Pool heat pump
Also called: heat pump pool heater, electric heat pump, pool heater.
A pool heat pump moves heat from the outside air into the pool water using a little electricity: a fan pulls air over a coil, and a compressor concentrates that heat and hands it to the water. A heat pump is the cheapest heater to run in mild weather.
Why it matters
Its one weakness is cold air. The Department of Energy's consumer guide, since taken down, said heat pumps work efficiently while the air stays above the 45–50°F range, so a heat pump that refuses to run on a cold morning is working by design. That is why Florida pools default to a heat pump and cold climates lean on gas. It also needs flow: Hayward HeatPro and Pentair UltraTemp both state a 30 GPM minimum, so a dirty filter or a variable-speed pump set too slow stops it. Whatever you heat with, a cover saves 50–70% of heating cost per the DOE. For an attached spa, gas is the usual call.
In numbers
From 80°F down to 50°F air, expect a pool heat pump to retain roughly 58 to 75% of capacity, with the coefficient of performance dropping from about 5.5 to 4.0.