Heat pump COP degrades as outdoor temps drop. Just a result of the fixed outdoor heat exchanger surface area and limited min refrigerant temperature inside of it. Basic thermal physics I suppose.
COP also increases as indoor temps drop, ironically. Colder indoor air flowing across the indoor heat exchanger increases the rate of heat extraction there. But, of course, noone wants cold indoor air..
Quote:
Originally Posted by abogart
Did I just pick a poor performing unit here or is it really just cheaper to heat with gas through the winter?
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The outdoor temperature at which it's cheaper to use a gas furnace will vary with your heat pump selection. I don't know the precise dynamics of how system component properties determine COP at various temperatures, so I can't say how a system could be modified or selected to give a better COP at lower temps.
Maybe a unit with a higher load rating will have a better COP at a lower temperature. But then you don't want to oversize the unit if you're using it for cooling as well, otherwise humidity control will suffer.
I would imagine a larger outdoor heat exchanger with a given compressor would extract more heat at lower temps. And if the indoor heat exchanger surface area remains the same, it should have the same dehumidifying capability when cooling.* But unless you're building your own heat pump, I don't know how to search for a unit by size of the outdoor heat exchanger.
*Furthermore, assuming the indoor heat exchanger surface area & internal volume is unchanged, then the volumetric flow rate of refrigerant and the refr temps must also be unchanged (to provide equivalent dehumidifying ability with same compressor). To meet these requirements with a larger outdoor surface area, you would need smaller refr channels** in the outdoor heat exchanger, larger metallic fins attached to the channels, or just longer runs.
**I believe parallelizing smaller channels, using smooth branches and junctions, would permit a design of equivalent pressure drop, whereas longer runs or parallel runs of the same size will introduce additional pumping losses. Of course I have no idea how much of a load the heat exchanger presents to the compressor in the grand scheme of things. It may be quite minimal compared to the power involved in producing such high pressures.
It may also be the case that as the outdoor surface area increases, defrosting occurs less frequently at the critical outdoor temps around 32F, since lower exchanger surface temps will be necessary to meet a given load. However, when defrosting is performed, it may require slightly more energy, given more surface area is covered with ice..
You could try the AHRI directory,
AHRI Certification Directory, and sort the result list by the 17F load rating, and use that to find units that are more efficient at low temps.
I hate to interject additional comments, but there is a type of heat pump system that uses less energy to perform defrosting, which does become a burden at temps hovering around 32F. I've read of at least one "reverse-cycle chiller" system (which is essentially a heat pump using water/brine to distribute heat indoors) that will direct waste heat to a water storage tank, and utilize that heat whenever a defrost is necessary, rather than resistance coils. This would only have an impact at temps floating around 32F, where defrosting occurs most frequently.
In one of Sanyo's technical manuals they specify precise losses due to defrosting, see
http://www.sanyohvac.com/assets/docu...ta_2007_08.pdf page 2-7 lists coefficients. It shows between roughly 23F and 37F, a 10% loss of capacity due to defrosting. This is in addition to the linear loss of capacity shown in the graphs above those coefficients.
I suspect COP decreases right along with the capacity, as at that point the compressor is maxed out and outdoor temps are the controlling factor. I suppose the heat capacity flat lines at warmer temps because the surface area of the heat exchanger become the controlling factor.. that is, fixed area, and the internal refr pressures are limited. The compressor would run slower and slower as outdoor temps rise, to prevent the internal temp from becoming unsafe..
I should really stop now because this is entirely too much conjecture. Maybe someone with more experience can chime in.