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Thick slabs

BillatbhBillatbh Member Posts: 2
i am not exactly a newbie for radiant systems. Been installing them for more than thirty years and used to be a regular on "The Wall". I've always avoided heating slabs over 5", figuring the problems of lag and overshoot/undershoot would be too great. Coming back to the question now. I have a customer who wants to install radiant heating in a 12" slab. Underslab and perimeter insulation are a given. I'm thinking that if I can keep the tube about 2" below the surface that will help a lot. Also, it seems to me that keeping water temperatures down using outdoor reset with the "boost" feature on the boiler may minimize overshoot/undershoot while allowing somewhat reasonable warm-up times from cold start (due to "boost" feature). I would welcome your thoughts.


  • Jamie HallJamie Hall Member Posts: 13,205
    If you can manage to keep the slab at close to the same temperature all the time -- moving only to compensate for changes in heat demand due to outside conditions -- you should be fine. And that's exactly what a properly calibrated outdoor reset will do for you, although it will help to have a slab or space sensor trim function, which most do.

    What you don't want to do with any slab is try to vary the temperature for such things as setbacks or other whimsies. It won't do it, and the thicker the slab is, the less inclined it will be to help. Slowly varying, if at all, water circulation, and always on.

    If your client is of a mind to change the space temperature according to occupancy, or use setbacks for some reason, or there is a space with wildly varying heating loads (like a south facing room with lots of glass) my own opinion would be that that space would be best served with the constant temperature slab augmented by panel radiators on their own zone and control.
    Br. Jamie, osb

    Building superintendent/caretaker, 7200 sq. ft. historic house museum with dependencies in New England.

    Hoffman Equipped System (all original except boiler), Weil-Mclain 580, 2.75 gph Carlin, Vapourstat 0.5 -- 6.0 ounces per square inch
  • sallaberrysallaberry Member Posts: 19
    I would imagine the thickness isn’t as important as the insulation you have below the slab. If you had 2” rigid it would decrease the amount of btus needed. I would be careful on putting the radiant to high in the slab due to saw cutting. I don’t believe it would benefit greatly just because it’s higher up. Maybe on 2” dobies. Just did a house with 5” slab on main floor. I fired the boiler up at 4:00pm set thermostat at 70 when I got back at 8 am it was at 68 no problem heating with the 2” insulation under the slab.
  • GordyGordy Member Posts: 9,523
    Keeping the tubing close to the top will help initially. Once the slab is charged that becomes mute point since the slab will constantly seek equilibrium. What you seek will be in the control strategy. Keeping the fly wheel in check. Usage details of the conditioned space are crucial, as well as the heating regions outdoor weather characteristics. Hi lows daily etc.

  • hot_rodhot_rod Member Posts: 13,737
    Bob "hot rod" Rohr
    trainer for Caleffi NA
    The magic is in hydronics, and hydronics is in me
  • GordyGordy Member Posts: 9,523
    I agree with that article. However I think with a 12” thick slab it’s not a typical Slab thickness to relate to the articles supplied data. A 4-6 inch slab yes.

    With that being said I would say the upper 1/3 of a 12” slab would more relate to the findings in the article.

    I think tightening tube spacing also would be a benefit.

  • hot_rodhot_rod Member Posts: 13,737
    Curious why a 12" slab? That amount of mass will be a slow moving train to start and slow down.
    Bob "hot rod" Rohr
    trainer for Caleffi NA
    The magic is in hydronics, and hydronics is in me
  • GroundUpGroundUp Member Posts: 1,023
    What could he possibly be running over this slab that requires 12" of concrete?? I think that may need reconsideration before worrying too much about the radiant, but if for some reason 12" is actually necessary I think you're on the right track keeping the tube toward the surface. Larger tubing and tighter spacing will help the flywheel due to the huge thermal mass, but also remember the weight of 12" of mud is going to compress the foam underneath considerably more than the standard 4-5" and thus requiring very high density foam to maintain R value
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