Heat pump effectiveness in cooler temps

[Neal]

The rule of thumb I have heard regarding when a heat pump is no longer effective is approximately 40 deg F. In my recent experience here in Mesa AZ where temps got to a low of 46 my believe is that 50 is the line and not 40 degrees. My heat pump will run continually seemingly below 50 degrees outside temps and not heat sufficiently, furnace should be used when temps will be below 50 in my opinion.

 

[Gronk]

The problem you run into with a heat pump is that in the heating mode itis trying to cool the outside air. The outside coil gets so cold it wants to freeze up and the freeze sensor keeps shutting the compressor down but the blower keeps running.

 

[TJ&LadyDi]

Neal:

I think you are looking at two issues here; "efficiency" and the auto-changeover set point. They are not necessarily the same.

Most heat pumps are set up to change over to an alternate heat source (furnace, etc.) at about 40 degrees. The actual efficiency drops significantly below about 45-50 degrees. As Mntom points out, the heat pump will start doing defrost cycles well before the outside temperature reaches 40 degrees. You are still getting some heat at that point, but it is intermittent due to the heat/defrost cycles.

Interestingly enough, the big heat pump at the S&B place changes over to the auxiliary heat (gas furnace) at the same 40-degree point. Our service guy was here at the house the other day and I asked him about that. He said most heat pumps (small or large) use the 40-degree changeover as it seems to be the best balance between efficiency and stressing the unit with heat/defrost cycles. He also pointed out that the moisture level in the outside air has an effect on when the unit starts to freeze up, Very dry air won't create as much of a problem as will humid air.

TJ