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Large slab

Tom_133 Member Posts: 820
Hey All,

I have a new customer who is planning a 60x80 shop. It has 2- 14x14, 1- 14x12 and 1- 14 x16 garage doors, with one man door. The insulation is average, typical R-19 in walls and R-40 in ceiling with 3" foam underneath. I have never done such a large slab. I was planning on running 5/8" tubing and installing a couple floor sensors, a wall hung 199 boiler and basically just doing the normal procedure, that is use outdoor setback. Initial heat loss is 113K.

The thing thats throwing me is at one point I thought I remembered reading in siggys book that certain things need to be thought about with larger slabs.

What am I forgetting? Should I go bigger tube and 18" apart? This is a heavy equipment shop and he is in there 10-12 hours a day.



Montpelier Vt


  • GroundUp
    GroundUp Member Posts: 1,399
    "A couple" floor sensors? Is it multiple zones? Bigger tubing gains nothing and wider spacing doesn't either. I do about a dozen slabs a year that size or bigger, usually 5/8" tubing in 400ft loops at 12" spacing for an average shop situation. As long as the tubing is laid out properly with exterior wall exposure balanced between loops, there's nothing really different about a larger slab than a small one. A dual function stat like a Tekmar 519 is ideal for these types of buildings IMO, running both floor and air sensors to keep the boiler from short cycling when they open the overhead doors.
  • Paul Pollets
    Paul Pollets Member Posts: 3,508
    I'd use either 5/8 or 1/2" placed on 3" chairs for a 6" slab. It's nice to put the design through a radiant heat loss program, and see what the loop temps indicate. The flow rate is also important so design can consider a low loss header, if required.
  • hot_rod
    hot_rod Member Posts: 16,286
    do a heat a heat load calc to determine the best spacing.
    Thinking about radiant cooling also? The larger diameter tube would be a good idea, if so.
    Occasionally you can use 15 or 18” spacing, but at the cost of recovery time, comfort, and higher SWT.

    Planning on saw cuts? Keep the tube low enough for the 10x10 typical cut grid.
    A drone shot of the tube layout is nice to have.

    place anchor bolts for any equipment that needs to be bolted down.
    Bob "hot rod" Rohr
    trainer for Caleffi NA
    Living the hydronic dream
  • Tom_133
    Tom_133 Member Posts: 820
    very smart responses all. I did the heat loss calc on uponor, and it was 113K. Water temp of 100 or so.

    Last thing, I was just told it was going to be an alaskan slab type. What do you guys do to not heat the footing?
    Montpelier Vt
  • GroundUp
    GroundUp Member Posts: 1,399
    There's nothing wrong with heating the footing/beam if it's insulated properly. Many shop applications like this, the outer edges are covered in workbenches and the like anyway, so keeping the tubing away from the edge does help to reduce heat loss. I did a 7200 sq ft shop a couple weeks ago with the thickened edge/Alaskan slab like you describe, and due to the usage of the space we kept the tubing 24" from the outer edge all the way around. One of my shops at home I put 2 runs of tubing in the 12x12 footer/beam (at 6" and 12" from the edge) and the other shop I kept the tubing 18" in from the edge. There isn't really any noticeable difference in terms of edge loss, but the actual surface temp near the edges is about 5 degrees cooler in shop #2
  • Hi Tom:
    Your heat loss sounds right on, so why such a large boiler? The IBC HC 20-125 would be spot on although the HC 33-160 would be more suited if they like to keep the garage doors open. Other manufacturers probably have similar choices.

    Often wrong, never in doubt.
    rick in Alaskakcopp
  • Tom_133
    Tom_133 Member Posts: 820
    thanks for the comments.

    The boiler size is based on DHW production. Its a tight budget job so he cant swing an indirect right now. A combi is ideal and that setup will give him about 5gpm. I may squeeze him down to a 150 if he can be convinced. He also uses the hot water in his pressure washer, I have tried to explain to him why thats not ideal and he doesn't want to listen.
    Montpelier Vt