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100,000 BTU house, air-to-water heat pump recommendations?

user-1034802 | Posted in Green Products and Materials on

My house’s load was computed at about 100,000 BTU.  I’m in zone 5b with lots of full sun and a pretty tight envelope.  It’s a barn conversion to resi, so it’s a lot of open space, and just a few small rooms. No PV, no pads or wiring for outdoor heat pumps.  Maybe PV in spring 2024.

DHW is taken care of – I’ve got a hybrid electrical unit already.

I had an LP Triangle Tube that worked great for 12 years, pumping water through my euro-radiators and concrete slab.  It met its end this spring, and I’m considering my next move.

If I stay LP, my easiest path, they’re quoting me $5,800 for a Veissmann BIHE-85, plus 40 hrs labor.

Or, should I invest in heat-pumps now?  Do I pair up or triple some Mitsubishis?  Do I consider the 100k+ units I see on the pool supply websites?  My best local installer seemed to choke a bit on the BTUs needed.  FWIW, I’ve got a wood stove to take the chill out if 100k BTU is just not feasible.

Are there solutions to do air-to-water for heat, and mini-splits for targeted cooling?

What direction would you pros go?

Thanks so much!

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  1. Expert Member
    DCcontrarian | | #1

    100K is a lot for an air source. One of the things that you'll run into is that with heat pumps the cost rises pretty linearly with the capacity. This is in sharp contrast with combustion boilers where increasing the capacity doesn't increase the cost much, often the only difference between a 70K boiler and a 120K boiler is a different fuel nozzle.

    The other thing to look at is low temperature capacity, heat pumps tend to be able to produce less when it's cold, which is exactly when you need that capacity. For example, if you look at the Chiltrix CX50 ( ) which has a nameplate capacity of 57K BTU. That's at 47F, here's how the capacity moves with temperature:
    47F 57K BTU
    32F 45K BTU
    23F 41K BTU
    17F 38K BTU
    5F 31K BTU
    -4F 26K BTU

    That's pretty steep. I'm not picking on Chiltrix, they just have their performance curve easily available.

    A new entrant that seems interesting is the Taco Series M ( ). This is interesting because Taco has nationwide distribution. My understanding is this is a rebadged European model that has been for sale in Europe for a while.

  2. Expert Member
    Michael Maines | | #2

    How large is your house and what is the thermal envelope? 100KBTU seems very high for a "tight" house.

    1. user-1034802 | | #5

      LOTS of glass. Big open space with a few small bedrooms. Nearly 5000sqf. Used to be a barn.

  3. Expert Member
    DCcontrarian | | #3

    The Taco System M has a performance chart here:

    Outdoor Temperature Water Temperature kW BTU/h COP
    47° F (8.3° C) 105° F (40.6° C)6 8.64 29,481 4.38
    47° F (8.3° C) 120° F (48.9° C)6 8.36 28,525 3.52
    47° F (8.3° C) 140° F (60° C)6 9.50 32,415 2.68
    45° F (7° C) 95° F (35° C)5 6.90 23,500 4.60
    36° F (2° C) 95° F (35° C)5 6.00 20,500 4.00
    19° F (-7° C) 95° F (35° C)5 9.00 30,500 2.80
    17° F (-8.3° C) 105° F (40.6° C)6 5.26 17,948 2.76
    17° F (-8.3° C) 120° F (48.9° C)6 6.24 21,292 2.33
    17° F (-8.3° C) 140° F (60° C)6 6.15 20,985 1.85
    5° F (-15° C) 110° F (43.3° C)12 5.24 17,880 2.21

    So it's similar that 5F capacity is about 60% of 47F capacity.

  4. paul_wiedefeld | | #4

    Was this a fuel usage heat loss calculation? 100kbtu is a lot. For what it’s worth, that boiler quoted only outputs 80kbtu.

    An easy compromise is installing a 4-5 ton heat pump with a larger boiler (non-condensing, low efficiency is fine) as backup. You’ll find that something 50-60% of the heat loss will cover much more than 50-60% of the annual heating needs, even more if your heat loss is lower than calculated.

    1. user-1034802 | | #6

      My LP usage over a typical winter has been around 1000 gallons on the Triangle Tube 110. Does that align with a 100k btu load?

      We're calculating the 80k btu with the wood stove taking out the chill.

      I'll talk to them about a less efficient boiler and consider PV and 5k heat pump for the spring. Sounds like a better plan.

      I need a roofer to come before I can pull the trigger on PV - and roofers are not getting many working days this very wet summer. Sigh.

      Thanks so much for your insights, Paul


      1. paul_wiedefeld | | #7

        It's not that close to a 100kbtu load, but it's hard to say right now.

        I'm using this method:, but I don't know your location, so the heating degree days are a guess for now. They should be around 5000, but if you're in a colder location, your heat loss will actually turn out to be lower (BTUs spread across more HDD 65).

        The wood stove is a wildcard though - it is much harder to track those BTUs and much harder to guess that efficiency.

        You're in a great situation for an air-to-water application with the in-floor and panel radiators. They usually can tolerate low water temperatures so instead of switching from heat pump to boiler at a certain outdoor temp, you can just use the boiler to further increase the water temperature from the heat pump.

        However, how important is cooling to you? If cooling is important, then you might be better suited using a ducted heat pump.

        1. user-1034802 | | #8

          Add another 1000 for the heating degree days. Near Albany, NY.

          Woodstove is an old Vermont Castings. Probably just 10k btu. Considering a new one with a cat.

          I'm doing spot cooling. It's been fine.

          1. paul_wiedefeld | | #9

            Oh even lower heat loss then, just under 40kbtu. It doesn't actually change much - the air-to-water heat pump can cover all/nearly all of the load and if you go with the backup boiler, the backup boiler can be pretty large if you want. It won't run much.

            The next step is determining what water temps you need on design day. You should be around 120 for the panel radiators to get the most out of the heat pump, but lower is better. The floor should be even lower than that.

          2. orange_cat | | #13

            Also, the new more efficient stoves have smaller footprints and higher BTU. Smallest Vermont castings is 16,000 BTU. There are many small footprint non-catalytic (or catalytic) with 15,000-40,000 BTU output (I had the opposite problem recently trying to find something I like with the small BTU, and even tiny Euro sized ones which take super short logs are over 10,000 BTU.) Here is the full list:

            If you are not opposed to continue using woodstove, it is much cheaper to get a sensibly sized heatpump and a more efficient woodstove (before I could persuade my engineers that their calculations were too high, I looked at sizes and prices and over 70,000 BTU is really expensive, bulky and noisy. I also have hydronic, so research AWHP as well)

        2. Expert Member
          DCcontrarian | | #10

          Don't we need to know the heating design temp?

          1. paul_wiedefeld | | #11

            Albany's highest HDD 65 for the last three years was 65. So around 0.

  5. orange_cat | | #12

    I had 2 different engineers calculate what to me was an absurdly high heat load (the third one came within the range I calculated - upon the advice on this cite - using this:, and the first two then reduced their calculated load upon corrections to the design temperature and so on. But it took citing pages of the building code to do that right and sharing the correct calculations - not mine, which they would not take, but the third engineer.)

    I would use your actual energy bills to estimate your true load before looking into 100,00 BTU unit.

  6. user-1034802 | | #14

    I finally found the heat load analysis done during construction of one of the best Passiv Haus people in my area. He said 42,000 BTU Passive, 2x that for non-passive. So, everyone's assertions that 80,000BTU is my number are correct.

    Thanks so much for this thread!

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