When is the compressor less efficient than auxiliary heating?


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Old 12-31-17, 08:36 AM
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When is the compressor less efficient than auxiliary heating?

First, let me make it clear that I do not have any problems to solve. I am asking this question that has been in my mind for years out of curiosity.

I have been wondering if the outdoor-indoor temperature difference can be so big that the compressor is even less efficient than the auxiliary coil. I am thinking the compressed refrigerant can be cooler than indoor temperature making the efficiency of the compressor zero. Let's use an extreme situation as an example: suppose the outdoor temperature is -30C and the indoor temperature is 20C, the difference is 50C or 90F. Under this condition, can the compressor help heating at all? The auxiliary coil has a constant efficiency immune to any environmental factors.

I have seen so many experts here that I hope one will clarify this once and for all.

The current super-long stretch of arctic air over New England finally prompted me to ask the question here.
 
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Old 12-31-17, 09:15 AM
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I am not a heat pump expert. I service them but they are not all cold weather performers.

-30C = -22F. That's pretty cold for an air source heatpump.

Different manufactures and different systems can squeeze some heat out of cold air down to roughly 5f. Some claim to go a little colder in ambient temps. It's not only how much heat can you remove from low temperature air.... it's the cost of obtaining that heat and the increased defrosting periods. Defrosting requires the electric reheats to be used. I use 10-15f as the lower limit here.

Coefficient of Performance: COP = energy out / energy in.

???? Btu/hr x 0.293 = ????? watts for the energy out portion.
???? watts of current draw is the energy in.

Electric heat is a COP of 1.
An air source heat pump is over 1. Can be upwards to 3 for a well performing system.
 
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Old 12-31-17, 09:59 AM
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Thank you for explanation. It looks like that getting the COP under different temperatures is the key to this question.
I have a 2A6H4030 heat pump:
2 - R-22 refrigerant
A - American Standard
6 - Split Heat Pump
H - Heritage
4- SEER 14
0 - Brazed
30 - 30K BTU

I have no idea what COP it has. Now in New England, night temperatures below -15C ( 5F) seem to have become the new normal.
 
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Old 12-31-17, 10:30 AM
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All things being equal..... that's just system efficiency.

30k x 0.293 = 8790 watt out
18 x 240 = 4320 watts in
8790 / 4320 = 2.04 COP
 
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Old 12-31-17, 11:33 AM
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Thank you, could you tell me how you got 18x240?

I assume COP varies with the temperature difference. One point that I was trying to make in my original question is that COP could theoretically be zero if the temperature gap is big enough.

The following is a figure from a DOE document for a 4-ton low temperature heat pump (LTHP) manufactured by Hallowell International :

 
 

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