I've been seeing some strange water pressure in my house, nothing has changed recently. I had always seen the pressure coming out hard at first, after the water heater had been running, but the pressure seems higher lately. I found a leak behind the fridge that I think was caused by the high pressure, ruined part of the floor in my kitchen.

City water comes in at about 90 psi, there is a pressure reducing valve that reduces it to about 40. Hot water temperature was about 130F

I just replaced the expansion tank with another 2.1 gallon expansion tank, it was precharged to 40 psi, I double checked it and left it there. Doesn't seem to be anything wrong with the old tank, it was only at 10psi once removed, but it still holds air.

Now, with the new tank on, I still see bouts of high pressure. I clocked it at 100psi this afternoon. 100 seems very high to me, I've never noticed it that high in the past.

Do I need a bigger tank? Just odd that this started out of the blue..

  • If the water pressure is exceeding 40 (your preset) it may just be a bad prv. perhaps if the hot water tank has calcium buildup in the lines, the expansion tank may be doing nothing for you at this moment(if clogged).
    – noybman
    Nov 19, 2018 at 4:36
  • When the pressure is high, if the expansion tank is functioning, I should see that same pressure on the tank, correct? I doubt the line to the tank is clogged, but that is something to look into, thanks.
    – Gary Bak
    Nov 19, 2018 at 12:36
  • not sure what pressure is the "same pressure" you believe you will see on the tank? If the tank is functioning, && the pressure is higher than 40, you should see a # greater than 40 on the tank.
    – noybman
    Nov 20, 2018 at 12:28
  • btw - what is your hot water set point at? How many gallons? With that we can calculate approx how much expansion could be expected. I wouldnt think 100. Still look at the prv. or some other thing connected INTO the water line. Do you have a cold water fed boiler?
    – noybman
    Nov 20, 2018 at 12:31
  • No boiler, 50 gal hot water tank only. As for the set point, I'm not sure. I know it's above the recommended point, but I have not measured the water temp right when I hear the heater cut off.
    – Gary Bak
    Nov 20, 2018 at 12:37

1 Answer 1


Either your PRV is bad, your gauge is incorrect or being used incorrectly, or the end points in question are old and just in need of repair as these things happen (and if all installed at the same time it is not unheard of that they fail around the same time).

It does not make sense that your water heater is a culprit nor the expansion tank.

  • I agree the PRV is a simple pressure regulator and the diaphrams go bad then they don't work well, with a flow they may be able to regulate but at static the pressure usually matches the incoming line.+
    – Ed Beal
    Nov 23, 2018 at 3:27

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