermostat at 68° 24/7 heat pumps work best when keeping a constant temp.

    Walta

  11. user-4053553 | | #11

    That PHPP document gives some interesting numbers. I expect one of our geniuses will come along and give you expert advice so i won't go there yet. We can only assume its accurate, who prepared it for you? Why did you decide to get this load analysis and how did you decide what levels of insulation to go with?
    That said I also assume its assuming 0.6ACH50 (find out) and please provide the square footage of your house. Finally did you ever have a blower door test done after construction?

  12. jiggyj | | #12

    Great conversation above and I'm learning a lot. Thanks.

    > Is your crawl space inside the thermal envelope of the home and approximately the same temperature as the rooms in the home??

    There are 2 registers for the ERV in the crawspace; no returns; so it gets some of the heat from the living area, more when the ERV is in high gear. The foundation has 2" of foam inside and out, and the floor is full of insulation (R30), and the crawlspace is sealed, and it's steady at about 55 degrees F down there, with lots of humidity in the summer. So the crawl is kind of its own system.

    > Why don't you trust the "safety" of your bathroom floor heating??

    I just never had a heated floor before; I've only encountered them in a fancy hotel once or twice, where I assume people turned them on to make their feet warm after showering. So it never occurred to me that you'd leave it on all the time, and I had no idea that was OK.

    > You should know the R-value of insulation in your attic and walls,

    There's no attic. According to designs, walls are 19; ceiling is 38. Reviewing these numbers, maybe "superinsulated" is ... an overstatement. Live and learn, as I say. :/

    > the U-value of your windows, etc and the areas of each.

    20% glazing on the south; far less on all other walls (but I don't have the exact %).

    In the brochure for the windows I see "Uf .75 W/(m²K)" and "Ug .54 W/(m²K)" for the standard windows and "Ug .63 W/(m²K)" for the south ones.

    > the average outdoor January temp

    High of 30, low of 11.

    > If he gives us his total square feet, we will know his heating loads

    It's 1,600.

  13. user-4053553 | | #13

    "There's no attic. According to designs, walls are 19; ceiling is 38. Reviewing these numbers, maybe "superinsulated" is ... an overstatement. Live and learn, as I say. :/"
    Does this house meet minimum code?
    Are these whole assembly R values?

  14. jiggyj | | #14

    I misread the plans! The numbers I previously gave were overridden by a system detail at the end.

    When I add up all the wall parts (concrete board, "studded" air gap, housewrap, 2.375" fiberboard, osb, 7.25" dense pack celloluse, osb, "studded" air gap, gypsum) or ceiling parts (same as walls, except 12" dense fill and no fiberboard) using the tool at https://ekotrope.com/r-value-calculator/, I get appx. R42 walls and R50 ceiling.

    1. user-4053553 | | #17

      How did you not calculate heating load when designing the house?

      1. jiggyj | | #22

        Well, if you're truly curious, here's how:

        We bought architectural plans online, in order to keep the project on budget.

        We had a fledgling pre-fab passive house panel builder build our panels on the cheap, and even then, we had to trim a layer of insulation from the outside to keep the project on budget and moving forward.

        We had a freshly certified passive contractor do the site work on the cheap, and even then, we had to make hard decisions about every system on short notice to keep the project in budget and moving forward.

        We built in a town where nothing like this (pre-fab extra-insulated framed panels) had ever been done before, and the town imposed some unexpected criteria on the project midway through.

        These various parties did not communicate extremely well together about what must be delivered or who would deliver what, so we occasionally had to pretend to be GCs or construction project managers. Certain systems (such as insulation, foundation details, structural engineering) changed significantly well after planning was done. We are not builders; we are not passive certified; and we most definitely are not rolling in dough. So we did our best, and we missed a few things that a more expensive project would include (like extremely experienced consultants or executive project management).

        In the end, we got a house that is imperfect but close to what we wanted, in a place we like, for a lot less than the going rate. And now as time and budget allow, we're bringing things up to ideal. Live and learn, as I say :)

  15. Expert Member
    Dana Dorsett | | #15

    >" If we leave it at 68, the ERV is blowing nonstop, and it can’t quite keep it there on the really cold days."

    It can't quite keep it there, but you could use the wattage of the in-line heaters and the temperature at which it stops keeping up as the measuring instrument a calibrated measurement of the design heat load.

    So, how many watts of heater is there, and at what temp does it start to lose ground?

    For "normal" insulated houses you can use these methods, with a presumptive 60-65F heating/cooling balance point:

    https://www.greenbuildingadvisor.com/article/out-with-the-old-in-with-the-new

    For a superinsulated house it's going to be more accurate to use 50F or 55F.

    For instance, let's say you have 1200 watts of heater, and it starts to lose ground at an outdoor temperature of -4F/-20C, in a location with a 99% design temperature of -13F/-25C. If the balance point is 55F/13C, with a linear approximation model there are (55F- -4F= )59 heating degrees, so the heat load grows by about 1200W/59= ~20 watts per degree-F (or 1200w/(13C - -20x) = 52W per degree C.) So at -13F the heat load would be about 20W/F x (55F- -13F)= 1360W.

    Converting that to BTU/hr would b 1360 x 3.412 BTU/ watt-hour = 4640 BTU/hr.

    Run this math on the wattage and temperature you think is where it starts to lose ground, and do it for both base 50F and base 55F. The watts/degree will be larger using base 50F, but the total heating degrees will be fewer.

    There are a few mini-splits fully specified down to -22F/-30C, and many specified down to -13F/-25C, even more specified at -20C. Since you have the in-line heat with the ERV it's fine to just specify the mini-split for the load at whatever it's output is specified, and use the ERV heaters as the auxiliary heat to make up the difference. Fujitsu's ducted mini-splits even have an output that can essentially behave as the thermostat for the auxiliary heat, or you could just set the auxiliary heat thermostat a few degrees cooler than where you set the mini-split to ensure that the heavy lifting is still being performed at higher efficiency with the heat pump.

    The COP at -20C or cooler is less than 2.0 even for cold climate mini-splits, and might be less than 1.5 at -30C, but at temperatures north of -15C the efficiency begins to grow fairly quickly.

    1. user-4053553 | | #16

      Is his PHPP numbers enough to work with for peak load assuming they are accurate?

      1. Expert Member
        Dana Dorsett | | #19

        >"Is his PHPP numbers enough to work with for peak load assuming they are accurate?"

        "...assuming they are accurate...", sure, but why assume?

        The accuracy could be verified using the balance point temperature at which it stops keeping up, which would be a MEASUREMENT rather than an assumption.

        1. user-4053553 | | #20

          I don't disagree, i would typically want a provisional answer when available but many will latch onto it instead of doing the extra work.

  16. walta100 | | #18

    Just to be clear do you have a box somewhere near your house that looks like the one in this photo?

    I have to believe every home in zone 7 has a heat source. So it seems likely you have a heat pump somewhere and no one has taken the time to explain to you how the your system work the heat pump and your EVR systems and or controls may be combined.

    Walta

    1. jiggyj | | #21

      > Just to be clear do you have a box somewhere near your house that looks like the one in this photo?

      I had the butler walk the perimeter and he didn't see one of these boxes, but he did find one of these (pictured below) in the drawing room -- do you think it could be related to the heating system?

      OK, that was a joke. I admire your skepticism. That said, as I've been installing baseboards, hanging doors, wiring lighting, and installing and plumbing the sump, I have not encountered a mini-split compressor or a mini-split evaporator (which, you have to admit, would be pretty hard to overlook in a 1600' house).

      Amazingly, the extra insulation and careful air sealing combined with all the individual heat inputs (bodies, lights, appliances, inline heaters, bathroom floors, and a *lot* of sunshine*) makes the house pretty cozy, even on cold nights, when occupied. We always knew we might need a little more heat, and I think we do, and I specifically think we need heat that runs efficiently when the house is vacant, and heats up quickly when the house is suddenly occupied. I also think this forum is a great place to learn how to solve for this, which is why I came.

      Thanks for all the great answers. I'll do my best with the math, as soon as I can figure out the size of the inline heaters. More ideas welcome!

      1. user-4053553 | | #23

        I am very confused about your situation:

        Why did you get a PHPP assessment done when you were not building a Passivhaus?
        How did you get a building permit/certificate of occupancy without having a furnace?
        How did no one give you a peak load calculation or demand you obtain one?
        Why did you build without a qualified heating source?
        What is the U factor of your windows? How many are there (total surface area)?
        Did you ever get a blowerdoor test done?
        Do you have a basement?
        Why are you not using your fireplace?
        What is the air source for that fireplace?
        What is the BTU rating of your fireplace?

        At this point having the plans would probably also be helpful. And you better hustle, with winter coming your screwed if you don't get this solved soon.

        1. user-4053553 | | #24

          I saw this after my post so i copied it here for simplicity

          "Well, if you're truly curious, here's how:

          We bought architectural plans online, in order to keep the project on budget.

          We had a fledgling pre-fab passive house panel builder build our panels on the cheap, and even then, we had to trim a layer of insulation from the outside to keep the project on budget and moving forward.

          We had a freshly certified passive contractor do the site work on the cheap, and even then, we had to make hard decisions about every system on short notice to keep the project in budget and moving forward.

          We built in a town where nothing like this (pre-fab extra-insulated framed panels) had ever been done before, and the town imposed some unexpected criteria on the project midway through.

          These various parties did not communicate extremely well together about what must be delivered or who would deliver what, so we occasionally had to pretend to be GCs or construction project managers. Certain systems (such as insulation, foundation details, structural engineering) changed significantly well after planning was done. We are not builders; we are not passive certified; and we most definitely are not rolling in dough. So we did our best, and we missed a few things that a more expensive project would include (like extremely experienced consultants or executive project management).

          In the end, we got a house that is imperfect but close to what we wanted, in a place we like, for a lot less than the going rate. And now as time and budget allow, we're bringing things up to ideal. Live and learn, as I say :)"

          Dear god, at this point you need a proper load analysis done. And your going to have to find someone who knows what they are doing, or you can give us all the details here but its going to have to be in far more detail then you have given us so far because of all your changes.

          If you can answer the unanswered questions above that would be great but i think a floor plan with details from what U factor windows, size and actual insulation in all walls floors, ceilings and air change rate are necessary.

          Or just do as Dana suggests above and calculate your heat load, which is probably the best way to go if you do it properly.

          @Dana, does he need to subtract appliances and human contributions?

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