At what (outside) temperatures is it efficient to run my windowbox heat pump?

Question

I am an electrical engineer that took two semesters of Thermodynamics about 4 decades ago. I am aware of the theory behind the Carnot cycle and such.

I live in a condo in the Boston area that is all-electric (except for a fireplace I am not using and has its flue closed up). Recently, I replaced the windowbox A/C with a windowbox A/C and heat pump. I have been using the heat pump solely to heat the apartment while the outside temps have been above 40 F.

The other heat is simple resistance heating, which I will call "100% efficient" even though the electricity was generated at a plant somewhere with a heat engine of far less efficiency. I look at the heat pump as being a way to recover some of the heat lost in that inefficiency because, theoretically, it is better than 100% efficient because the heat coming out of it is the sum of the electric power running it plus whatever energy taken from the outside air. I also know that this efficiency decreases as the outside temperature falls.

But now as winter is starting to close in, outside temps will become consistently below freezing. At what point is it not worth it to run the windowbox heat pump? Just asking if some HVAC sorta people know a rule of thumb regarding this.

Answer 1

Recently I reviewed the technical specifications for a number of heat pumps (reverse cycle air conditioners) with a view to recommending one for purchase.

The lowest outside temperature for a heat pump (reverse cycle air conditioner) to operate varies according to the manufacturer and the unit. According to the manufacturer's technical specifications, in heating mode, some units I reviewed were capable of operating at outside temperatures of $-15$ $^oC$ ($5$ $^oF$) and other had their lowest outside temperatures at $-10$ $^oC$ ($14$ $^oF$).

The lowest outside operating temperature for your unit should be stated in the operating manual, which you should have received. If you don't have one, look at the technical specifications for your unit on the manufacturer's website.

Answer 2

The operating performance of air-source heat pumps (ASHP) will vary by model: mostly, but not entirely, driven by the choice of working fluid.

Put an individual plug-in electricity meter on the ASHP power supply: you might be surprised. There are two things that can bite you. One is that some ASHPs have a built-in resistance heater that they will switch to when the source temperature gets low. The other is that they will periodically run a defrost cycle too, which will be both energy-hungry and noisy (some defrost at 72dB).

You'll get higher efficiency from your ASHP if you can lower the temperature that it delivers heat to you: turning it down a couple of degrees can make a big difference. But watch out for the dew point temperature in your room: running your room cooler raises the risk of mould, and the consequential risks to health and property.