I had a zone off my boiler specifically to heat my pool through a heat exchanger. It worked wonderfully on my small pool(11 k gallons) in MA to bring it from 60 to the low 90's in about 20 hours. Oil use was minimal, about 20 gals to heat it for the weekend. not sure how gas would work but we were hap with the setup we had. During the cooler months like may,june and sept to keep it on say 82 the boiler would run a few hours a day depending on ouside temps of course.