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Thread: Leaking main line inside house

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  1. #1
    DIY Junior Member Jack Lynch's Avatar
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    Default Leaking main line inside house

    So the water comes in my house here.

    The red knob shuts the water off to everything in the house. Before the knob you can see the connection with the small pipe

    Then you can see the small pipe that goes around the back of the room in the corner

    Then that pipe goes into the condensation line for the AC system

    The condensation line exits out the back of my house.
    Now my question is the little pipe that runs into the condensation line has water running out of it and that water is coming out of the back of my house. When I turn the main off to the house it stops the water from leaking. What is this pipe for? How can I stop the water from coming out of it?

  2. #2
    DIY Junior Member Jack Lynch's Avatar
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    This is the picture of the condensation line that comes out the back of the house.

  3. #3
    Moderator & Master Plumber hj's Avatar
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    It is a pressure relief valve. Either the pressure in the house is higher than its setting, or it is defective and leaking at a lower pressure than it should.
    Licensed residential and commercial plumber

  4. #4
    DIY Junior Member Jack Lynch's Avatar
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    Is there anyway to test it to see if it the due to the valve or to much pressure?

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    In the Trades Gary Swart's Avatar
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    You need a pressure gauge. This sounds like you have a closed system which causes the T/P valve on the water heater to open to relieve the excess pressure caused by the water heating and no place for the expansion to go. Just a quick basic explanation may help you understand. A closed system exists when there is a check valve in the water supply. It may be in a pressure regulator valve or it could be in the water meter. When the water heats in the tank, it expands quite rapidly. The expanded water can not be absorbed by the water main, so the pressure rises in the tank and trips the T/P. This prevents the water heater from exploding. To deal with this, a thermal expansion tank is added between the pressure regulator and water heater to provide a temporary home for the expansion. An inexpensive pressure gauge attached to a hot water faucet will demonstrate this quickly and graphically. The T/P valve is working exactly like it is supposed to do. There is a video floating around somewhere that shows what happens when the T/P is wired closed. The resulting explosion looks like a bomb...it's no joke.

  6. #6
    Retired Defense Industry Engineer jadnashua's Avatar
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    A pressure relief valve like that could be used as a poor-man's pressure reduction valve, but a PRV doesn't dump water to reduce the pressue, it changes it mechanically via internal valve mechanism.

    Wheter you have a closed or an open system is somewhat irrelevant - the valve is either doing what it was designed to do - open if the pressure exceeds its setting, or it has failed, or your static pressure is now higher (maybe the city put in a new water tower or pump station).

    As said, the easiest thing to do to check things is to pick up a $10 or so pressure gauge. Find one that has a second hand that acts like a peak reading indicator (a tattle tale). You can find them with a hose connection, and screw it onto say the WH or a hose bib, or say the washing machine supply. Check the instantaeous pressure, and leave it connected for say 24-hours to catch any peak readings. Anything steady over 80psi should have a PRV added. Peaks above that not caused by a closed system would also require one. Peaks above that caused by expansion means if you do have one, your expansion tank is shot, and you need a new one, which is also required on a closed system (a prv makes it a closed system as would a check valve that the city may have on your meter or supply line).

    The pressure relief valve may still have a tag on it indicating the pressure it will open, or it may also be adjustable.

    If you have a closed system, (essentially, a check valve so water cannot be pushed back into the municipal supply), then you also need an expansion tank to account for the fact that heating water causes it to expand. Pipes aren't very elastic and water doesn't really compress, so a little expansion will cause the pressure to rise quite a bit. A water heater will have a T&P valve that typically would open in this situation, but if the pressure relief valve opens first, then the T&P may not reach the point where it will open (typically around 150psi).
    Jim DeBruycker
    Important note - I'm not a pro
    Retired Defense Industry Engineer; Schluter 2.5-day Workshop Completed 2013, 2014

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