I just installed a new water pressure regulator as documented in Water Pressure Regulator not regulating after rebuild.

I left the pressure gauge attached to a hose bib with the faucet turned on so I could monitor the pressure over the course of a cpl days. My gauge has a red, maximum pressure, hand to record the highest measured pressure during that measurement period.

So far everything is working as expected, and it's able to control the pressure to 60psi where I set it, except sometimes when I leave and come back several hours later, I see the maximum pressure hand read high over the 60psi.

Once it was up at 80psi, the other time it was at 110psi (from other tests I've done, I assume 110psi is the city water pressure level, and this is the same pressure I was reading on the old regulator that went bad before I replaced it).

From reading other articles on the site, I understand one possible cause is the water heater heat cycle expanding the water and increasing the pressure in the system, although I'm not sure about this.

I don't see an expansion tank on my water heater, and I don't think there's any pressure release valve anywhere in my house water line.

Any ideas what the issue could be, and how I can solve it?

Update Pics of my TPR.

And it looks like where it connects to the wall is leaking. Any idea how to fix that leak. Is that solder?

  • The answer is in your question. Self answering is acceptable here, so put the 5th & 6th paragraphs in an answer box along with the thing you need to add that they imply. Though if you can stand a little while without much hot water, you could turn the water heater off to verify (you'll still have the hot in the tank, until it runs out.)
    – Ecnerwal
    Commented Feb 21, 2023 at 13:30
  • There should be a pressure release valve on the hot water tank. Other thing to check is the thermostat on the hot water tank(~ temp setting around 130/140F).
    – crip659
    Commented Feb 21, 2023 at 13:37
  • @crip659 Your suggested temp setting is a little high, 120-125 would be safer. Commented Feb 21, 2023 at 17:40
  • @GeorgeAnderson There seems to be a debate about the right temperature now. 120 is right for not burning people, but might be too cool to stop nasties growing in the tank. Those new flangle anti-scald devices should keep both camps happy.
    – crip659
    Commented Feb 21, 2023 at 17:47
  • 2
    You can find solid defense for nearly any temperature between 120 and 140. Wandering in to that debate is likely not what OP wants! And @Ecnerwal is spot on, turning the tank off is the super quick diagnosis path.
    – KMJ
    Commented Feb 21, 2023 at 20:00

3 Answers 3


Sounds like water hammer causing very brief pressure spikes as water flow is shut off abruptly to something (ice maker, washer, dishwasher, etc.)

  • 2
    If it still happens with the water heater turned off, might be this sort of thing. But I'd bet on the missing expansion tank.
    – Ecnerwal
    Commented Feb 21, 2023 at 17:20
  • Hi, thanks for the suggestion. If you read the Answer I just posted, I can confirm that flushing master bedroom toilet causes pressure spike up to 110psi... suggestion? Should I start a new query for that?
    – SDSHuge2.0
    Commented Mar 6, 2023 at 21:50

Presumably your water heater already has the standard temperature/pressure release valve installed (which will typically be set at 120-150 PSI for pressure) so what you need is an expansion tank - that goes anywhere on the cold water supply between the PRV and the water heater.

Or anywhere it's convenient on the cold water supply after the PRV, really - if you don't have any oddball check valves that would not be typical, the entire cold supply is connected, so the expansion tank will work anywhere on it.

  • Thank you so much! That's the conclusion I just came to myself, but I really appreciate the confirmation also. I just checked the pressure guage again just now, and the red hand showed it had been up to 100psi at some point. What I don't understand is if all residential water systems will suffer from high water pressure over where the regulator valve is set, then why isn't an expansion tank a standard installation? Doesn't that mean people are damaging their waterlines and appliances without one?
    – SDSHuge2.0
    Commented Mar 4, 2023 at 13:42
  • It's not all systems. Not everyone has a PRV or check-valve, so in many cases excess pressure went right back out the water meter. I have a well system, which has a pressure tank which is basically a much larger expansion tank, so I don't need one. They have been added to code requirements recently due to the proliferation of PRVs and water meters with check valves built in, but systems that predate those updates to code won't commonly have them. You are unlikely to "damage your pipes" below the point where the water heater's TPR valve will let go. Toilet fill valves tend to be the weak point.
    – Ecnerwal
    Commented Mar 4, 2023 at 14:00
  • I see. That makes sense. Thanks very much for the explanation.
    – SDSHuge2.0
    Commented Mar 4, 2023 at 14:02
  • If you want to edit in pictures of your water heater we can point you at the TPR valve - it's highly unlikely not to have one. Usually either on top, or on the side near the top.
    – Ecnerwal
    Commented Mar 4, 2023 at 14:11
  • Thank you! I found the TPR valve. It was stuck closed. I had to use a little force to open it the first time, but it opens smoothly after that. But I understand that the TPR releases at about 150psi, and I though it's reccommended to set the PRV mo higher than 80psi to protect appliances and pipes, no?
    – SDSHuge2.0
    Commented Mar 4, 2023 at 14:16

Thanks for everyone's suggestions and guidance!

I forced my water heater into heat mode by increasing the temp to the max setting, and I confirmed that in fact the water heater was in fact causing the water pressure to rise

I watched this YouTube video as the blueprint for my installation: Expansion Tank Installation on a water heater - What is an expansion tank? - How to install it

Before/After: enter image description here enter image description here

After installation, I no longer observe the temperature rising due to water heater heating.

One thing that I did when I was refilling the water heater w/water after installing the expansion tank was to leave the TPR open to let air escape. My logic was that I didn't want trapped air to put pressure on the expansion tank diaphragm and reducing the efficacy for the actual purpose of absorbing the pressure of the expanding water. Not sure this was necessary, though. But I did it.

I also detected that flushing master bedroom toilet spikes the water pressure up to 110psi.

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