Sizing a Heat Pump for Adjoining Rooms
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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