Home water pressure consistently exceeds 150 PSI

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Randy Savage

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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?
 
Maybe worth seeing if you even really need that PRV on the incoming cold line.
If you could eliminate it, the high pressure could relieve back towards the city supply line.

Turn the heater off, let everything cool, run a few gallons of water to release any high pressure.

Then adjust the incoming PRV way up and see how high it goes.

If it stops going up below 75 psi, maybe just eliminate it or disable it?

I am not a pro, so this might not be correct advice.

Maybe your incoming pressure is too high, or has occasional high spikes, so the incoming PRV might be needed.
 
After a bit more troubleshooting I was getting 150 PSI on the cold water line with the water heat isolated. Replacing the PRV on the cold water supply line straightened things out.
 
After a bit more troubleshooting I was getting 150 PSI on the cold water line with the water heat isolated. Replacing the PRV on the cold water supply line straightened things out.
Doesn't sound like it's straightened out, if you're still getting 150 psi on the cold water line. Or did the new PRV(pressure reducing Valve) bring the pressure down to 80 psi or less?

Make sure your expansion tank is precharged to closely match the typical cold water service pressure, not just as it came from the factory.

It also sounds like a 2 gallon expansion tank may very well be too small for an 80 gallon water heater.

If you don't know how to properly check set the tank pressure to closely match your water pressure, google it and ask if you have any questions. Properly setting the correct exp tank air pressure will allow you to obtain the maximum amount of expansion capabilities.

Here's a Watts guide but the bottom line is typically the same.
Exp Tank 1.jpg Exp Tank 2.jpg
 
BTW...when talking about the pressure relief valve mounted on a water heater, it is a Temperature & Pressure Relief Valve and typically referred to as a T&P Relief valve. Not a PRV. Same with a separate HW pipe mounted relief valve. Although it's possible someone installed a Pressure Relief Valve there to avoid the expansion tank when a water service PRV(Pressure Reducing Valve) was installed.
 
I agree with Diehard, I don't the expansion tank is set up correctly. I would think it should be pre-loaded to 2 psi less than the minimum pressure in the system, same as with a well pump.
 

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