Heat pump

A heat pump pulls warmth from outside air and moves it into the pool. It uses electricity, not gas, and is efficient in mild weather. It works slower than gas, and it loses strength as the air gets colder.

In North Texas, that makes it a good fit for extending spring and fall swimming. It struggles during a hard freeze. Keep the air coil clear of leaves, and keep scale out of the water path. Problems with the compressor or wiring need a pro. See also gas heater.