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Thread: How to detect leak under a slab with underground pex

  1. #1
    DIY Junior Member madonnab's Avatar
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    Default How to detect leak under a slab with underground pex

    Hi,
    I am new here and was hoping to get some advice. I am building a home and right now it is in the stage of waiting for sheet rock so all the plumbing is exposed. We have encountered a leak somewhere and it has to be under the slab since the top out is not leaking. We have checked the supply line to the house and no leaks. Our plumber will be coming later this week but he already stated that it must be where the water enters the house. He doesn't seem to know too much about checking for leaks if it is under the slab.

    Basically, my question is how do you check for leaks in the slab. My husband said if we could isolate it he would just cap it off and run the line overhead. The question is what equipment do you use? I wish we had the plumbing run overhead but the plumber kept insisting that we should run it in the ground because that is the normal practice and it would not be noisy.

    By the way the house is in Texas.

    Thanks,
    madonnab

  2. #2
    Moderator & Master Plumber hj's Avatar
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    Default leak

    You need a professional leak detector company, NOT a plumber who advertises "leak detection". He will have the means to test each line, although, I also have a problem believing the PEX is damaged, unless the concrete company drove a stake through one of them.

  3. #3
    I&C Engineer (mostly WWTP) Lakee911's Avatar
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    DIY ... pressurize it with hot water and borrow a thermal camera ... look for the hot spot.

  4. #4

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    Quote Originally Posted by Lakee911 View Post
    DIY ... pressurize it with hot water and borrow a thermal camera ... look for the hot spot.
    Through a 4" concrete slab and a foot or so of soil? Isnt that an awful lot of thermal mass to get a reading through?

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    Plumber Cass's Avatar
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    If you run enough HW into the area you can feel the heat with your hands...although it is much more difficult through carpet...

  6. #6
    Moderator & Master Plumber hj's Avatar
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    Default leak

    The harder part may be getting that much hot water and connecting it to the pipe. Leak companies use co2, helium, and other means to locate leaks in pipes, even when they are NOT connected.

  7. #7
    Journeyman & Gas Fitter Doherty Plumbing's Avatar
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    Quote Originally Posted by madonnab View Post
    Hi,
    I am new here and was hoping to get some advice. I am building a home and right now it is in the stage of waiting for sheet rock so all the plumbing is exposed. We have encountered a leak somewhere and it has to be under the slab since the top out is not leaking. We have checked the supply line to the house and no leaks. Our plumber will be coming later this week but he already stated that it must be where the water enters the house. He doesn't seem to know too much about checking for leaks if it is under the slab.

    Basically, my question is how do you check for leaks in the slab. My husband said if we could isolate it he would just cap it off and run the line overhead. The question is what equipment do you use? I wish we had the plumbing run overhead but the plumber kept insisting that we should run it in the ground because that is the normal practice and it would not be noisy.

    By the way the house is in Texas.

    Thanks,
    madonnab
    If it's just pex and a fairly simple job then just do what you suggested yourself and run a new line. If you have a bunch of lines that it could be then I'd hire a professional company as well.

  8. #8
    DIY Junior Member madonnab's Avatar
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    Default Thanks for advice

    Talked with some leak detector companies and they said to just cap it off and put a pressure gauge to see if it loses any air pressure. The plumber is testing to see if it the cold or hot water lines.

    I was wondering if the meter is possibly reading wrong. The leak detector dial spins forwards and backwards in spurts probably due to pressure changes in the main line. Could this be causing the meter to read water usage, 3/4 gallon an hour?

    Thanks for any help,
    Madonnab

  9. #9
    Homeowner Thatguy's Avatar
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    Quote Originally Posted by madonnab View Post
    We have encountered a leak somewhere and it has to be under the slab since the top out is not leaking.
    Basically, my question is how do you check for leaks in the slab.
    We put a valve on one pipe at a time and shut it off. Once we found our 3 minutes per gallon leak we capped it off and ran the pipe elsewhere.

    Running the pipe in the soil may eventually cause pinhole leaks.

    You also might listen with your ear pressed against the candidate pipe. If the leak causes turbulence this might be reasonably loud. I heard our leak in an upstairs bathroom faucet from across the room. Metal pipes transmit sound pretty well.
    Last edited by Thatguy; 10-19-2009 at 06:44 PM.

  10. #10
    Moderator & Master Plumber hj's Avatar
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    Default meter

    If the meter is indicating usage, then you have a leak. Oscillations would just cancel themselves out which means one time the reading could be higher, but the next time it would be less, or vice versa.

  11. #11
    DIY Junior Member madonnab's Avatar
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    Default Need some help

    okay, my plumber has put air pressure gauge and he can not find a loss of air pressure. We are stumped. He did both cold and hot supply lines and still no loss in air pressure. We have changed the meter to make sure it wasn't a faulty meter. We are still losing about 8 gallons in a 24 hour period now. The plumber said if there is no loss in air there is no leak but how can you account for the meter ?
    Any suggestions? Is it possible to have a leak even if air pressure doesn't show any loss?

    Thanks for any help,
    Madonnab

  12. #12
    Homeowner Thatguy's Avatar
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    Quote Originally Posted by madonnab View Post
    okay, my plumber has put air pressure gauge and he can not find a loss of air pressure. We are stumped.
    We are still losing about 8 gallons in a 24 hour period now.
    This doesn't make sense to me, which means I am assuming something that isn't true. Figuring out what that faulty assumption is, is real work.

    "When one has eliminated the impossible, whatever remains, however unlikely, must be the solution."

  13. #13
    DIY Junior Member madonnab's Avatar
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    Sorry if it sounds confusing.
    What I meant to say was that the air pressure showed no loss in pressure. So we thought a possible faulty meter. After changing out the water meter and turning the water on the meter still shows water usage about 8 gallons in a 24 hour period.

    Now my plumber is currently testing again with air pressure but he states that if there is no loss in air pressure there is no leak. So how do you account for the water usage from the meter? The water is running out somewhere?

    Anythoughts?
    Madonnab

  14. #14
    Homeowner Thatguy's Avatar
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    I guess you could install valves at different locations downstream of the meter and turn each one off until you have isolated the leak?

    This is a very strange problem.

    If you can find an area where the electrical conductivity of the ground is higher than in other areas, this may be where the water is going, but doing this test is pretty complicated.

  15. #15
    Moderator & Master Plumber hj's Avatar
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    Default leak

    If you are "losing" 8 gallons per day, you DO HAVE a leak, but must not be testing the section where the leak is. Air will leak from holes that water won't, but the opposite is not true. We do not have enough information to diagnose your problem remotely, but can assure you that if there is a leak, it will NEVER get any better, unless you fix it.

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