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Pex infloor heat question - concrete

Discussion in 'General' started by GRH, Sep 5, 2019.

  1. GRH

    GRH Well-Known Member

    I'm looking to build a bigger shop next year and have been buying some materials along the way. I came across a deal on 3/4" pex tubing that was leftover from a snow melt project. The tubing would be free to me. I have installed a few infloor heat systems previously but used 1/2" pex in every application. While getting free pex is enticing, is using 3/4" for a 5" concrete slab ok? With 1/2" I was installing at 12" centers, due to the bend radius of 3/4" I'd likely do 18" zip tied to concrete mesh. Slab would be poured on slab shield insulation and with 2" foam lining the perimeter.
    Thanks
     
  2. I'll ask my buddy, he owns a pretty large concrete company and does a fair amount of shops with heated floors.
     
    GRH likes this.
  3. Motofun352

    Motofun352 Well-Known Member

    Given the same flow rate the velocity thru the pipes the 3/4 inch will be less than half as fast. This might lead to uneven heating though that's just a guess.
     
  4. eggfooyoung

    eggfooyoung You no eat more!

    Another thing to consider is, the 3/4" tubing has an increased volume of about 120%.
     
  5. eggfooyoung

    eggfooyoung You no eat more!

    I would definitely look in to a larger heat source, and recirculating pump. It might be a small financial savings upfront, but I think long term, the 1/2" will be cheaper, even with purchasing the tubing.

    @JBraun
     
  6. ChemGuy

    ChemGuy Harden The F%@# Up!

    I was worried about flow too but if the output of the pex was necked down to 1/2” then the flow would be similar but the 3/4 pipe would make a large reservoir. Depending on slab size I could see the last bit not have as much meat as the first and be cooler....maybe.

    I don’t think you would have a catastrophe on your hands though if you did it. Worst case maybe non optimal heating or running flow from 2 sides of slab to even the heat out some.
     
  7. Motofun352

    Motofun352 Well-Known Member

    Volume will be 2.2 times as big (for the same length) if my head math still works...too lazy to search for the claculator.:D
     
  8. eggfooyoung

    eggfooyoung You no eat more!

    Too late...
     
  9. Motofun352

    Motofun352 Well-Known Member

    120% ? I guess I misunderstood this as 1.2 times bigger. In any case...Get the friend to address, experience counts for more than guessing.
     
  10. eggfooyoung

    eggfooyoung You no eat more!

    Increased volume of 120%.
     
  11. Tristan

    Tristan Well-Known Member

    Are these setups not normally done with multiple zones with separate flow controls? If so, I can't see any disadvantage.
     
  12. GRH

    GRH Well-Known Member

    More detail:
    Building will be 32'x70' 15' ceiling, The plan in either case for the floor (1/2" or 3/4") is to run a 32' long manifold down one side of the building so I minimize supply/return line congestion, heat will come from a 500kbtu outdoor wood boiler using a flat plate heat exchanger and mixing valve(s). A hydronic fan coil will be installed as a separate zone to be used as a booster.
     
  13. ChemGuy

    ChemGuy Harden The F%@# Up!

    Jimmeny Christmas....500K BTU...thats like needing 1.21 Jigawatts

    I aint done the math but if your running a manifold and just sending lines across (and maybe back??) to a return manifold the extra hose size shouldnt kill you. But maybe talk to the boiler people, or whoever is specing your pump and let them say yeah or nea.
     
  14. GRH

    GRH Well-Known Member

    3 people could fit in this boiler, not kidding, I throw 36" long 16" dia chunks of wood in it which is about the max I can lift, it's a beast and already in place. It has 3 separate tap off points and I'm only using 1 currently for my house so it'll be heating both places.
     
  15. ChemGuy

    ChemGuy Harden The F%@# Up!

    Nice.
    Where you at in case I need to um....dispose of some....wood...yeah thats it...wood scraps.

    TIA.

    :D


    Does each tap have its own pump/mixing valve setup?

    What brand? Issues?
    I may be looking at these for the house and pole barn for next season.
     
  16. R Acree

    R Acree Banned

    OK, I know little to nothing about heated slabs....could you run the manifold(s) down the center of the slab to reduce the length of the runs and reduce the delta t on the return? That might give more even heat distribution.
     
  17. GRH

    GRH Well-Known Member

    Near Syracuse, NY
    Wood boiler output is 180*F at 0 psi, no mixing valves, they are in the house and would be in the future shop. I have a fair amount of land so burn mostly blown down/dead trees. I've used old pallets at times when I've gotten low, construction scraps, junk mail, it doesn't discriminate, it's all BTU's....
    Mine is a Central Boiler, it's my second one, first one was smaller and eventually wore out, this one hold 400 gals of water and I have (2) 80gal tanks inside for additional capacity. I like it but it's not for everyone and they should be used in a rural setting. Nothing is for free when heating, you either pay for it out of pocket or pay for it in labor.
     
  18. Banditracer

    Banditracer Dogs - because people suck

    The joy of wood heat. But you can't beat it when it's zero and the winds ripping.
     
  19. lazlo

    lazlo Stand up guy who corners low.

    3/4" pex is fine, you will have lower head loss. Nothing else changes, boiler size, manifold placement, etc., etc. Size the pump gpm for the load, the pump head loss for diameter and length of tubing. You can change the center to center, but I wouldn't.

    The only difference between the 1/2" pex and the 3/4" pex is the head loss.

    Placement of the manifolds will not affect how even the heat is distributed.

    This works best with a high-efficiency boiler. A standard efficiency boiler can be made to work, but with the additional controls a standard efficiency boiler would require for snow melt, high efficiency is best.
     
  20. My buddy stated just about the same as this. It’s fine to use, you will have more surface area but less pressure, without getting into too many math equations it’s kind of a wash. He said “it’s not like you are flowing 200psi through them, it’s not that big of a deal” he said you can use a standard center to center or move it a bit further apart but since you have the 3/4 might as well use it to your advantage.
     

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