We have a hot water heating system where this winter every morning in
cold weather the pressure rises to around 30 psi and water gets dumped
on the floor from the pressure release valve. It was originally an old
gravity system with old-fashioned expansion tank - about 10 years ago
we got a new boiler and they replaced the expansion tank with one of
the smaller compression-type tanks and added a circulation pump. But
we still have big oversized pipes and radiators.
The mysterious thing is that the pressure during the day stays in the
high teens and 20's and there is no leak, even when the water temp is
fairly high - say 170 deg. The overpressure only happens in the
morning when the system has been running hard bringing up the temp
from the 55 deg we set it for the night.
The plumber first replaced the pressure release valve. That had no
effect. He then replaced the expansion tank - problem continued. Then
replaced the expansion tank with a bigger one, as he felt the small
one was undersized for the house (couldn't understand how it had
worked for 10 years without incident, though - thought maybe that
there had been some air in the radiators acting as an additional
expansion tank). Still not fixed.
I'm wondering if there could be an intermittent leak in the water
refill valve when the system has been running for a while - can't be
a continuous leak as the pressure does go down to around 15 when the
system is cold. Plumber is quite skeptical - claims never to have seen
such a thing.