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Community and Q&A

Sizing a Heat Pump for Adjoining Rooms

maine_tyler | Posted in Mechanicals on

I’m wondering if there is wisdom in sizing a ductless heat pump for the ‘load’ from an adjacent room or for the room only.

This is an addition with 3 exterior walls and a shared wall with an existing, older structure.

I am dealing with an installer that wants to install an oversized Mitsubishi heat pump (oversized for the room). I say oversized based on my own Cool Calc, which I don’t necessarily trust 100%, but on the other hand, the installer did nothing more than eyeball the room. I came up with around 7,600 BTUH for heating. I think it’s possible it’s even less given the constraints with Cool Calc inputs.

One could argue I may even get away with a 6k unit, but I will not have auxiliary heat, and the ‘slight oversizing’ principle in cold climates leads me to believe the 9k unit would be ideal.

BUT the installer came back with a proposal for a 12k unit. One of his justifications was that there is a wide door (that will likely often be left open) to the older part of the house on the shared wall. He thought the larger heat pump (which is on the opposite wall and pointed, albeit not quite directly, at that open door) could heat part of the adjacent room (a kitchen).

It seems there must be a temp differential between those rooms for this concept to apply, no? In other words, that adjacent room would have to be heated (it has hydronic baseboard) to a lower temp by that baseboard for the heat pump to contribute to that load. So unless the setpoint for the baseboard system is lower, the heat pump won’t be able to contribute to that kitchen load.

Is there wisdom I am missing to oversizing in this way? It’s true that the house also has a pellet stove which is not in the kitchen, so when running the pellet stove, conceivably the heat pump and pellet stove could (from opposite directions) heat the kitchen without the baseboard running at all. But I don’t know if it’s wise to size the unit based on this… especially because the load of that one addition will be quite small in mild weather.

From what I can tell, the 9k unit modulates down to 1600 btu/h at 47 degrees, while the 12k unit modulates down to 3700 btu/h at 47 F. That seems like a significant difference.

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  1. Expert Member
    Akos | | #1

    What is your climate zone, size of the addition and what is the exact construction? Even 7000BTU seems on the high side. 12k is about the heat load for 1000sqft of older insulated 2x4 construction in my area.

    If unsure , the best way to check is to to put a space heater with a power meter (ie KillAWatt) and run it for a week or so with the doors closed. Once you have the usage, you can do the math from here to figure out your exact heat loss:

    Even without doing the math, if you can keep the space comfortable in cold weather with a single plug in space heater the load is under 5000 BTU.

    There is a small efficiency improvement gained by a bit of oversizing but way oversized might cause the unit to cycle which will kill efficiency and limit humidity removal.

  2. maine_tyler | | #2

    Hmm. Yeah I am not sure how much to trust the Cool Calc numbers I am getting. While I'm sure it's off, unless I've completely missed the boat, I don't know how it would have gotten so far off.

    I'm climate zone 6a, its about 540 sq. ft. of double wall (10 inch) construction and flat ceiling with cellulose. I do only have 2 inches of foam under the floor (build atop a slab on grade) and 3" of foam around the slab perimeter, 4' down. Glazing is pretty middle of the road.

    Can't test with a heater for a few reasons, one of which is that it's not yet insulated and winter is about over.

    Assuming I can trust, to some degree, my numbers, I suppose I still wonder about whether I should consider an adjacent room part of the load calculation-- with the parameters being that the room has its own hydronic baseboard. If that baseboard was on its own zone, perhaps I could try to rely on the heat pump to heat that kitchen? Currently its on a single zone with the rest of the downstairs though. I am also unclear how well a heat pump can throw heat into another room entirely (the room is about 27' long, i.e. the heat pump head is about 27' from the door joining the rooms).

  3. maine_tyler | | #3

    Thought I might give this one more bump.

    Let me ask a more general question:

    How effectively can a ductless mini-split throw heat into an adjacent room through a 48" wide door when the head is on the wall opposite the door, roughly 27' away? Room being 20' wide or so.

    If it doesn't do that well, then I see no point in sizing the unit for an adjacent room load. If it can do that while maintaining only a degree or two difference between rooms, than maybe there is value in that?

  4. Expert Member
    Dana Dorsett | | #4

    A 48" door 27' away from the head isn't going to get much direct heating from the mini-split.

    The mini-split may have enough throw to achieve reasonable mixing of the air at the far end of a 27' long room but not enough to drive warmer air into the adjacent room through a 4' doorway.

    The heat load of a 20' x 27' room with sides of exterior wall could conceivably hit 7800' @ -5F (or whatever your local zone 6 design temp is) even with the 10" double-studwall, depending on the size & performance of the windows, and the air leakage assumptions. It could easily be less than that too. But it's not likely come anywhere near needing an FS12 or equivalent cold climate mini-split.

    Overall your thinking regarding size is correct.

    The FS09 is good for 9.6K @ -5F, plenty big enough, not too big, a 1.23x oversize factor for a 7.8K load which is nearly ideal.

    The FS12 can deliver 12.3K @ -5F, and which would be nearly a 1.6x oversize factor, thus suboptimally oversized for a 7.8K design load.

    The half-tonner could still make it at -5F, but might leave you cold when it hits negative double-digits.

    If you're not quite trusting CoolCalc, try the BetterBuiltNW tool as a sanity check:

    1. maine_tyler | | #6

      Nice to hear from you Dana. Thanks for your input. I did try out the betterbuilt tool and got a pretty similar heating load.

  5. exeric | | #5

    I can only give you my own experience trying to do the same thing. While I live in a much milder climate, cz4, and there are physical differences in my house from yours, I think there are enough similarities to take note of. I have my single ductless mini split head mounted on my living room wall and it was put in that location to not only condition the living room but to also condition the two bedrooms. Those bedrooms are on the opposite wall from the MS head and the two doors to them are 32" wide. So far it has worked very well to keep the temperature in those bedrooms during the daytime within 2-3 degrees of the living room. The distance from the head to the BR doors is 19 ft.

    I think it's best to have the position of the head in the room you spend the most time in. It also helps a lot to have all rooms well insulated and air sealed. This gives time for the natural averaging of temperate in a large enclosure to work. If you have a big temperature sink in the more distant room, it won't work well.

    I think another important feature to think about is to have a good-sized wall jump duct that is well separated from the door. This allows any pressure differential that builds up as air gets thrown into the adjacent room to have a separate return path to the living room. This will allow a smooth flow of air to the adjacent room without causing turbulence as the air tries to move in two directions through the door. Sort of like an adolescent trying to chug beer by creating a second opening in the can. (How do I know this?)

  6. richmass62 | | #7

    My building is a good example of what you are trying to do. We have an 18K heat pump serving 900 sq. feet just on the 1st floor. There is a laundry room (no door) that didn't warm up enough with the heat pump, but fortunately it has ductwork from our old gas heater. The laundry room is about 25 feet away from where the head of the heat pump is located.

    We recently beefed up the insulation and air sealing of the 1st floor and I can say that the laundry room now can get heated by the 1st floor heat pump when that was not the case before.

    If you have the ability to reside your house anytime soon it is a great solution to add a serious amount of exterior insulation. In my case i was able to add one layer of half inch foam (new foil faced, $20 per sheet at bix box stores) and then top it off with a layer of 2 inch reclaimed polyiso (that was about $15 per sheet). It was pretty labor intensive due to the need to add pressure treated lumber to frame the walls on the outer edges of the foam board assembly, and then seal gaps between sheets with spray foam. But the impact on the house has been enormous.

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