Mysterious Water Leak

I would suspect faulty water heater pressure relief valve. I've had that problem a couple of times.

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.

Last night I did a load of wash in the basement I'm going to do annother load today to see if the leak could somehow be caused by the washing machine.

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.

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.

This is a basement. Any chance it came through a drain, the floor, the wall? Lots of rain lately?

How can I check the water heater pressure relief valve?

Whirlpool water heater

You guys are great.

Will do. As soon as I get the rest of the pipe cleared out in the ceiling the plumber is going to fix it.

Yes, lots of rain around here but none where I live last night. 70 y/o house with no drain in floor. I'll look for other drains.

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Or, w/ the water supply off, somebody ran some hot water and since the heater had no resupply it overheated and blew off some steam during the night.

I'd fix the upstairs drip....which was I'm betting, albeit indirectly, the cause. :)

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Bingo. Last night I BELIEVE I left the hot water tap open in the kitchen befor going to bed. About nineti minutes before going to bed I did a load of wash which, I assume, filled the water heater with cool water. I'm going to try to get the plumber here tomorrow to fix the leak in the cold water pipe going to the upstairs bath tub.

It was about eight - ten gallons of water total. I did a load of wash about ninety miniatures before going to bed. I hope to get a plumber here tomorrow or Monday.

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.

Where is the pressure relief valbe? Is it the valve on the side near the top of the heater cylinder with a long copper oipe going to the bottom of the heater?

Should I also leave a cold watewr tap open?

The toilet tank will fill with cold water. That will help with the WH tank expansion?

Hi, Yes. You can't test it. It either works or leaks. Replace it if in doubt. It's called P & T relief valve.(Pressure and temperature)