Heat pump
also: air-source heat pump · ground-source heat pump · mini-split · inverter compressor
A machine that moves heat from one place to another with a refrigerant cycle, providing both heating and cooling far more efficiently than resistance heat.
A heat pump uses a compressor, heat exchangers and an expansion device to pump heat "uphill": from cooler outdoor air, ground or water into a building in winter, and the reverse in summer. Air-source units, including ductless mini-splits, are the most common; ground-source (geothermal) systems use the steadier ground temperature for higher efficiency at higher installation cost. Heat pump water heaters and air-to-water units serve domestic hot water and hydronic systems.
Variable-speed ("inverter") compressors modulate output to match the load, improving comfort and efficiency and extending cold-weather performance. Controls matter a great deal: defrost strategy, staging of backup heat, setpoint behavior and avoiding short cycling all affect energy use and comfort.
Where it bites: poor sizing, poor installation, and controls that bring on backup resistance heat too eagerly can erase much of the savings. Energy.gov advises against conventional setbacks on heat pumps unless the thermostat is designed for them, because recovery can bring on inefficient backup heat.
Founder takeaway
Size from a load calculation and design the controls for heat pump behavior (variable speed, defrost, backup heat), not as if it were a furnace.
engineer review pending