> A heat pump in the winter is literally just air-conditioning the outside, and produces similar condensation in the unit outdoors. It produces condensation, y

> A heat pump in the winter is literally just air-conditioning the outside, and produces similar condensation in the unit outdoors. It produces condensation, yes, but the total heat transfer spent on that condensation is still pretty small, because now the condensation isn't removing moisture from the inside air, it's removing moisture from the outside air that is blowing past the heat pump's outside heat exchanger. So the amount of water vapor being condensed is only based on the difference between the ambient outdoor air temperature and the temperature of the outdoor air just after it's blown past the outdoor heat exchanger, which might only be 5 degrees C or so--say -10 C (ambient) to -15 C. From the Engineering Toolbox [1], that's 0.00231 - 0.00158 = 0.00073 kg/m^3 of air. With a latent heat of 2.6 x 10^6 J/kg (since we now have to count the latent heat of both condensation of vapor to liquid and freezing of liquid to ice), that's about 1900 J per cubic meter of air. So it would take about 5000 cubic meters of air being blown past the outside heat exchanger for the heat transfer due to ice formation to equal the heat transfer to heat up the inside of the house from -10 C to 22 C (which I calculated in my previous post a few minutes ago). I don't know the exact outside airflow of a heat pump, but that seems like a lot more outside air than would be needed for the necessary run time of the heat pump for that amount of heating. [1] https://www.engineeringtoolbox.com/maximum-moisture-content-air-d_1403.html https://www.engineeringtoolbox.com/maximum-moisture-content-... > As far as I can tell, the cost of running a heat pump is basically the same no matter which direction you're running it. Not in terms of heat transfer, no; comparing the calculations in my previous post with the above, it should be evident that it makes a big difference which heat exchanger is doing the condensation of water vapor (inside or outside).