Oops, that should have read central heat and air. But, no, I don't leave the under tank heater on at this time of year. Even when we run the A/C it doesn't get below 75 which is an 'ok' temp for them. I keep thermometers in each tank and just keep an eye on them. This is pretty easy at this point with just three tanks but a thermostat would be a better way to go.