Not sure what is going on - need an advice:

40 year old house with incoming water pressure regulator.
Pressure with water running about 50 PSI, with faucets closed 60 PSI.

When I had no expansion tank with all faucets closed pressure was slowly going to about 100 PSI while water heater was heating water.

So today I installed expansion tank, set tank air pressure at 50 PSI before installation.
Now I see that pressure going to 120 PSI+ within an hour.
I have WATTS gage permanently installed right next to the tank so I can monitor pressue.

I know expansion tank is working because if I open faucet I see no water meter movements for 10-15 seconds until it empties the tank.

So question - what I did wrong?
Tank too small?
I can get a large one tomorrow, but want to get an advice first.
Thanks

Update:
- Overnite pressure went up to 140PSA even with hater heater set at "vacation", so something else is going on.
Can this be a faulty pressure regulator?

Update:
- Overnite pressure went up to 140PSA even with hater heater set at "vacation", so something else is going on.
Can this be a faulty pressure regulator?
That would be my guess. I think the expansion tank was just coincidence and that the PRV valve went bad about the same time you installed the tank. Replace the PRV valve and I bet your problem will go away. Regards, Tom

That would be my guess. I think the expansion tank was just coincidence and that the PRV valve went bad about the same time you installed the tank. Replace the PRV valve and I bet your problem will go away. Regards, Tom

Did you mean pressure regulator or Pressure Relieve Valve on the water heater?
I did replace Waterheater pressure relieve valve recently because it was not closing and making sipping noises. I guess it was holding pressure lower than 140 PSI I have now.

I did another test and now I am 100% convinced that incoming pressure regulator is bad: I turned water heater off and closed cold water valve at water heater- cold line pressure still going up - only one source - incoming street water, I guess regulator is leaking.

I'm sorry if I confused you with my answer. I meant the pressure regulator valve. I call the water heater valve " the T & P Valve" (temperature and Pressure valve) so the two don't get mixed up. Replace the pressure regulator valve and set the house pressurs between 45 and 50 PSI. Too much pressure putse a strain on the water pipe system and the valves and faucets.. Good luck, Tom

I replaced pressure regulator this morning and now pressure is stable, I set it at about 55 PSI and gage is not going over 60 with water heater on.

Thank you