Yes. Turning off the main water valve could cause the water heater's
relief valve to open. Here's why: water expands when heated. The extra
volume has to have somewhere to go. With your main valve open, this extra
volume goes back into the water main feeding your house. With the valve
closed, this extra volume has nowhere to go but out the pressure relief
valve on your water heater.
This also points to the water heater relief valve. Turning the main
water supply on and off as a solution to a leak is not good practice.
It should only be done temporarily to get you through a weekend until
a plumber (or yourself) repairs the problem. Sometimes a small leak is
a small leak, and sometimes it's a warning. It should not be ignored,
or "worked around". Hard to diagnose a system that has other faults
that might be contributing. You may even find that if you fix the
bathtub leak and stop shutting off the main water supply, the other
problems may be cured.
Meanwhile, put a bucket under the pressure relief and you'll know for
sure if that is where the water is coming from. I really think the
shutting off of the main water supply due to the small leak on the
bathtub is the root cause of the problem. The pressure relief might be
operating properly and doing exactly what it is supposed to do.
After readfing the posts I agree. The plumber can't come 'til Monday so I
think I'll cut the herat setting back to "vacation" setting when I leave
then reset it to the normal setting and turn the water main back on when I
return to the house or when I wake up in the morning.
Is the bathtub leak accessible? If so, and we are talking about a
pinhole leak, you might be able to temporarily seal it by wrapping it
several layers thick with tightly wound electrical tape, which is then
wrapped tightly with string (each wrap of string directly alongside
the preceeding with no gaps) to keep the tape from bursting or leaking
sideways. Obviously not a permanent repair, but you might eliminate
all the other contortions until the plumber gets there.
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