A heat pump runs on electricity and operates by removing heat from the air and using it to heat the pool or spa water. A gas heater runs on propane of natural gas. Gas heaters generally provide much more rapid heating of a pool or spa but are more costly to operate than a heat pump.
If your intentions are to heat the pool for long durations then heat pump. Solar heating is effective at raising the pool water temperature 10 degrees or more in all except the coolest months. More.
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