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Community and Q&A

Underslab leak?

jackofalltrades777 | Posted in General Questions on

Is it possible to develop an underslab plumbing water line leak months after all lines checked out OK? On a slab on grade design. PEX lines were used.

How hard are they to find? Is special equipment needed?

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  1. GBA Editor
    Martin Holladay | | #1

    Q. "Is it possible to develop an underslab plumbing water line leak months after all lines checked out OK?"

    A. Yes.

    Q. "How hard are they to find? Is special equipment needed?"

    A. Good questions. Here are two more questions for you:

    Do you have your own drilled well, or is this municipal water?

    What symptoms led you to believe that you have a leak?

  2. jackofalltrades777 | | #2

    Yes, I have my own well. I am able to shutdown the water line from the well pump to the well air tank inside the garage via a shut-off valve. That line is holding pressure.

    I can't find anything on the interior and everything looks dry. Both toilets are solid and no running toilets or faucets.

    With NO water being used in the home. The pressure starts to drop about 1 psi per 15 minutes.

  3. JC72 | | #3

    This may give you an idea of what's involved in locating the leak.

  4. user-2310254 | | #4

    Could your plumbing polybutylene instead of PEX? Polybutylene is typically a gray color pipe.

  5. Stockwell | | #5

    Air tank is a possibility. You could have a leaking bladder there. I have had that before. Also had well piping that the morons used galvanized connectors on that corroded and started leaking, but you have shut the valve from well to air tank, so that won't be a cause for your loss of pressure.

  6. jackofalltrades777 | | #6

    It's definitely PEX.

    Could it be the water heater as it is cooling off? Since hot water expands, as the water tank cools it begins to draw in cold water.

  7. GBA Editor
    Martin Holladay | | #7

    If your water heater is overheating, you may be losing water through the pressure & temperature relief valve (which, if it is plumbed to an invisible location with a drain, you may not have noticed).

  8. Randy_Williams | | #8

    I've seen bad check valve, I believe they're called a "foot valve" that causes the water to flow back into the well. You would have to have a well driller check the system. Way easier to fix than a broken line under the slab.

  9. jackofalltrades777 | | #9

    I tested it for 2 days in a row and it held pressure overnight (10 hours). Strange. Really strange.

    Regarding well check valves. The drop pipe has 4 check valves (every 100 feet or so) so even if 1 valve failed, there would be 3 backup valves to prevent water from draining down.

    I will keep on eye on it but if there was a slab leak, it would NOT be intermittent. Either it would leak or not, especially when under constant 60 psi of pressure. Right?

  10. jackofalltrades777 | | #10

    So after 5 days of testing, my lines are holding pressure even after 12 hours. The ONLY drop is in the hot water tank of about 2 psi but that is after I turn off the hot water heater. The cold lines are holding pressure.

    So my hypothesis is that when the hot water tank cools down, the water table drops inside the tank and this causes the 2 psi of water pressure drop.

    If it was an under slab leak, it couldn't be "intermittent" and the pressure drop would be higher than just 2 psi.

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