Water pressure high after faucets are off for a while

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Old 01-28-20, 10:17 PM
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Water pressure high after faucets are off for a while

I started a thread about this in the hot water tank section because my TPR was leaking but but after checking the pressure. it might be worth mentioning in this section, I have a closed system (there is a backflow device by the water meter and a PRV next to my shutoff inside the house) and today I hooked a pressure gauge to a couple of different faucets and it read 60. I attached it to a hose bib outside and left the water on without opening any other faucets and after an hour of it being on, the gauge rose to 120. As soon I relieved pressure by opening another faucet it went down to 60. Since the gauge is reading 60 initially, does that mean the PRV is working and set at 60? I do not have an expansion tank on the hot water heater. Would that cause the pressure to rise that much? Any other thoughts?
 
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Old 01-29-20, 05:22 AM
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Not sure of your sequence of PRV, shutoff valve and backflow valve, but I assume PRV is closest to the hot water tank. Assuming the PRV is designed/adjusted for less than 120 psi, my opinion is the PRV is malfunctioning and needs replacement. The 60 psi is from the water supply and the added pressure is from the heated water in the hot water tank, Most TPR valves on a hot water tank operate at 150 psi and/or 210 degrees F. The tank pressure can be lowered by lowering the water temp in the tank.
 
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Old 01-29-20, 05:41 AM
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It sounds like your pressure reducing valve needs to be replaced.
 
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Old 01-29-20, 07:07 AM
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Did someone just take a shower or was the dishwasher or other hot water consuming appliance used just before you conducted the test?

You need an expansion tank.

Every new tankful of water in the water heater will expand when heated, increasing the system pressure unless you have an expansion tank or you let a little water out of a faucet every few minutes.

I take it that after you let some water out and the 120 PSI dropped back to 60, it stayed at 60 for several hours or at least until the water heater kicked on again..
 
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Old 01-29-20, 08:35 AM
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I can't say for sure but the dishwasher and washing machine were run yesterday and I am not sure if they were done right before I ran the test or not. I will say that the TPR valve on the tank only leaks after hot water has been used for a while. We have gone away for the weekend and the bucket under the TPR is empty so I know it hasn't leaked. I was thinking though, and this might help narrow things down:

There is another shutoff on the line that goes into the water tank right before it. If I shut that off, that will isolate the hot water in the system, and I can conduct the test again on the hose bib. In that scenario, only cold water pressure would be tested and there will be no thermal expansion in play. If the pressure rises when I do that, the only explanation would be the PRV but if it doesn't rise, it would have to be the expansion tank. Am I figuring that out right?
 
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Old 01-29-20, 10:11 AM
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Yes you figured it out right.

The expansion tank must go in the cold water line with no valve or other gizmo between it and the water heater.
 
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Old 02-03-20, 04:43 PM
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Looks like it is the PRV. Shutting off the supply to the hot water yielded the same results. Will trying to adjust it do any good or do these things just go totally bad when they do?
 

Last edited by vpn33; 02-03-20 at 05:00 PM.
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Old 02-03-20, 05:42 PM
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Replace the PRV. Then, install a thermal expansion tank.
 
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