David wrote:
Problem: House pressure rises during the night from 40 PSI to 70 PSI.
Is incoming pressure regulator failing?



Based on the OP's thorough description I'd say it HAS to be an imperfect
presure reducing valve.

My mind started running off in the direction of the pressure reducing
valve being "perfect" but the incoming water was real cold, and the
house warmer, which could cause a pressure increase as the water warmed
up and expanded during the night. Then I went back and read that he had
a thermal expansion tank in the system, which killed that bright idea.

The pressure reducing valve must be weeping more than just a tiny bit,
because that thermal expansion tank (assuming it IS in proper shape)
would have to accept quite a bit of water before it'd "bottom out" and
permit the full line pressure.

Chances are a new valve, or a rebuild of the present one would reduce
the weepage to the point where the expansion tank could easily handle
it, certainly during an overnight idle period.

Jeff


--
My name is Jeff Wisnia and I approved this message....

(W1BSV + Brass Rat '57 EE)

"As long as there are final exams, there will be prayer in public
schools"