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Can This be Anything Other Than a Leak Under the Floor?

505BEN Member Posts: 9
I have a 4 zone baseboard system with 2 upstairs and 2 downstairs zones. System has been off but we got a cold snap and when the system started I heard telltale air in the pipe noises coming from my upper 2 zones. I checked the boiler and the outlet pressure gauge was reading around 6 PSI. After some trouble shooting I found that my autofill valve was only filling to 6 PSI. My pump is going towards the pressure tank so I thought maybe I had dropped system pressure enough to pull in air through the air separator. I set the autofill to 12PSI cold, then purged the system of air from the 3 way valve in front of the expansion tank. It seemed like I got all 4 zones purged but as soon as I ran the boiler I had a good amount of air in the system again. I purged again from the boiler drain with the same results.

Now suspecting a leak I filled the system to 14 psi, then shut off the ball valve to the water feed. The system drops to approx 2 psi within minutes. I checked the relief valve and it appears to be operating normally (no water discharging) and I checked the expansion tank to make sure it was not water logged. I then checked the house for wet spots and find no wet on the carpets, walls, ceilings, etc. I would think a leak that takes the system down so fast would make some wet somewhere but I cant find it. This leads me to believe it is under my slab somewhere but I thought I might ping the board and see if there is anything else worth checking before I call the leak detection folks.


  • bburd
    bburd Member Posts: 912
    If you have sub slab piping and this is an older house, that is likely where it's leaking unfortunately. You might try walking around barefoot and look for a warm spot on the floor while the zone with the sub slab piping is calling for heat and with the auto fill valve open. An infrared camera would make quicker work of this.

    Dan H told a story about a contractor who would let a cat loose in houses with leaking radiant floors, and the cat would find the warm spot!

  • HomerJSmith
    HomerJSmith Member Posts: 2,433
    edited March 2023
    An IR camera could see hot water spread on the floor and perhaps indicate a leak. Ya, you're pumping into the Ex-tank.
  • mattmia2
    mattmia2 Member Posts: 9,621
    Do you have valves to isolate parts of the system and see if it holds pressure?
  • hot_rod
    hot_rod Member Posts: 22,100
    That is a big drop in a short time period.
    The baseboard piping is under the slab?
    Bob "hot rod" Rohr
    trainer for Caleffi NA
    Living the hydronic dream
  • 505BEN
    505BEN Member Posts: 9
    Unfortunately all I have on the zones is the control valves on the return side, no isolation valve on the individual feeds.

    For the first floor of the house all the piping is under slab. I agree that the pressure drop is quick that is why i figured if the leak were above ground I would see the results. I thought there might be enough flow to use a mechanics stethoscope to determine which zone is flowing so I re-pressurized the system and listened but couldn't tell.
  • hot_rod
    hot_rod Member Posts: 22,100
    If you could isolate out the boiler, you could increase pressure in all the piping. Every component in the system except the boiler could handle 60 probably even 100psi.

    Sometimes that extra pressure helps find leaks.

    A “Leak Buster” tech showed me a trick for finding under slab leaks. In addition to the pressure from a garden hose he had a small Venturi fitting that would suck in air with the water stream, so you could better hear where the leak was.

    Outside on a snowmelt system he had a headset and a cup to slide across as there was a lot of noise around the job.
    He claims inside a building you can often pinpoint leaks without a head set, listen for the gurgling 😳

    He used this technique when he had a Leak Buster franchise in Hawaii to find leaks in the piping around swimming pools. He also found leaks in piping system on Navy ships with the same method. I used him on two different slab jobs and he put an X on the slab and was within inches when I chopped away.

    Infrared cameras work well also. Let it cool down, hit it with some hot water the next day, it is usually pretty clear where the hot puddle shows.

    Inexpensive ir attachments for I phones these days, there have been some examples show here over the years.
    Bob "hot rod" Rohr
    trainer for Caleffi NA
    Living the hydronic dream
  • FStephenMasek
    FStephenMasek Member Posts: 88
    You may wish to find an environmental consultant or home inspector who has an infrared camera, or rent one. Hope you find it soon and it is in a location which does not require demolition of ceramic tile or other such finishes.
    Author of Illustrated Practical Asbestos: For Consultants, Contractors, Property Managers & Regulators
  • 505BEN
    505BEN Member Posts: 9

    Thanks for the help all. Had a leak detection company come and infrared camera showed warm spot near one of the baseboards. We were able to isloate the piping from the boiler and at about 40 PSI I could hear hissing from a leak under the floor at the same register and leak guy was able to pinpoint to about a 12" diameter. Plumber is coming out tomorrow to cut the slab and repair.

  • EdTheHeaterMan
    EdTheHeaterMan Member Posts: 7,816

    @505BEN said:

    I thought there might be enough flow to use a mechanics stethoscope to determine which zone is flowing so I re-pressurized the system and listened but couldn't tell.

    You needed to get a plumbers Stethoscope to find water leaks…. Mechanic's Stethoscope only find oil and grease leaks

    Edward F Young. Retired HVAC ContractorSpecialized in Residential Oil Burner and Hydronics