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Radiant slab inlet temp completely under joists

rosy Member Posts: 9
An addition is served by two 70k btu air handlers west and east and happens to have under each zone 5" of insulated radiant slab with about 250 ft 1/2" pex run 12" OC. On the East, the slab is in an insulated crawlspace about 4 feet under an uninsulated (there is no further insulation blocking heat transfer through the floor) 3/4" plywood floor supported by TJI's and on the West the slab is 12" below the same type of floor.

The question is whether to mix the water such that it enters the slab at about 120-130, but it occurs to me that since floor temps arent an issue, and transfer is likely only fair, that there doesnt seem to be a reason not to just run it hot, since it will lose easily 5-10 degrees coming and going because it is 30 feet from the mechanical room. Does anyone have a reason not to run the slabs at a higher temp than mixed?


  • Jamie Hall
    Jamie Hall Member Posts: 22,187
    I doubt you'll get much floor heat from either side -- no contact, too much space. You might get some...

    On temperature, the thing to watch is expansion. Running up to 120 or so may not be a problem. Running up to 180 you may get cracking in the slab or some weird problems with the PEX trying to expand in the concrete and not being able to.
    Br. Jamie, osb
    Building superintendent/caretaker, 7200 sq. ft. historic house museum with dependencies in New England
  • John Ruhnke
    John Ruhnke Member Posts: 882
    According to RPA guidelines the max temp allowed to enter a slab is 140 degrees.

    I would be careful running any higher than that.
    I am the walking Deadman
    Hydronics Designer
    Hydronics is the most comfortable and energy efficient HVAC system.
  • rosy
    rosy Member Posts: 9
    I was thinking of running it near 110-120 just so the floor above is warm.
  • hot_rod
    hot_rod Member Posts: 20,827
    When you run high mass slabs warmer then required you may experience hard to control "flywheel" effect. It could take days to reduce the slab output when the heat load decreases.
    Bob "hot rod" Rohr
    trainer for Caleffi NA
    Living the hydronic dream
  • nibs
    nibs Member Posts: 500
    Our boiler is set at Max 115 deg and it works beautifully.
    Much of the time it is running below 100.