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Community and Q&A

Heat Pump capacity

deanberg | Posted in Energy Efficiency and Durability on

I’m hoping that you can confirm my belief or tell me that I’m missing something…
I’ve had several people tell me recently that unless a home is well insulated, a heat pump will not adequately heat the home (this is related to forced hot water conversion to ASHP).
My understanding is that as long as the heat pump (let’s assume ductless for now) is sized properly and has a BTU output equal to the BTU output of the current boiler, that the heat production should be equal to that of the boiler. With the insulation level being equal in both the before (boiler) and after (heat pump), why would the ‘ability to heat the home’ be any different?

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  1. paul_wiedefeld | | #1

    You are correct, that is nonsense. However, most boilers are larger than the largest heat pumps, so there’s some merit to insulation bringing down the load to the range of heat pumps. However, most boilers are also much larger than the heat load too and your typical installer will be tempted to replace the boiler with the same size, so take their advice with a grain of salt. Here’s an easy way to correctly size a replacement heat source.

  2. DC_Contrarian_ | | #2

    Here's where I think this idea comes from: Heat pumps don't heat the air as hot as fossil-fuel based heating systems. In a heat pump the efficiency is determined by the temperature difference between the input and the output. In a forced hot air system, the ducts will be designed to move the amount of air that needs to be moved to heat the house. If you replace a furnace with a heat pump, even if the heat pump has the same BTU rating, the air coming out won't be as hot so more air will have to be moved to provide the same level of heating. However, if the ductwork isn't upgraded the ductwork won't be able to handle that airflow and comfort will suffer.

    I think where the bad reputation of heat pumps came from is from people trying to use them as drop-in replacements for furnaces.

    1. paul_wiedefeld | | #3

      Agreed but electric resistance strips fix this for the few days it matters. Also, this was comparing a boiler to a heat pump.

    2. Expert Member
      Akos | | #5

      The bad reputation for heat pumps is installing one that is not rated for the climate. A regular heat pump falls flat on output bellow freezing temperature and COP quickly drops to near 1.

      A hyper heat unit in my climate (-2F design temp) will provide near nameplate rating, for example a 2.5 Ton Mitsubishi SUZ-KA30NAHZ produces around 30kBTU of heat at -2F. The supply temperature while running on high speed into a 70F room would be 105F. Not something I would consider cold, run at at lower fan speed and it will be even warmer.

      The nice benefit of one of these is they are always supplying some heat not cycling like a fuel burner which makes even older uninsulated homes much more comfortable.

      It is still a good idea not to have any vents directly blowing at people, a defrost cycle without strip heat is noticeable.

    3. walta100 | | #6

      I tend to agree with what you have said but it seems to my you need to go step further. As you said HPs deliver cooler air so deliver the required number of BTUs, they do need circulate more air. If the house in question is a drafty one and a half story uninsulated farm house built around and over the original log cabin heated with a monstrous 300k BTU furnace and needs most of them to heat the place (Yes, I did live in that house) That house heated with a heat pump would have been like living in a wind tunnel.

      Some of HPs bad reputation was well earned in the 1980s there were a lot of poorly installed systems that failed to deliver on comfort and efficiency.


      1. Expert Member
        Akos | | #7


        Most houses have a furnace that is at least 2x oversized. This generally means that even if you account for the lower supply temperature, the existing ducting is big enough for a right sized heat pump. Maybe have to update the return and the filter a bit but that is pretty much it. High static pressure air handlers (ie Mitsubishi SVZ) have enough oomph that they make most retrofits pretty simple.

        For example, I have an existing furnace that is 3x oversized. I checked flow rates and pressure in the system, and when it dies I can simply replace it with a right sized mid static unit. With a bigger filter, it will only need .45" WG from the blower.

  3. BirchwoodBill | | #4

    For hot water systems you need to transition to low temperature water, I.e. under 100F for heat pumps to start become cost effective, achieve the COP. Very doable with the new equipment out there. Siegenthaler also describes “Exergy”, which is related to the efficiencies of the heat exchanger.

    Moving air through ducts or hallways also involves energy loss, so you need to calculate the heat loss for moving the mass from point A to point B. Changing the temperature of the transport media changes the math.

    As to the question, heat loss is heat loss, but changing from hot water heat to a mini split changes the distribution of heat through the house. You really need to model the house and the system to create a comfortable IEQ. Do the math!

  4. adrienne_in_nj | | #8

    “With the insulation level being equal in both the before (boiler) and after (heat pump),”
    there should be no difference in “ability to heat the home.” However, in an older, drafty house with lower insulation levels and older windows, I believe the hydronic heat has a distinct comfort advantage due to the location of, and type of heat emitters. Radiators are usually located under windows and baseboard heat (either fin/tube or baseboard radiators) is usually located along outside walls. These are the locations of the greatest heat loss and locating ductless mini-split heads away from these locations may be less comfortable. Depending on how many heads you are planning, you may also have less total heat emitters. Radiators also provide radiant heat which is very comfortable when the hot water or steam is running through them, especially to someone who is cold all the time (like myself.) I’m doing a similar conversion on my house and time will tell if it’s less comfortable. But I also made improvements to the air sealing and insulation so I won’t be able to do an exact comparison on comfort. I have no doubt that either system could provide adequate heat.

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