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Will tube inside existing radiant tube heat floor?

Hello all,

I have radiant floor heating installed in a slab under a new kitchen remodel. After a couple of years it became apparent that there was a hole in the tubing (1/2" PEX). To fix the problem, I have spent the last three months pulling some smaller diameter tubing through the existing tubing. I used 5/16" OD nylon tube, so there is a 5/32" air gap between the inner tubing and the wall of the existing tubing. I am wondering, will what I have done even work once I get water flowing through? Will the small tube inside, heat the air inside the larger tube, which will then heat the slab? Or will that air gap prevent the heat from radiating to the slab?

I guess I will find out in 3 months when winter comes, but I'm hoping to know now if all my efforts are going to be in vain or not. Thanks.



  • GordyGordy Member Posts: 9,530
    I think you will be very disappointed.
    kcoppSolid_Fuel_ManRich_49delta TDan Foley
  • JBahr2xJBahr2x Member Posts: 6
    Thanks Gordy. Are you saying it isn't going to do anything to heat the floor? or are you saying it might heat a little, but no where near what the original did?
  • GordyGordy Member Posts: 9,530
    Some things that will happen.

    The tube in a tube will be insulated. Due to air space, and the wall of the old pex.

    The 5/16" tube would require much shorter loops to obtain a reasonable flow rate with a reasonable head loss for the existing circulator.

    That being said unless you swapped out the existing circulator for a mining pump your flow rates will be very low, or non existent.

    Due to the above hurdles much higher water temps would be required to even try to see any performance.

    Did you try to isolate loops if there is multiple loops?
    Finding the leak is not impossible, and is repairable.

    How long are your 5/16" loops now?
  • JBahr2xJBahr2x Member Posts: 6
    The system is a single loop, 150' long. I do not know how many bends are in it.

    I was hoping the existing pump would be fine since it was pumping less water than before. You think I might have an issue with the pump now that I have smaller tubing?

    Just out of curiosity - besides ripping up the floor, is there another way I could have fixed this? Thanks.

  • GordyGordy Member Posts: 9,530
    edited September 2016

    If this is pex a you used
    .5 gpm is about 17' of head. What pump are you using?
  • ZmanZman Member Posts: 6,310
    Pex damage in floors is usually caused by nails. An infrared camera can help pinpoint the spot, then you tear into it and make the repair.
    I agree with Gordy. Your flow rate will be very low, There will be less surface area and, the extra air and tubing will have an insulating effect.
    "If you can't explain it simply, you don't understand it well enough"
    Albert Einstein
  • JBahr2xJBahr2x Member Posts: 6
    The small tube is 5/16" nylon (rated to 225 PSI, 150 degrees)

    Here is what is printed on the pump:

    Bell and Gossett
    NRF-25 103417 1K21
    225 F/107 C
    150 PSI/10 BAR
    A W
    3 1.1 125
    2 .8 90
    1 .7 72

    There is a switch on the side that has 1, 2, 3 with a line next to each, 1 being the shortest and 3 the longest. The switch is set to 1 right now.

  • GordyGordy Member Posts: 9,530
    edited September 2016
    Speed 3 with out flow check will give you some flow.
  • JUGHNEJUGHNE Member Posts: 7,861
    Is this a good application for the toe-kick space heaters?
  • GordyGordy Member Posts: 9,530
    JUGHNE said:

    Is this a good application for the toe-kick space heaters?

    I would be curious to the results. However kick space, or panel rads.
  • JUGHNEJUGHNE Member Posts: 7,861
    If a kitchen on a slab, could be possible to fish pex under all cabinets to get 1 or 2 kick space heaters in place. Just need to get homeruns under there from boiler room.
  • hot_rodhot_rod Member Posts: 14,374
    That will be an interesting test. I suspect the inner tube touches the wall of the outer tube in many spots, so you have some conduction transfer, but an additional layer of tube wall to drive the energy through. You may make up some of that by running higher supply temperatures.

    Was the original zone calculated accurately to the heat load? Did it ever run during design conditions? How did it heat?

    I'm amazed how well some of the Lieutenant Dangle, suspended pex tube systems work. Bare pex heating a big air space, you at least have some convection going for you.
    Bob "hot rod" Rohr
    trainer for Caleffi NA
    The magic is in hydronics, and hydronics is in me
  • JBahr2xJBahr2x Member Posts: 6
    I just wanted to post my result after a full winter. The new tube did give off some heat, but it was only enough to "take the chill off" the floor. It was not enough to heat the air in the kitchen and we need to wear slippers to be comfortable.

    So bottom line is, it certainly did not "fix" it, but it is better than doing nothing. Thanks to all who replied.
  • kcoppkcopp Member Posts: 3,596
    thanks for the report back.
    It may be best in the long term to come up w a long term solution.
  • GordyGordy Member Posts: 9,530
    Thanks for getting back to us. About as I expected.
  • GordyGordy Member Posts: 9,530
    Curious of what you actually did though. Circulator etc.
  • JBahr2xJBahr2x Member Posts: 6
    I switched the circulator to the highest setting, and upped the temperature to 120. That helped but because we have an indirect hot water tank we were running out of hot water in the morning. So I switched the thermostat to only run during the day. This kept the chill off but it ran (called for heat) constantly until the thermostat said it was time to shut down.
  • Mark EathertonMark Eatherton Member Posts: 5,837
    I'd caution you to maintain a proper pH because I don't think the nylon has an oxygen barrier. But again, my assumptions are all oxygen barrier pipe and ferrous components throughout...

    It's not so much a case of "You got what you paid for", as it is a matter of "You DIDN'T get what you DIDN'T pay for, and you're NOT going to get what you thought you were in the way of comfort". Borrowed from Heatboy.

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