Pressure builds up with Thermal Expansion Tank

I just had a plumber install a hot water recirculation system with a dedicated return line to the water heater cold inlet and a pump at the water heater. The system woks great and I now get instant hot water at the most distant sink. A backflow valve was put into the cold feed to prevent hot water going back into the cold line from the return line (some say this isn't needed). A 2 gal. thermal expansion tank and a pressure gauge were added. The water heater has a 50 gal. capacity. Now the pressure gauge reads 68 psi after it settles down after opening a faucet. Then over time a few hours with no water use it creeps up to 92. Do you think the expansion tank is working or is it the Wilkins BR4 Pressure Reducing Valve? Since I never had a pressure gauge in the system (other than using a hose bib type occasionally), I may not have known this was happening. How can I isolate the cause of this.

Thanks for any advice.

Pressure creeping up is only an expansion issue if this happens as the water heater burner or elements are ON, heating up an infusion of cold water. It sounds like this is NOT your case.
One question is this: Which side of that check valve is the expansion tank located on? Try turning off the pump and see if the problem recurs. If it does, it points to your PRV, and these do go bad. If the problem goes away with the pump off, that would point to an issue with the location of that check valve.

Good tip. I will turn off burner, use some hot water and watch the gauge. The expansion tank is between the backflow valve and the tank (i.e., upstream). It does happen with pump off or on .

There should be a 68 psi air charge in the tank when the tank has no water in it.

Turn off the water supply, open a faucet to let the pressure out, and charge the air valve in the expansion tank to 68 psi.

Close the facuet and turn on the water. Then watch your pressure gauge. It should reach a much lower pressure than 92 psi.

If you still get 92 psi, then the PRV has failed or the expansion tank is too small.

The pressure does creep up (to 80 psi so far) with the pump and water heater burner off so it looks like the pressure reducing valve (Wilkins BR4) is bad.

Here is my plan. I will leave the pump and water heater burner off all night to see if I get 92 psi in the morning. If so I will have the PRV replaced. (My wife says don't even think about trying to put a new cartridge assembly in the PRV myself, but I'm tempted.....) Once the PRV issue is resolved I will proceed with checking the pre-charge on the expansion tank and make sure it is at the same pressure as the supply.

Similar to problem I am having

My new electric 55 gallon water heater has been leaking from the TPV since I installed it, come to think of it my old one may have been too. I had a plumber install a 5 gallon thermal expansion tank, but it did not take care of it so I took it off, made sure it was empty, matched the air pressure with the static water pressure (55 psi) and reinstalled it----and it worked better for a bit, but over time (2 hours) it creeps up to 145 psi. My old PRV is outside of my house in a covered pit and was installed 14 years ago when the house was built. Would you folks recommend that i put a new PRV in the cold water line just downstream of the main water shutoff (in the basement)? The other evidence that I think may indicate that the old PRV is not working is that, 3 times I went on travel for work and was gone 5 days each time. Before I left each time, I turned the water heater off at the circuit breaker, opened a faucet to get the water down to 55 psi--even took a shower one time to make sure to use up some already-heated water; then closed the faucet and when I came back from travel the pressure was back up to 145 psi. Any thoughts on anything I might be overlooking? Thanks, Jim