This question concerns "regular" heat pumps not a mini split. How does the COP vary with regular heat pumps? I understand how it varies with temperature. For example the unit I am looking at is rated at 3.54 @ 47 degrees and 2.46 @ 17 degrees. On a mini split I understand that if they aren't sized correctly and not modulating the COP can suffer and to some extent have quite a drop. How does a regular heat pump compare? If the unit is oversized or undersized does this change the COP? BTW the heat pump above is a single stage RP14 from Rheem. Thanks

With single stage heat pumps it's all about duty-cycle. It takes at least 8-10 minutes for the heat pump to reach it's steady state COP at whatever the indoor & outdoor conditions happen to be, so longer cycles and less standby-idling the better.

The higher the seasonal average duty cycle, the higher the average as-used HSPF, but it still needs to be sized to at least to the 95% or 97% load to avoid relying excessively on the heat-strip backup power, which will reduce the as-used COP at low temperatures. So far as I know there are no 1-2 stage heat pumps in the US using the vapor-injection scroll compressor technology of the type used in cold climate mini-splits to give it more capacity and higher efficiency at low temperatures. At +5F most cold climate mini-splits will have a COP of 2 or a bit more (depending on modulation level), whereas most standard split system heat pumps won't muster more than 1.5, some may not even hit 1.2. This is true even of those that hit the mid-2s @ +17F.

Posted By Dana1 on 10 Feb 2019 09:17 PM
With single stage heat pumps it's all about duty-cycle. It takes at least 8-10 minutes for the heat pump to reach it's steady state COP at whatever the indoor & outdoor conditions happen to be, so longer cycles and less standby-idling the better.

The higher the seasonal average duty cycle, the higher the average as-used HSPF, but it still needs to be sized to at least to the 95% or 97% load to avoid relying excessively on the heat-strip backup power, which will reduce the as-used COP at low temperatures. So far as I know there are no 1-2 stage heat pumps in the US using the vapor-injection scroll compressor technology of the type used in cold climate mini-splits to give it more capacity and higher efficiency at low temperatures. At +5F most cold climate mini-splits will have a COP of 2 or a bit more (depending on modulation level), whereas most standard split system heat pumps won't muster more than 1.5, some may not even hit 1.2. This is true even of those that hit the mid-2s @ +17F.

Thanks again! I will have a propane furnace for backup so I will have the heat pump set up to shut off at the point that either it quits working efficiently or wants to turn on the heat strips which will be disabled or not installed. The propane furnace will be on the propane backup generator circuits so if power fails it will automatically switch to propane. Sometimes the power can go off in my area for several days. A 9kW Generac Guardian is about $3000 which will run off propane. I can have the propane company put me on their "keep full" service so I don't have to worry about running out. It uses 1 gallon per hour at half load so with a 250 gallon tank (with 200 gals in it) should run a week or more. The genny is WIFI enabled so I can check on it from home. My main concern will be to keep the pipes in the basement from freezing so this should work. When I head towards CO I will turn on zone 1 which is the first floor and let it start heating up. If the heat pump can't do it I should be able to switch over to the furnace with the wifi enabled thermostat. The plan is to probably to use Verizon for my internet access since they have backup generators and I suspect the ISP's don't. Thanks

In many markets propane is going to be more expensive heat than the aux heating strips, but not more expensive than running the propane generator to run the heating strips.

Colorado surprised me. I expected propane to be high. Here in Texas it's $2.15 there it's $1.90

1 Million BTU's of propane heat is $26.38
And electric $48.72 for resistance heat
Heat pump at 47 degrees with COP of 3.5 = $13.92
17 degrees where it's COP is 2.45= $19.88
Once the COP reaches 2 where cost is $24.36 I am just as well to switch over to propane because I figure the defrost cycles will decrease the COP even more

Whether defrost impacts COP depends on how the defrost cycle works, but there are more defrost cycles when it's in the 30s and low 40s F than when it's in the teens. With ductless heat pumps that reverse the cycle using indoor heat rather than resistance heat for defrost the impact on COP is small, but the impact on capacity can be high. In the relatively dry winter air of Colorado the impact of defrost cycles is smaller than say, Seattle, where it's in the 30s and low 40s all winter, with outdoor dew points lingering close to the outdoor temperature.