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Same issue with both 3/4" PEX and 3/4" PERT in 2 different snow melting systems

We recently observed manifolds for snow melting 2 different systems that were being pulled off the wall by 3/4" type A pex on an 11 year old job and 3/4" PERT on a 9 year old job.
Both had 40-45% glycol
Both were were 10 port manifolds with 20"-30" of tubing between the slab and the manifold
Both were powered by mod/con boilers
Both systems are tight and have performed and been maintained well through the years with minimal issues

It appears that the tubing in both cases has gotten shorter over time. The slabs have not sunk or dropped in elevation. You can tell by the fact that both are attached to exterior walls and there is no gap below the enclosures (attached to the walls) and the slabs. if it were slab drop caused by settling, there would be an obvious gap and I wouldn't be posting. It really appears that the tubing itself has shrunk in length with both the PEX-A and the PERT installation.

Has anyone else observed anything similar?


  • hot_rod
    hot_rod Member Posts: 22,472
    Got some pics? I’d certainly sent them to Lance MacNevin down at the PPI for his thoughts.

    Www.plastic pipe.org
    Bob "hot rod" Rohr
    trainer for Caleffi NA
    Living the hydronic dream
    GGrosskcoppErin Holohan Haskell
  • Ironman
    Ironman Member Posts: 7,397
    Both types of tubing expand when heated. It could be the tubing has pushed the ground away and then settled when it’s cooled.

    I’ve not seen it to that degree in a SIM application, but it happens in geothermal systems due to freezing and thawing.
    Bob Boan
    You can choose to do what you want, but you cannot choose the consequences.
  • khclifton
    khclifton Member Posts: 3
    We install pex and pert, "sleeved" through 1" PVC conduit 90's at the transition from slab to manifold so it seems unlikely the tube could be dropping into the substrate below the slab and if it did it would also be able to return to the height at the time of installation. It really seems like shrinkage of the tubing, but that also seems like a radical amount of shrinkage, considering the short amount of space (less than 30") from the conduit to manifold.
    The 2" copper BV-Barb manifold is clamped on unistrut, the screw holes from strut mounting and the amount the manifolds have come out of level indicate a change of 1/2'+ lower.
    We have not felt the need to put eyeballs on the remote manifolds much over the years because both systems have not lost any pressure and neither has seen a drop in volume of their glycol feeder systems.
  • khclifton
    khclifton Member Posts: 3
    We are now wondering several things:
    1. how much do 3/4" Pex and 3/4" PERT expand during the 100 psi air test that is code mandated and that we have always left on during concrete pours to ensure the tube is not compromised during the concrete pour? Does a ballooning occur that makes the tubing loose inside the cured concrete after it is cured and our system is filled to it's lower 15 psi operating pressure?
    2. how much do 3/4" PEX and 3/4"PERT expand with ambient temperature? Both these jobs were laid and poured during the summer in Colorado and the tubing could have been 90F or more with solar gain.
    3. Is it a combination of both factors above?
    4. I initially wondered if this could be a chemistry issue with glycol but now think not

    I had always thought of tubing as in complete contact with concrete or gypcrete. I had been told PEX expands and contracts primarily in length and when trapped by wall friction in concrete or gypcrete and length-wise expansion is not possible expansion is then expressed by increases in wall thickness. It seems to be the case that expansion is leans primarily toward length as i can almost always get crimp rings on without to much struggle and have done so at ambient temperatures between 5F and 105F. The slight differences in difficulty with crimp rings I have attributed to variations in manufacturing but within spec's even if sometimes difficult. I now wonder about temperature as a diameter variable.

    We have never observed the same problem on indoor slabs that remain at relatively stable temperatures. Installed in the exact same manner none have ever pulled manifolds toward a slab.

    I'm pretty sure what we are observing is tube contraction and lack of contact with the sidewalls due to the 2 factors at the beginning of my comment. I would guess that when heated during snow melting operation the tubing expands to the walls. I think what is pulling the manifolds down is when it is quite cold with an idle slab approaching the ambient air temperature and tubing sidewalls that are contracted from the concrete we are seeing tubing creep towards its contracted length.

    We can control the air test expansion and only keep 100psi in it just long enough for inspection but reduce to 15 psi for the concrete pour. This takes pressure ballooning out as a factor. There is not anything we can do about outdoor temperature during installation because of constuction schedules. We will add some flexibility to our manifolds and relief 90's to above slab tubing.

    We are curious to know if anybody else has observed this happening and/or is willing to talk about it if they have.
  • MaxMercy
    MaxMercy Member Posts: 515
    edited February 2023
    khclifton said:

    2. how much do 3/4" PEX and 3/4"PERT expand with ambient temperature? Both these jobs were laid and poured during the summer in Colorado and the tubing could have been 90F or more with solar gain.

    The manufacturers of the pipe all list expansion per degree of change. I would't think ambient temperature of the pipe would be any different than the temp of the water mixture it carries.

    To my memory, it's something like an inch per 100 feet for every 10 degrees the temp is raised for most PEX, not sure about PE-RT but I suspect it's similar or the same.

  • LMacNevin
    LMacNevin Member Posts: 15
    It is not likely that either the PEX or PE-RT tubing actually shrank in length over time, as that would require molecules to bunch up and get tighter together while under pressure. That is contrary to the physics of pipes. But here are a few thoughts:

    1. The rates of longitudinal thermal/expansion for PEX and PE-RT are practically the same, approx. 1 inch per 10 degrees per 100 feet length. You can check the values at www.plasticpipecalculator.com
    2. It is most likely that the tubing was installed on a warm day and has shrunk due to temperature change. When you start up a SIM system, the fluid is at outdoor ambient air temp, so very cold.
    3. The 100 psi pressure test should not affect the tubing length to any measurable degree. Even at 200 psi internal pressure, the expansion in diameter for the tubing is negligible (maybe 0.001").
    4. There is a thing called "Poisson Effect" which says that if a material stretches in one direction it will shrink in the perpendicular direction. This applies to very large HDPE plastic water mains which can expand when highly pressurized and then reduce in length over miles of pipeline. But I've never heard of this happening with PEX or PE-RT tubing.
    5. There should be no chemical effect of glycol on either tubing material. Both materials have been tested for that and used successfully for decades.

    Feel free to each out to me directly with any questions on pressure pipes and I'll help if I can. lmacnevin@plasticpipe.org

    GGrossErin Holohan HaskellIronman