Q: I have a second home with a gas hot water heater that is only used a couple days a week. When not in use, I turn the thermostat on the water heater all the way down. There is a copper overflow pipe down the side of the heater that seems to drain slowly. I get maybe one-half to three-quarters of a cup over 4-day period. What is causing this leak and is there any way to prevent it? This hot water heater is in Norristown on town/city water. It seems to leak mostly (maybe only) when the temperature is set as low as possible and we are away/not using it. When we are in the apartment and need hot water, we turn it to high, heat the tank, and then turn it all the way down to low. I do not see much if any drainage when I am there. When I return 4 days later, the heater has been on lowest possible setting and I notice the leakage.
A: Gas water heaters when turned low will form condensation in the burning chamber of the heater and will make for puddles of water. In fact, many people suspect their heater has a leak when this occurs. If you are sure that it is coming from the relief valve then it is overpressure in the system at certain times caused by heating and cooling of the heater and that the water company puts back flow preventers on your system. Thus, when the heater is not in use and cools, it draws in cold water as the water in the heater contracts. When the unit comes on and heats the water, the water expands and since no one is there to use the water it creats an overpressure which forces water out of the relief valve rather then rupturing your heater. This can be prevented by putting in a small expansion tank. We sell ones that require no tools and are easy to install if you want. If it is condensation, you can either turn the heater off, or leave it at the full temp.