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Community and Q&A

Sizing a heatpump

eoindeb | Posted in General Questions on


I’m hoping to get a second opinion on the correct size for my heatpump. I’m in a 2,400 sq ft colonial in zone 5. I have an oil boiler as backup heat, and want to install a heatpump to cover as much of the heating season as I can. My contractor completed a manual J, and the heating load @ a design temp of 3F is 46k BTU – 25k total for the bedrooms upstairs and 21k total for the living area (quite open rooms) downstairs. The house has ductwork that was retrofitted some years ago, and runs through two closets to deliver air to the downstairs vents.

My contractor has proposed a minisplit (MSZ-GL12NA-U1) for the living area downstairs, and an air handler (SVZ-KP24NA) for the ductwork for the bedrooms upstairs. The outdoor unit would be a MXZ-SM36NAMHZ. 

My concern is that the minisplit unit for the downstairs may be undersized, and won’t be able to keep up in the coldest days. I asked if we could bump it up in size to maybe a 15k unit, which would still allow me to keep the same outdoor unit if I understand its submittal correctly (max connected capacity of 46,800 btu). My contractor says that if I want the heatpump to run in temps below freezing, I’ll need to change both the indoor and outdoor units. Does that sound correct?

Given that my heat load and the capacity of the outdoor unit are quite close, why wouldn’t I be able to keep the same unit and just run it at colder temps to cover as much of the heating season as possible?

Thanks for any advice or help you can give.

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  1. paul_wiedefeld | | #1

    "My contractor says that if I want the heatpump to run in temps below freezing, I’ll need to change both the indoor and outdoor units. Does that sound correct?"

    No, that's nonsense. That outdoor unit can work down to -14, and you're correct, the max connected capacity is very high.

    A quick way to double check your heat loss is using your oil receipts using this method: It might be lower than the contractor found.

  2. eoindeb | | #2

    Thanks. I had run my heat loss using that link (thanks Dana!) and it was ~39k. That was based on 4 heat zones (old part of the house, addition, primary bedroom over the garage, and basement) but I also have a bunch of the zones turned off during the day, so I'm assuming that's where the difference in that method and the manual J may be explained (not that both of them are supposed to match).

    I also ran my own manual J using coolcalc and it came out very close to the manual J from the contractor, so I think those numbers should be fairly accurate.

    1. paul_wiedefeld | | #4

      You'll be good to go then. You can decide if 2 1 outdoor-to-1 indoor units or 1 1-to-2 units makes the most sense, there are advantages and disadvantages to both. You're well on your way to slashing carbon emissions.

  3. walta100 | | #3

    Have you done a fuel use study to calculate you heat load?

    I am not a fan of the multi head systems. Separate system can operate at much slower speeds so they cycle much less. Sooner or later every system fails when it does the multi head system will give you zero heat while one of the separate systems is very likely to still be operational.


    1. eoindeb | | #5

      Yep, I did the fuel use study and it came out to ~39k.

      My preference would be for a 1:1 to system as well, but the quotes I got for those systems were much more than the multi head system (I got 4 different quotes for multi heads). So I agree with all your points, but given those numbers, I decided to go with a multi head system instead.

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