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My problem started a few months ago. I started to notice my cold water pressure was spiking to over 140 and broke one of my Whole home water filter tanks.

I had the pressure-reducing valve replaced thinking that was the problem. Then I continued to notice it happening, so I tried shutting off the supply to the hot water heater at night and would check the pressure in the morning and it was fine. I did this for several nights and every night it was fine, so obviously the pressure increase is migrating back out of the hot water heater into the cold water supplying it.

Why does this happen and how the heck do I fix it?

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  • Is this on a well or city water? With most city water systems the pressure just feeds back into the mains and does not rise when the water heater heats the water. – Ted Mittelstaedt Jan 9 at 18:29
  • @ted mittelstaedt a pressure reducing valve itself acts as a check valve. If there is no check valve or PRV in the system the water being pushed backwards through the meter snd since water meters only measure movement volume you get charged both directions. Many city water systems can have pressures well above 80 psi depending on location and above 80 most homes do have PRV or regulators for potable water. – Ed Beal Jan 11 at 0:57
  • @Michael-where is the new reducing valve located in your plumbing system? And can I assume your water heater has a Temperature + Pressure Relief Valve? – ojait Jan 11 at 2:54
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This is a problem on systems that have check valves or pressure reducing valves (PRV).

What is happening? The system fills up to line pressure or what the PRV is set to. With no leaks and the water heater at its lowest temp the pressure is say 60 psi and now the water heater starts heating the water starts expanding but has no place to go so the pressure builds until the water heater turns off and cools a little and the cycle repeats.

How can this be prevented ? Addition of a small pressure tank literarily only about 3 gallons on the cold water inlet will completely eliminate the problem. The pressure tank has a air space that the water expands into and the pressure only slightly increases a few PSI and things won’t be breaking.

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  • Alternately, get a professional plumber to come out and inspect your hot water heater. They're quite dangerous machines, and the last thing you want is for a botched DIY project to cause it to literally explode and destroy your house. – nick012000 Jan 10 at 10:41
  • @nick012000 this is a common problem and there is no need for the heater to be investigated with the information provided. – Ed Beal Jan 11 at 0:42
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The best way to check the water pressure is with a pressure gauge which I believe you have. I would start by using the pressure reducing valve again. Get it in service with the main supply line coming off the meter.

Connect the gauge to a near by spigot and dial the reducing valve adjusting bolt "up" or counter clockwise to reduce the water pressure. You may have to rlieve pressure at the gauge and than apply it again to see it lessen. Household water pressure is considered normal at between 60-70 PSI.

Other suggestions:

-Make sure your water heater tank has a functioning Temperature and pressure relief valve

-if you believe the cold water supplying the hot water tank is back flowing it would be wise to install a check valve on the cold supply pipe. This would allow the cold water to enter and prevent it from back flowing.

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  • Actually checking the pressure after the PRV with a valve flowing water and then at static will show if the valve is properly sized. Measuring at the spigot will show a very small pressure. – Ed Beal Jan 11 at 0:47

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