that sounds like the most likely cause. An expansion tank would cure that
problem, IF in fact that's what's happening. ANOTHER cheap way of dealing
iwth the problem would be to flush a toilet AFTER turning the water off for
the night. THEN when the WH causes expansion, it could just flow into the
now emtpy toilet tank.

s

"Bob" wrote in message
...


do you have pressure tank on hot water heater? closing off the main cut
off may give you less room for heat expansion of the water, so it blows
out the relief valve.