Originally Posted by RegeSullivan
It is most likely the water heater is building pressure when heating water after you shower. Many communities require backflow preventers causing the pressure to build in your plumbing system. Some regulators even have them built in so you may have one and not know it. To compensate install an expansion tank anywhere it is convent in the cold water side of the system. Just make sure any valve between the water heater and the expansion tank is normally open.
hmmmm, that might explain why yesterday I would run the shower and not find pressure built up immediately after turning water off but then a bit later pressure had built.
I will investigate more - thx!
edited to add:
pressure at hot water tank after shower was 150.
Flushed a toilet and it dropped to 50.
will look into installing expansion tank.