My pressure relief valve was started leaking. I changed it out and it still leaked. The water heater was 10 years old so I bought a new one and installed it. The new one is doing the same. The pressure relief valve is letting out about 1 gallon every 8 hours or so. What could be causing this?
Thermal expansion causes this problem. Check out http://www.fwbehler.com and look on the Tips page for a great explanation and details on how to correctly deal with this problem.