Thank you for writing to my blog. We have 1000+ heat pumps from PA to Maine, so I know your pools and weather.
In PA, I like to heat from May first to October first. Some of my friends in northern NH were still swimming well into October last year. It depends on the weather. I know that you will love your new swimming pool heat pump. Gas is too expensive in PA to heat a pool and solar doesn't work in the cooler weather. For your size pool, 20 X 40, about 30,000 gallons, look at heat pumps in the 100,000+ BTU range. I would stick with companies that have been in the heat pump industry for 20 years or more. Then you know they will be there for you. Since you have a spa I would get the biggest heat pump that you can find so that you don't have to wait too long for the water to heat up. Heat Siphon has one with 122,000 BTU's and they are made in PA, Jandy and Gulfstream have one with 130,000 BTU's and Rheem has one with 137,000 BTU's. If you might like to jump in the spa when it is below 50 degrees at night in the spring or the fall consider a heat pump that will defrost because if you don't then most heat pump shut down at about 50 degrees.
Heat pumps will save you a lot of money over gas. For instance, if the heat pump costs you $75.00 a month in electricity, then propane and electric element heaters would be close to $375.00 a month to operate, and natural gas and oil about $250.00 month.
I hope that helps.
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