Re: Water Heater Pressure Relief Valve
When you initially fill the water heater at the beginning of the season, it doesn't fill completely with water. There is an air pocket at the top and this air pocket absorbs the pressure when you heat the water.
As time goes by, you can slowly lose the air pocket and when the water heats and expands, the pressure increases and the water comes out the pressure relief valve. You have to recreate the air pocket.
As markopolo said, you have to remove some water from the heater (when it's cold and the pump and city water supply are off.).
I've read you can do this by simply opening the relief valve until water stops coming out. Then close the valve, turn the water supply back on and open a hot water faucet to allow the heater to fill to its normal level.
You will probably be OK for a while, but might have to do it later in the season.