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Background:

When I bought my home it had a 50 gallon gas water heater in the garage without an expansion tank. On the hot water line there is a Watts adjustable pressure relief valve (this NOT the PRV on the tank - it's an additional valve). The water pressure as measured on the hose bibs was frequently around 150 PSI and the Watts PRV or the PRV on the tank would frequently release water. There is a pressure regulator valve on the cold water supply line to the house and the pressure when measured without the water heater heating is 55-60 PSI.

Problem:

The water heater survived 20 years under those conditions but it finally started leaking. I replaced the water heater with another 50 gallon unit and added a 2 gallon expansion tank vertically on the cold water line feeding the tank. I expected this to solve the high pressure problem but it hasn't. Even with the water heater turned down to around 115F, when it heats, it brings the pressure over 150 PSI and the PRV on the tank opens and stays open until I run the water somewhere to release the pressure. ~30 minutes later, the pressure is back up and the cycle continues.

I'm baffled on what the problem is. The expansion tank is the recommend size for the size of my water heater and line pressure. The only thoughts I have are to get a bigger expansion tank or potentially replace the pressure regulator valve on the cold water supply line. The latter is a bit of a hail mary IMO and will be a major project due to the location.

Anyone have thoughts on what else could be the problem or potential remedies?

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    Sounds like the PRV is failing (slow leak), expansion tank is failing, or the expansion tank isn't sufficient. Check the expansion tank for half air (should be able to hear the difference with a light tap on the side with a wrench). Turn off the hot water heater and see if the pressure eventually rises or stays low for over an hour. – BMitch Jan 10 at 14:06
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    Thanks @BMitch. It was the PRV. I replaced it and the pressure is normal now. – DSS Jan 10 at 23:26
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From the comments, it appears the PRV failed, likely with a slow leak on the valve. A good way to test for this is to shutoff the hot water tank, close all valves in the house while monitoring pressure over time. If the pressure slowly climbs even without the hot water heater running then replacing the PRV would be my next step.

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    I agree and this is a common failure PRV are regulators they have a diaphragm that moves some are metal and some are rubber, both fail over time, another failure is rust or scale blocking the orifice allowing water to flow.+ – Ed Beal Jan 11 at 17:58
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It could be due to a backflow prevention valve somewhere downstream the expansion tank. This BPV must be upstream the expansion tank, if the expansion tank is upstream of the heater. I.e., there must not be any blocking device between heater and expansion tank. Sometimes the BPV is only a small insert in the connection of another device/valve, not to be recognized without opening the connection. Those BPV inserts can be as small as a coin. Some water heaters have a BPV insert in the incomer connection, should be mentioned in the manuals. There could be also more then 1 BPV between expansion tank and heater.

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