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Discussion Starter · #1 ·
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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The pressure tank in not the issue, the pressure relief valve.
Often times they can be rebuilt in place.
 

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I was thinking about the possibility that joed mentioned. It’s unlikely, but since you’re trying to diagnose what is causing the pressure rise you can completely eliminate the house inlet PRV as the culprit by shutting a valve someone on the incoming water line while you’re running your next test (and nobody can use water in the house during the test, of course).

Assuming that the expansion tank is pressurized and not isolated by a closed valve, what else could it be?

Having a PRV downstream of a HWT is odd, so I wonder if the previous owner may have also put in a check valve for some reason that is preventing the water from the HWT from expanding back towards the new expansion tank. They don’t look like much so you may not have noticed one, if it was there (see below).

Chris
 

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If it's the PRV on the supply line, wouldn't there be pressure increases even with the water heater isolated or the heat turned off?
Yes there would.

FYI it is not a PRV on the hot water tank. It is a TPR, temperature and pressure relief valve. It is there to release the pressure if the tank was to overheat and become a bomb.
 

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Discussion Starter · #9 ·
Yes there would.

FYI it is not a PRV on the hot water tank. It is a TPR, temperature and pressure relief valve. It is there to release the pressure if the tank was to overheat and become a bomb.
Thanks. I did get 150 PSI on the cold water side with the water heater isolated. Replacing the PRV valve did the trick.
 

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Good to hear that solved the problem. I was going to say the pressure regular on the cold water pipe from the street in. I replaced like 3-4 of them on my rental properties. I am curious how did you replace your? If it was CPVC, then I glued it but if it was copper in tight space then I just used Sharkbite. It's just to risky to torch it.
 
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