Re: Water Heater Temp/pressure valve leak
If there is any "air dome" at the top of the tank, that air will eventually be absorbed by the water and the dome -- which would be a very minimal volume -- will be full of water. When I was growing up, our well pump system was connected to a non-bladder pressure tank. It required regular injection of air to maintain an adequate "head" to prevent the pump from short cycling. A water heater is no different. To maintain this air dome would require either partially draining the tank periodically, or having some means of injecting air. I believe the idea of an air dome at the top of the tank to provide for thermal expansion is a myth.
As for the expansion tank, if the OP is on a private water system with a well pump and pressurized cold water storage tank, an expansion tank is not necessary because the cold water storage tank will be able to handle the expansion of heated water, unless there is a check valve between the storage tank and water heater. If the OP is on a municipal water system, an expansion tank will likely be necessary since many meters and all pressure regulators prevent backflow into the main.
If the OP indeed does have an expansion tank, it may have lost its precharge. The OP should relieve all pressure in the water heater (shut of the inflow valve and open a hot water faucet) and check the precharge pressure of the expansion tank. It should be equal to the static water pressure of the system. If the precharge pressure is low or zero, the expansion tank may not be able to absorb the expansion of heated water, causing a rise in pressure forcing the T&P relief valve open.
Expansion tanks are usually constructed with a water bladder to separate the water and air, preventing absorption of the air by the water. Even so, expansion tanks may eventually lose pressure through micro-leaks in their snifter valves (rubber is porous). Water heaters require regular maintenance including:
- Flushing sediment out of the tank via the drain petcock
- Inspecting the sacrifical anode and replacing if necessary
- Checking the precharge of the expansion tank
This maintenance should be performed annually (but almost no one does it, myself included).
You could monitor this pressure rise by attaching a pressure gauge to a hot water faucet, then trigger the WH to begin heating. Do not open any other faucets while it is being monitored. The gauge must be connected before you will see a high pressure: when you attach the gauge and open the faucet the pressure in the system will drop as the faucet body and gauge fill. Water is fairly incompressible; in a closed system with no airspace a very small change in available volume can result in a very large change in pressure.
The "Senior Member" designation under my name doesn't mean I know a lot, it just means I talk a lot.I've been a DIYer since I was 12 (thanks, Dad!). I have read several books on various home improvement topics. I do not have any current code books I can refer to. I was an apprentice plumber for two years.