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Question DetailsAsked on 2/24/2017

Should hot water be coming from the cold tap after turning off main water line to home?

When I shut the water off the main water supply to my home to make a faucet repair, I turned on the cold tap to drain the line but hot water started coming thru the cold tap from my water heater and would not shut off. What would cause this. I replaced another faucet last year and never had this problem.

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2 Answers


I presume you used the main shutoff rather than under-sink shutoffs because you were replacing one of them too ?

Sounds like maybe a single-handle faucet. Shut off the pressure to one side and some of them will bleed over from the other side oncethe balancing pressure is gone - though since you said you shut off the main water supply shutoff valve, this should not be the case except for maybe a possible small amount of pressure in the hot water tank.

Otherwise, the hot water should have stopped flowing as soon as any residual pressure in the water heater dissipated. This would almost certainly mean you have a backflow preventer or pressure regulator in the line somewhere so you have an expansion tank to protect the heater - so any residual pressure in the hot water heater (including pressure in the expansion tank) would flow not only through any open hot water faucet, but also back out of the inlet pipe on the water heater to the cold side lines - until the pressure equilized, which would probably be in about 10-15 seconds of flow with a normal modern "football" expansion tank, though if you have an old water-heater sized one tucked up into the joists (sometimes up to 40 gallons in larger houses) it might push all the cold water out of the lines plus up to another 10-20 gallons worth of hot water from the tank if it was holding a significant amount of water in it. The flow should have tapered off fairly quickly (noticeably in a minute or so) - if it kept flowing indefinitely then either it was draining an attic water heater (might be able to basically drain the whole thing - which could damage it if it turned on), or the shutoff valve you used was not the "main" valve for the house - perhaps only for the cold lines or some intermediate shutoff valve.

Of course, opening hot faucets would drain the excess pressure out a lot more quickly.

One question - you said you turned on the cold tap to drain the line - I presume you mean another lower-elevation faucet - because turning on the cold tap on the one you were working on would not drain the line to it.

I would guess if you did not have this happen last year then you must have shut off the suppliy valve under the sink - not the main supply valve. Or the pressure go bled off elsewhere - maybe someone opened a downstairs faucet or flushed a toilet or such after the shutoff valve was closed.

Answered 3 years ago by LCD



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Answered 3 years ago by Member Services

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