Thread: Main water shutoff mystery

We shut off the main water shutoff whenever we leave our vacation home. This time, when we came back, my wife happened to put on the kitchen sink (cold side) BEFORE I turned on the main and found that it flowed CONSTANTLY. Puzzled, I checked the main shutoff and it WAS off. I couldn't understand what was going on. Then I noticed that the water coming out was warm! So I experimented and found that if I opened the main, cold water would come out and if I closed the main, warm would come out. (We leave the hot water tank ON while we're gone). I thought that I always found that once I turn the main OFF only a little cold water would come out of the faucets and would slow down to a dribble pretty quickly. I CAN'T understand why ANY water is coming out of the kitchen faucet when the main is off, never mind WARM water - on the COLD side!
Any ideas!
Happy New Year to all on this helpful board!
-Sonny

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should always turn off water heater when shutting off water supply.

Originally Posted by Mike Clarke

should always turn off water heater when shutting off water supply.

I know....and I understand the reasons why. But, it's a vacation condo in NH where it gets extremely cold in winter. We are required to shut off Main water when we leave. I do shut off water heater other than in winter. I'm just wondering WHY it's acting as I described. I don't believe it used to do that. Trying to figure out what has changed....and is it an indication that something is problematic?
Thanks Mike for your input!

If it is plumbed properly, the service shut off valve should do just that - turn the water service OFF ... which means nothing gets past it (unless the main service shut off valve is not working and it is not shutting the water off).

If you have pressure to the water heater ... which is the only way you would get warm water out of any faucet ... with the main water service shut off closed - then the main water service shut off is not the main water service shut off ... the supply to the water heater would be plumbed in upstream of that valve, making it not the main water service shut off.

I suspect that is your case - the water heater is plumbed into the water service upstream of that valve and that valve is not really the main water service shut off valve (maybe it should be, but it isn't).

Originally Posted by Sonny Daye

We shut off the main water shutoff whenever we leave our vacation home. This time, when we came back, my wife happened to put on the kitchen sink (cold side) BEFORE I turned on the main and found that it flowed CONSTANTLY. Puzzled, I checked the main shutoff and it WAS off. I couldn't understand what was going on. Then I noticed that the water coming out was warm! So I experimented and found that if I opened the main, cold water would come out and if I closed the main, warm would come out. (We leave the hot water tank ON while we're gone). I thought that I always found that once I turn the main OFF only a little cold water would come out of the faucets and would slow down to a dribble pretty quickly. I CAN'T understand why ANY water is coming out of the kitchen faucet when the main is off, never mind WARM water - on the COLD side!
Any ideas!
Happy New Year to all on this helpful board!
-Sonny

Sounds like it could be pressure caused by thermal expansion, especially if you have an expansion tank.
A small expansion will easily hold 2 gallons of water, some can hold 5.

Originally Posted by Jerry Peck

If it is plumbed properly, the service shut off valve should do just that - turn the water service OFF ... which means nothing gets past it (unless the main service shut off valve is not working and it is not shutting the water off).

If you have pressure to the water heater ... which is the only way you would get warm water out of any faucet ... with the main water service shut off closed - then the main water service shut off is not the main water service shut off ... the supply to the water heater would be plumbed in upstream of that valve, making it not the main water service shut off.

I suspect that is your case - the water heater is plumbed into the water service upstream of that valve and that valve is not really the main water service shut off valve (maybe it should be, but it isn't).

Ahhhh! THAT explains perfectly what must be happening! Obviously, I'm sorry to learn that it is plumbed upstream of the main shutoff. But, if there's any good news out of this, the fact that the hot water tank has constant water available, even with the main valve shut off, means that if the hot water tank develops a leak.....at least the heating elements won't get burnt out from lack of water.....just the house would get flooded! 😢 Oh well!
Thanks so much Jerry for the diagnosis!
Sonny

Strange things can happen when pressure is removed from the system. You could be siphoning warm water through the single handle control valve(s) due to gravity, hot water buoyancy, or a defective expansion tank.
Where is your water heater located relative to the house; attic, basement, Multi-story or single level?