Will an expansion tank and water pressure regulator work over periods of non-use?

Question

I have high water pressure coming into my house (~110psi) and a backflow preventer at the source. I do not currently have a pressure regulator or an expansion tank installed, though I'm considering doing this.

Currently my T&P valve is leaking via a slow drip (maybe 3-4 cups/day). I've taken a reading of the pressure and if I'm not using the hot water (i.e., overnight), the pressure gradually increases to 150psi when the T&P valve releases. My understanding is that as the water heater heats water in the house, pressure increases and has nowhere to go because of the backflow preventer. The T&P valve releases at 150psi, which is why my pressure spikes to this level overnight or when I'm not using the hot water.

I have purchased an expansion tank and a water pressure regulator, and plan to install them. My question:

Will the water pressure regulator prevent the pressure from increasing over ~50-70psi (or whatever I set it to) during times that I'm not using water? If not, how can I prevent my hot water heater from leaking at the T&P valve during periods that I'm not home?

My fear is that because the system is closed the water heater will slowly build up pressure until the air bladder in the expansion tank reaches 150psi, and then the T&P valve will leak anyway. Perhaps it would increase the time before the leak begins, but if I leave for any period of time the leak will recur.

Jim Stewart · Accepted Answer

When you get your expansion tank installed this leakage at the P&T relief valve will stop. The pressure regulator will keep the pressure at your set point of 50 to 70 psi.

The pressure is currently building up in your system because as the water in the tank is heated (especially after a large drawdown) the VOLUME of the trapped water increases more than the volume of the tank as the water is heated in the tank. The volume does not increase indefinitely but stops when the water in the tank is heated to the set-point and the burner or heating element in the heater turns off.

You could prevent this increase right now by letting a cold tap drip until the heater turns off after large draw-down. Probably letting out as little as 3 or 4 cups out a cold tap would keep the pressure down. This would cause this same small volume of hot water to back up into the cold supply, but this is of no consequence.

David Hahn · Answer

Okay a pressure regulator will NOT prevent the pressure rise from the heated water expansion.  The regulator will prevent pressure from the source to exceed the regulators max setting. Once the cold water is in the system downline from the check valve and regulator, the water will expand as it is heated in the trapped system.
The expansion tank is like a buffer, absorbing the added pressure, and releasing it less abruptly if you will.  Taking the stress off of your fixtures, pipes ECT.  It is just a rubber bladder in a metal tank.

Will an expansion tank and water pressure regulator work over periods of non-use?

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