As a (small) reason, the controls involved in operating a heat pump plus furnace/fan are more complex than the controls involved in operating a plain air condit

As a (small) reason, the controls involved in operating a heat pump plus furnace/fan are more complex than the controls involved in operating a plain air conditioner plus furnace/fan. (Possibly more than a little more complex if the heat pump is only operable under certain conditions and the heat pump and/or furnace is multi-stage or actually variable speed.). HVAC controls are very solidly stuck in the 1980s (or 1950s or whatever), with the main exception of highly integrated and proprietary variable speed heat pump controls. I say this as someone who recently helped undo the damage after an inept HVAC technician replaced someone’s thermostat (for $500, thanks California prices) and did it wrong. The quality of the instruction manuals is low, and the quality of online resources about how conventional HVAC controls work is lower. As far as I can tell, the theory is that HVAC systems used to consist of a bunch of discrete components, strung together, with control power coming from one or two transformers or thermocouples [0], and essentially no logic in the equipment. A relatively modern heat pump may well rely on a mechanism in the thermostat to protect the compressor. The actual equipment contains nothing resembling a microcontroller. [0] Yes, the actual power available from the equipment to the thermostat may be negligible, and old thermostats were mercury switches on bimetallic strips that were powered solely by temperature changes in the room.