I've been chasing the cause of this mysterious high water pressure for several days, and I'm now finally able to reproduce it consistently, and say it's caused by flushing the hallway toilet.

Note this is a different issue than a simlar report that I wrote Water pressure spikes when flushing Master Bedroom toilet

I have my PRV set to 60psi, and I can see it holding steady over a long period of time (proof that it's working as expected)

When the hallway toilet is flushed, I can see the pressure drop as expected for a flush and the subsequent tank filling, but then after the fill valve shuts off, the pressure bounces back to about 63psi (not sure if this is a useful datapoint or not, but I mention it because it's a little higher than the 60psi that I have my PRV set at), then over the course of about 10-20 mins, I see the pressure slowly rise to about 80psi. The pressure will hold steady there until something else releases the pressure like a faucet or shower or something.

I've reproduced this several times now. I made sure the water heater is not in a heating cycle.

Turns out this can be repro'd by both the Hallway and downstairs toilets (not the Master that I wrote the earlier post about)

Here is a picture of the inside of the hallway toilet with Toto Aquia II Fill and Flush valve internals:

Flush Valve

Fill Valve enter image description here

The Downstairs toilet has different internals that I'll have to get a pic of later when I have access.

Some tests that I've done:

  1. Let the cold water run at the same location then shut it off - I ran the cold water in the tub that sits right next to the toilet for about 1 min, then shut it off. Pressure decreased while the tub faucet was open, then went back and stayed as expected at 60psi where my PRV is set.
  2. Shut off valve for the toilet and flush - I shut off the toilet valve and flushed. I saw no change to the water pressure while the toilet was flushing or filling with the valve shut off. When I open the the toilet valve, I see pressure decrease while the tank is filling, then when the fill valve shuts off, I see the pressure bounces back to about 63psi, and then start to slowly rise it's way up to the 80psi as described earlier.

Does anyone know what could be causing this?

  • Comments have been moved to chat; please do not continue the discussion here. Before posting a comment below this one, please review the purposes of comments. Comments that do not request clarification or suggest improvements usually belong as an answer, on Home Improvement Meta, or in Home Improvement Chat. Comments continuing discussion may be removed.
    – Michael Karas
    Commented Mar 15, 2023 at 2:17
  • While I agree that it's wholly unsatisfying, the answer that you posted then deleted is useful information for anyone who may encounter an issue like this in the future. I would request that you undelete your answer for others to see.
    – FreeMan
    Commented Mar 21, 2023 at 14:24
  • 1
    @FreeMan I deleted it because immediately after posting the answer, I saw the pressure go up to abt 100psi. I had been watching closely for about 2 days, and the pressure had remained steady, then right after I posted the answer, I saw the pressure rise to about 100psi, so I deleted the answer. But after more inspection, I found that it is the washing machine that for some reason adds pressure into the system. So at least for this particular issue, it is resolved, and I just undeleted.
    – SDSHuge2.0
    Commented Mar 22, 2023 at 9:22

1 Answer 1


I can't fully explain this, I can only describe the steps that I took, and confirm that I no longer see the issue.

Based on another user's suspicion that it was a bad PRV. I wanted to prove him wrong, so I bought and installed a brand new PRV, and I no longer saw the issue ocurring.

Just to double check, I went back and re-installed the previous PRV, the one in question, and now I ALSO do not see the issue ocurring.

Toilets flush, and pressure goes back to 60psi. Every time, multiple times, all toilets, faucets, fixtures, appliances...

I'm perplexed.

The only thing I can think is that I installed the expansion tank after I installed this PRV, and I tried to 'bleed' the system of any air while filling the water heater back up... maybe I shouldn't have done that?

Regardless, I'm no longer seeing the pressure rise after flush.

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