Take a look at this thread where there is a discussion about gas heaters vs. heat pumps. In northern climates where the air temperature gets below 50 degrees regularly during periods that you *might* want to heat your pool for swimming, a heat pump using the ground as its source of heat might be better. However, May and September should still be in the 60's so a regular above-ground heat pump should work well. Heat pumps are for slow maintenance of temperature while gas heaters will generally heat more quickly, but much more expensively.
I have a gas heater that is mostly used in mid-April to mid-May and in mid-October to mid-November (it's used other times as well, but not continually) and it is VERY expensive. I live in the San Francisco Bay Area.
Richard
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