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PEX in-bedded In concrete

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Phil_8
Phil_8 Member Posts: 42
There is only one part of the house that does not have a basement and it is a 15' x 20' slab on grade construction family room. Coldest part of the house. I want to install in-floor radiant heat to this room I have installed 2" of styrofoam insulation on the floor. Plastic on top on the insulation then wire mesh.
Supply and return manifolds will be installed for each of the loops that will be buried in the concrete. Manifolds will remain external to the concrete
(a) Is there any type or brand of PEX that is best suited to for this application?
(b) what diameter PEX should be used?
(c) can the PEX loops make a 180 degree bend in the far end of the room? how sharp of a loop or bend can be made? If not what is the solution (multiple PEX fittings?)
(d) how many of these loops should be installed?
(e) Is this 30' run (15' out and 15' back) to long? What should be the spacing between these two legs of the loop (out and inbounc)
(f) how many of these loops should be installed?


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  • Jamie Hall
    Jamie Hall Member Posts: 23,554
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    Most of your questions would be answered -- or at least an understanding of what more information was needed could be had -- by looking at design manuals for in-floor radiant heating. A LOT more information is needed.
    Br. Jamie, osb
    Building superintendent/caretaker, 7200 sq. ft. historic house museum with dependencies in New England
    Rich_49
  • Solid_Fuel_Man
    Solid_Fuel_Man Member Posts: 2,646
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    A 15x20 slab would only need one loop of 1/2" pex at 12" on center. I'd likely split it into 2  ≈150 foot loops. You absolutely do not want any fittings buried in the concrete, just 2 continuous loops of pex. I use wirsbo hePEX for slabs, it bends the best on those 180 degree bends. I use regular zipties to hold thr pex to the wire mesh. I also always put 75 psi of air in the pex a few days before the pour. That way you know it is tight and in the unlikely event of a puncture during the pour you will see and hear the leak and repair it with a coupling wrapped in heat shrink to keep the concrete away from it. 


    Your 30 foot run is no problem, the standard maximum length for 1/2" pex in a slab is 300' and 400' for 3/4. So split the runs into 2 of 175' or so and don't worry about length. 

    How will you be controlling this? What is the heatloss of the room? How is it heated now? What do you have for a boiler? Is the rest of the house heated by water?
    Serving Northern Maine HVAC & Controls. I burn wood, it smells good!
  • hot_rod
    hot_rod Member Posts: 22,382
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    A load calc. for the room would be a good first step to assure it works. I like to tighten up the tube spacing. For a room that size 6” on center if 1/2” is workable, maybe a couple 250’
     The A type Pex is a bit more flexible. Uponor, Rehau, Mr Pex are some well know brands

    Bob "hot rod" Rohr
    trainer for Caleffi NA
    Living the hydronic dream
    Rich_49
  • TAG
    TAG Member Posts: 755
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    In an old house we have alway done 6' on center using the mesh -- this would be with an over pour for tile or some type of stone. I have also had good luck with Warmboard S over foam and 2x on the flats when I wanted wood floor and there is a slab or I am pouring a new one (too late for that here)
  • Phil_8
    Phil_8 Member Posts: 42
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    Thanks everyone for the very useful data.
    The house is currently heated by a gas fired forced air systems for both heating and AC. I am adding the boiler system for better heating and zoned temperature control
    I have salvaged and refurbished many cast iron rads that I will use in the other parts of the house (that have a basement) with several independent zones each with its own zone valve. I have not yet selected the boiler. However the choice will probably be a non-condensing cast iron Burnham.
    All of the drywall has been removed and all of the walls and attic (underside of roof sheathing) have been insulated with closed cell spray in foam.
    So the only part of the house for which I have questions is the family room which is slab on grade construction. All other parts of the house have a full basement.
    So thanks again for the valuable information. I will be installing the PEX tomorrow atop the 4' x 8' sheets of 2" foam that has been glued to the concrete slab.
    Will I have to use a lower water temperature (mixed down with separate circulator pump) for the tubing in the slab or will I be able to use the same temperature water that is going to the CI rads?
    Thanks again
  • Solid_Fuel_Man
    Solid_Fuel_Man Member Posts: 2,646
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    The temps which you operate the rads and the slab are completly dependant upon the size of the rads relitive to the heatloss of the rooms which they are installed. The temp which you run the slab is yet again dependant on how much tubing you use (distance on center) and the heatloss of the room. Cannot answer your questions w/o you doing the homework. 
    Serving Northern Maine HVAC & Controls. I burn wood, it smells good!
  • Phil_8
    Phil_8 Member Posts: 42
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    I was wondering what type and "mix" of concrete is best for embedding (covering) in floor PEX over foam insulation sheets. I assume that a mix of only sand and concrete with not gravel to minimize risk of damage to PEX. In the pictures that I have seen it appears as though these mixes are of such a liquid slump that that it appears as it is self leveling. Is there a special type of concrete mix that is used to cover PEX over foam panels?
  • hot_rod
    hot_rod Member Posts: 22,382
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    Regular 5 or 6 bag mix is fine.
    If you are doing a thin pour, 2" or less they can blend a mix with a smaller aggregate, pea gravel is considered a 3/8" rock, compared to the more typical 3/4 rock.

    A small grout pump can move those small rock mixes if you need to pump it.

    You want 1-1/2 times the aggregate size over the tube.

    To minimize cracking some installers add fiber to the mix, some concrete guys suggest beefing up the fiber in thin pours, 3 times as much.

    All sorts of additives available including some that make the mix flow better. Fly ash, if you can find it :) it's been in short supply, helps the flow or superplastizer additives.

    Gyp pours are a non aggregate mix, sand, water, portland mainly, not typically recommended for finish floor or a wear surface.
    Bob "hot rod" Rohr
    trainer for Caleffi NA
    Living the hydronic dream