We still have 4 pools that are ocean front that are rented with pool heat. Every Monday the phone rings and its one of two complaints. Either the pool is not heating because the night time temps are to cold for the heat pumps. The other is the water level is three inches down from yesterday because the pools are evaporating so much in the night from the gas heaters.
The reality of pool heaters is that, they should only be used up to a certain point. At which point they either can’t keep up, or should not be used unless you live there and know what to do when a problem arises. http://www.heatsiphon.com/