I looked into it a while back, and came to a similar conclusion to yours (if I read you correctly).

Heat pumps may be cost-effective for heating in areas with little or no freezing temps, but a large fraction of modern civilization lives in areas with substantial freezing.

For warmer climes, I’d suggest a simpler approach to lowering HVAC costs: “smart” cooling systems capable of varying the pressure/temp at both the condenser and evaporator, and spraying purified water on the condenser coils so that the high-side temperature can be kept much lower than the outside ambient temp. (Water purification systems are already very cheap compared to a few decades ago, and probably due to become much cheaper as the cost of manufacturing bipolar membranes continues to come down.)

My “back of envelope” calculations suggest that this (both innovations combined) could cut the cost of cooling air by 50% or more.

Another way to cut costs would be to replace thermostats with systems that measure/control both temperature and humidity, and focus on the higher-temp, lower humidity end of the comfort zone.

In addition to taking better advantage of natural human (mammalian) temperature regulation, such systems would reduce losses via conduction/diffusion through walls/windows. Diffusion of water vapor is much easier to reduce than diffusion of heat…

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