Questioning Heat Pump size in our just completed HVAC Design
Hello and thanks in advance for considering this question. We have just received our HVAC Design plan for our 2900 sq. ft. 1.5 storey home to be built on the shore of Georgian Bay west of Collingwood, Ontario. We are in CZ6. The plan calls for a fully ducted system with two separate heat pumps, a 2 ton on the main floor and a 1.5 ton on the upper floor. There will be a conditioned crawlspace below most of the building (the remainder will be slab on grade). The crawlspace will be heated by the 2 ton main floor unit There will be a conventional wood burning fireplace which could take the chill off on the coldest days, but the heat pumps need to meet code without taking into account that potential auxiliary heat. There will also be an ERV and a Makeup Air unit.
The design was originally spec’d for R22 (nominal) walls (Mineral Wool batts between 2×6 studs on 16″ centres). (Not sure if we can afford better than double-pane windows.) Then the heat loss calc was updated to reflect the addition of R10 ComfortBoard 80 on the exterior and a few other small details. This dropped our total heat loss numbers from 62,536 BTU/H to 55,508 BTU/H.
I was hoping that the addition of the ComfortBoard 80 might allow for smaller heat pumps. If I understand correctly, it is important to undersize rather than oversize heat pumps to avoid the inefficiency and reduced comfort that come with frequent cycling. IOW, in relatively cold weather, the heat pump should be working almost constantly, not intermittently. The HVAC designer has kept the same Mitsubishi units and made this comment relative to the addition of ComfortBoard 80:
“The equipment remained the same, although the heat loss and heat gain values went down (more so the heat loss). The good thing is that the equipment won’t run at full capacity on the coldest day (-25 degrees Celsius) and in turn will use less power on those days.”
Maybe he means the electric resistance component won’t be required as much on the coldest days, so it will be less costly to run. I’ll try to get more info from him on that.
I am attaching a chart showing the recommended heat pump details. Please chime in with your thoughts on his choice of equipment and his comment above. Thanks very much.
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