Drip, Drip, Drip

We have a single lever faucet on our shower that has a continuous drip when the hot water heater is on. I carefully replaced all the parts within the faucet with a kit, but to no avail. I understand that as water heats it expands, but where does the water normally expand to and how does it seek relief? I know that some water leaks through the hot water tank relief valve, but that is not an option that I like either.
Leon, (Belleville, ON, Canada)

Leon, I too, would suspect a faulty seal inside that faucet, but if you’ve replaced the guts with the proper components we can probably rule that out as a cause. And you are correct that as water heats it expands. Normally this expansion occurs inside the water heater. A cushion of air must always be present in there to accommodate the expansion. In optimum circumstances this cushion of air inside the tank above the water level acts as an accumulator of sorts. Over time, this air space (oxygen) is eventually fully absorbed by the oxygen molecules in the water. This results in no place for the expanding water to move into since the tank is, at that point, completely full. Like you mentioned, usually the integral pressure and temperature (P&T) relief valve then performs one of its safety functions and opens, expelling the heated water through the P&T valve. This also allows additional cold water to enter the tank (thus lowering the temperature) and causing the valve portion to close. Perhaps the lesser resistance to this expansion is through the shower faucet instead of the relief valve. It is normal for the P&T valve to drip a little during each heating cycle.

I’d try renewing that air cushion inside the water heater. There’s a good chance that air space has been eliminated over time. To reestablish the cushion of air inside the tank, remove the water source by either turning the demand pump off or by interrupting the flow of city water. Open any two hot water faucets in the coach. Next, open the manual lever on the relief valve until the water flow completely stops. Close the P&T valve and the faucets, and then turn the pressure back on. There is now a cushion of air trapped above the water level inside the water heater once again. Hopefully this will eliminate that pesky leak in the shower faucet. I’d be tempted to install another faucet if the leak persists.