Plumbing: do I need to replace my PRV?

I installed a Flo shutoff valve last year, so for the first time I have data on my (domestic) water pressure. The average of 55psi is perfect, but the pressure here seems to vary considerably, and there are transient peaks occasionally as high as 95psi (measured downstream of the PRV).

The PRV is a this Watts model, rated 25-75psi. It’s 9 years old.

I have no other issues or symptoms with my plumbing. Is this within the range of normal expectations for a PRV rated 25-75, or should I replace it?

WAG:

Water hammer somewhere?

Thermal expansion from a water heater in heating mode ?

https://inspectapedia.com/plumbing/Hot_Water_Expansion.php

ETA: another good link:

Best to eyeball the static pressure with the water heater in vacation mode/shut off.

That’s step one.

Is the PSI varying solely UP or does it vary down, too ?

IOW: if you watch your pressure, and then flush two toilets simultaneously, and run a tap while the two toilets are refilling, how much does your PSI drop ?

[providing the pressure from your actual water supplier isn’t fluctuating, explaining the variability you’re seeing. Sometimes worth pinging the water company before driving yourself nuts <grin>]

I should amend that: start this test when your pressure is about normal – your 55psi average.

Obviously I’m not very experienced with plumbing - I had assumed it was just variation in delivery pressure from the mains. I’ll do some reading of your links and study the exact pattern of fluctuation, then I’ll do some experiments and report back. Thanks so much!

How transient are the pressure spikes? Momentary spikes or spikes that last many minutes or longer?

It seems to fluctuate up and down on a timescale of an hour or two. Here are graphs of the last 7 days (divisions are 1 day) and the last 24 hours (divisions are 1h).

Plumbing - Google Drive

Unfortunately I can’t find any info on how frequently the pressure is saved, and somebody obviously decided it would look more impressive if they put a smoothing function in the graph. But it’s clear that these are not just quick spikes.

Pressure this week doesn’t look like it has gone about about 70psi, but I have seen it get as high as 95psi.

You nailed it. I watched it carefully this morning, and saw the pressure rise from about 50psi to 75psi over 10 minutes or so. Sure enough, it was when the boiler was running to heat the water tank. I waited 5 minutes, turned on a faucet to release the pressure, and it came back at 50psi, did not rise again - which it surely would have done if it was a result of fluctuating mains pressure.

I have a thermal expansion tank - so I guess this is what I need to look at replacing.

This looks like a reasonable list of expansion tank issues to check for:

SOURCE

Avoid Common Problems – Expansion Tank Maintenance

Unfortunately, thermal expansion tanks don’t last forever. Most tanks eventually become waterlogged, making them unable to perform their function. Professional plumbers recommend replacing them every five to ten years. Here are a few ways to look after your expansion tank:

  • Watch out for condensation. This can be caused by a waterlogged expansion tank or could just be the result of a very humid climate. Either way, it could pose a danger if your water heater is situated near any electrical devices.
  • Do the “tap test.” Simply tap or knock on your expansion tank and listen to the sound it makes. If it makes a hollow, ringing sound you know there’s still air inside it. If it makes more of a dull sound, it means your tank is full of water and needs to be replaced.
  • Feel the tank. Feel the top and the bottom of your expansion tank. The top of the tank, which should be filled with compressed air, should be cool to the touch. The bottom of the tank, where the water goes, should be warm. If the top of the tank feels the same as the bottom, you know you’ve got a problem.
  • Pay attention to your water heater’s pressure release valve. Remember the pressure release valve? It starts releasing water if there’s too much pressure for the expansion tank to handle. If you notice the release valve is constantly dripping, it could mean your expansion tank isn’t working anymore.
  • Upgrade your tank before it fails. You don’t have to wait until you notice a problem to invest in a new expansion tank. A good rule of thumb is to replace your tank whenever you replace or service your water heater.
  • Replace a faulty tank as soon as possible. If your expansion tank isn’t doing its job, your home’s at risk for water damage or, even worse, a water heater explosion. As soon as you notice that something’s not right, get a hold of a plumber and start taking steps to get a new expansion tank installed.

[bolding mine]

If your expansion tank is the same age as your PRV, I certainly wouldn’t rule out that it needs replacing, but no sense starting there.

Thanks. The expansion tank I have (Zurn XT-8) only costs $60, and it’s 9 years old. If it were $300 or something, it would motivate me to troubleshoot, but at that price…