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radiant tubes in a concrete slab-dumb question

Post in 'The Boiler Room - Wood Boilers and Furnaces' started by captaintone, Feb 21, 2009.

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  1. captaintone

    captaintone New Member

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    I am looking into radiant heat in a basement slab and thought about the concrete cracking. Does the cracking affect the tubes? Meaning would I have to worry when the concrete cracks. Is it going to destroy the tubes?

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  2. mtnmizer

    mtnmizer New Member

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    Not a dumb question at all.


    Small cracks haven't seemed to bother my system. I
    would imagine if major cracks were to form, ie shifts in
    levelness or large ones would. The older systems with
    copper pipe were known to be problematic from cracks.

    We really like our system.little .cracks will cause problems to
    tile if not put down correctly
  3. captaintone

    captaintone New Member

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    Well thats good news. Is there any room for expansion when the tubes are installed? I mean are they layed tight or do they allow for some movement? Did you do anything different when pouring the floor. Like extra re-bar, thicker slab...
  4. deerefanatic

    deerefanatic Minister of Fire

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    What was done on our shop by the previous owner was he laid a 6" sand bed down, put the pex in that, then poured the concrete over the top...... but, it takes a LOT of heat to get that system up to temperature initially.......
  5. sweetheat

    sweetheat Member

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    My slab-on-grade is somewhat overbuilt because of the point loading going on there from a post located every 14 feet. We used 2X12 form boards around the perimeter. The honch or outer 2 feet is thicker and has more rebar. #4 bars 12" OC. The inner slab comes up to approx. 7" thick with #4 rebar 2' OC. Blue styrofoam 2" thick is under all surfaces. I now wished I'd placed 2" styrofoam around the outside of the edges of concrete as I see lost BTU's melting the snow about 6 inches out. The floor has small drying cracks visible close to the bundled pex close to the manifold. Nothing structural, just a small thin surface crack. I've kept my eye on it and it seems to be OK to date. Concrete pour was in done in 1991, 2009 it's still the same. We used 3400 PSI concrete. sweetheat
  6. DaveBP

    DaveBP Minister of Fire

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    Any place PEX tubing runs across an expansion joint in a slab, the tubing should be run through a sleeve of black poly pipe a foot or two long to prevent pinching the tubing. It should be taped to the tubing to keep concrete from getting between the tubing and the sleeve. Not a dumb question. It can be a real problem.

    Welded wire reinforcement is the standard way to protect slabs from moving when they crack. It doesn't keep them from cracking, but it keeps it from becoming more than a visible crack. Good place to tie the PEX down, too.

    To minimize cracking, the mix plant can throw in various "admixture" chemicals that reduce the amount of water that has to be added to make the concrete easy to work with. More water added to the mix, more contraction in the slab as it cures. Most crews just keep adding water as they work to keep the mix fluid and easy to screed off. Check with your contractor for the price of whatever your local plant offers. There are a number of different ones available.

    Fiber can also be added. Short polyester fibers that are added to the mix at the plant. The fiber can be almost as good as welded wire. Or no good at all according to some. Our local septic tank manufacturer switched to fiber, replacing wire. It leaves little hairs in the surface of the finished slab that some people hate the look of, though.

    Whatever you do, insulate beneath and around the outside edge with rigid foam. If your contractor is unwilling to use anything but bubblewrap, look for another contractor. Put heavy poly film under the insulation. A basement floor doesn't have to be damp in the summer and it should be easy to heat in the winter.

    Some of my opinions, anyway.
  7. mtnmizer

    mtnmizer New Member

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    A picture being worth a thousand words, here's how mine was done
    before the pour..The mixture was a standard 6 bag with fiber. The
    expansion cuts were made a few days later. So far so good. The
    guys who did this have many under their belts..

    Attached Files:

  8. djblech

    djblech Feeling the Heat

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    I agree with what is being said. Poly under 2" of ridgid foam. Wire or steel reinforcement is not needed in a basement slab because it is at a constant temp. You can put fibers in but not necessary. Minimum of 8" of compacted sand under foam, 12" or more is better. Make sure you have drain tile outside if loose soil, outside and inside to a sump if heavy soil. I am a contractor and have learned some of this the hard way. Unfortunately on my own house.

    djblech
    Greenwood 100
    Stihl 170 and 360
    Kioti dk45 and dump trailer
    120 acres of woods
  9. oldmilwaukee

    oldmilwaukee New Member

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    ditto on putting pipe insulation around the pex extending 1 foot into the slab on either side of any expansion joints. that's what I did. our slab (2200 square feet) was poured in 3 different sections. I don't trust the fiber stuff - I've seen it crack, and once it cracks, what's keeping the slab from moving further? go with steel. I used 3/8" rebar instead of the mesh, but mesh would probably work fine.

    My parent's house has copper radiant heat in the slab - 50 years old and still working. 1/8" cracks in the slab have messed up the original tile, but not the radiant heat. I presume it still works today because of rebar in the slab. I wish my grandfather were still alive so I could ask him how he did the slab.
  10. djblech

    djblech Feeling the Heat

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    One other thing, my masonary contractors are saying that the water temp should not exceed 140* and 120* is better. If you have metal reinforcement and the temp is to hot the metal could expand faster than the concrete causing cracks. Makes sense if you think about it. Could also be problems with hot and cold zones expanding differently.

    djblech
    Greenwood 100
    Stihl 170 and 360
    Kioti dk45 and dump trailer
    120 acres of woods
  11. captaintone

    captaintone New Member

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    Hey thanks for the info. There is a lot ot consider. I am in the planning stages of building and want to get things right the first time. I plan on using a garn system to heat the home. I have heard you can heat the slab at very low temps. 100 or less. is this true? one more question I plan on having a garage not attached to the home. It will also have a concrete floor and I have thought about also using radiant in there. any suggestions on the garage floor, or is it the same as the basement slab?
  12. 2.beans

    2.beans Minister of Fire

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    theres a new rubber mat ive seen installed that has a higher r value then the blue board. blue board doesnt lay flat on the ground unless you spend lots of time with a rake causing air pockets which contributes to cracking. definitely do not use bubble wrap. you can heat the floor with real low temps but i wouldnt worry about cracking it with to hot of water. i send 140* plus water to my outside walk ways that are well below freezing with no cracks. once you get the mass hot it holds the heat for a long time. make sure you have a real good plumber/engineer figure out your heat loss and tubing arrangement before you start because once its in there no going back. i have a 1600sqft build with radiant floor that currently cant keep up, that sucks. hopefully ill have it working for next winter.
  13. tom in maine

    tom in maine Minister of Fire

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    Be advised whenever you are looking at any insulation, it is the R value per inch that is important and "mats" are going to have a lower insulation value than rigid foam insulation. I think people get freaked out over Styrofoam cracking when they have to walk over it. This is not a big deal. If you are concerned about slab cracking, use fiber reinforcement and proper grading/compaction.

    Most mat-like materials are going to be R-1-3 per inch of thickness. In northern climates, R-10 is typically used for insulating slabs and I think R-15 is advisable.
    The other issue is the true compressive strength of any of these materials.

    Most bona fide insulation materials are certified by Factory Mutual, which is an accredited industry standard for rating insulation value.
    I have seen a lot of floppy mat manufacturers who use someone who skews numbers and makes a lot of bogus claims about their insulation value.

    Be wary of "new" insulation products. If they sound too good to be true, there is usually a reason for that.
  14. djblech

    djblech Feeling the Heat

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    Slab on grade like in a garage or slab house has different issues like frost protection. A typical basement is in the ground 5' to 7' or 8'. This and the constant temp eliminate frost problems. A slab on grade should have the foam down 18" to 24" and out 3' to 4' around the perimeter as well as underneath to keep the frost from going under the slab and lifting it. This also prevents heat from leaching into the ground.

    djblech
    Greenwood 100
    Stihl 170 and 360
    Kioti dk45 and dump trailer
    120 acres of woods
  15. in hot water

    in hot water New Member

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    the slab is only as good as the subgrade below it. That is the biggest problem I see when slabs crack and move. Uniform thickness in the pour helps also. A good base of level and compacted base rock on a stable excavation is the best way to start out.

    2" of foam under and around. Have a metal flashing company bend some Z flashing to cover and protect the edge of that perimiter insulation that may be exposed. Crete-Heat is a great foam product with knobs to snap the tube into, and a vapor barrier covering. Pricey but a real labor saver, no need for unwieldly wire mesh :)

    Small shrinkage cracks are common in large slabs. Don't water down the mix when it arrives will help eliminate small cracks.

    Most large slabs get control joints sawed into them after the pour, then fill the cuts with butyl caulk. Sawed joints are much nicer looking than a troweled control joint. Unless it is a very large slab I doubt you will have expansion joints. I've installed up to 16,000 square foot slabs without expansion joints. But they were cut on a 12 foot grid right after pouring.

    Fiber added to the mix can help stop small cracks from traveling across the slab. It is tougher to finsh with all that "dog hair" in it. A power trowel at the end will burn off the fibers for a smooth finish.

    Not too smooth for a shop slab or it gets slippery when wet.

    It's nice to sleeve the pex where it comes through the slab to the manifold. I use scrap 1" PAP tube to bend some protectors.

    Take pictures and document the layout if you need to anchor into it later.

    hr
  16. heaterman

    heaterman Minister of Fire

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    Depending on the heat loss of the building the water temp can be very low. I have a couple structures running 95* water at design temps here. (-8*) Lower yet at "normal" winter temps.
    Designing and installing a system that can use very low water temps has a huge number of advantages in the long run regardless of the source of heat input..................It is snowing so hard here right now I can't see the big church only a block away from my house. :)
  17. Der Fuirmeister

    Der Fuirmeister Member

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    If installed correctly pex radiant floor tubing is a terrific way to heat. And the hairline cracks that will appear should not be a problem. Our 10,000 sq ft shop has pex throughout. We used 2 inch rigid insulation around the perimeter of the slab inside, and along the wall down 4 feet on the outside. A closed cell poly foam is recommended under the balance of the slab interior (away from the walls). The largest heat loss is the perimeter where the temps drop along with the outside air. Once you get more than 8 feet in from the wall sub-slab soil temps run what they do 8 feet under the surface outside, or around 50 degrees around here. The sub-slab heat loss is far less than at the wall. The idea is to maintain 80 degree floor temps in our shop.

    Not all "rigid foam" is the same. There are three types commonly used in construction. It comes with different compressive strengths and water absorbing properties. I would not use 15 psi foam under a floor. 15 psi foam would be better inside a wall. If you are going to use rigid foam, I'd recommend the 25 psi foam. Better yet, use a closed cell foam designed for use under concrete. See links attached.

    Some fibers work. However, not all fibers are the same material. Some are plastic and some fiberglass. Not all steel mesh is the same. It comes in different wire gauge and spacing. We used both fibers and #6 mesh 6" X 6" (what's referred to as highway mesh) in the bottom inch of the pour. We put the tubing on top of that and tied it with "zip ties". Then we added another layer of 6X6 mesh on top of that. Then we used 5000 psi concrete and cut it on 10 foot centers. The engineers told us 100 sq ft is recommended for cuts. No more than 150 sq ft (ie 12x12=144). We poured in sections and used the recommended two foot pipe sleeve centered over the joint. We also sleeved the PEX inside long radius pvc pipe where it comes up out of the slab (electrical conduit radius, not plumbing radius). We also added re-bar dowels at the joints. Under the slab we compacted 2 feet of 1-1/4" "traffic bond" (which is limestone with fines), in 6 inch lifts, compacted to 95%. It was like concrete before we poured. 185 yards of concrete all brought in by wheelbarrow to protect the tubing. We've run many 50,000 lb loads over the 6" thick floor. No surface cracks. Any cracking has been controlled with the relief cuts.

    The PEX spacing is used to determine the heat output. 120 degree water temp is recommended max. It does not work to space the tubing wider and run hotter temps. Space tubing closer on the perimeter where the heat loss is greatest. Wider in the center of the room. There's more not much on this web site for in slab radiant heat info., and not to hijack the post, but you can find lots of information through some of the links below.

    I'm looking forward to connecting a gas'er to the shop in-slab radiant heat system some day.


    http://www.backwoodshome.com/articles/hackleman64.html
    http://www.pexheat.com/
    http://www.radiantcompany.com/faq/#cost
    http://www.thebarrier.com/
    http://www.slabshield.com/index.php
    http://www.radiantsite.com/
    http://www.pexsupply.com/categories.asp?cID=355&brandid;=
  18. yooper rich

    yooper rich New Member

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    Great question captaintone. I hope to be doing the same thing on our new home construction project this year and like you said, we want to get it right the first time. Thank again for all the info from you experts.
  19. AOTO

    AOTO New Member

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    I have a 32' x 48' barn that has Radiant heating. I used 2" blue board with Tekfoil bubble on top of that. I also used Pex-Al-Pex 1/2" tubing. So far this year it's worked out very well. It is nice and cozy in there at 52-54 degrees. I should note that I have a few stress cracks near the corner of the doors and I used 2400 PSI (I think) Cement with no rebar.

    Attached Files:

  20. GKG-MO

    GKG-MO New Member

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    I have poured literally hundreds of pex floors and the cracks will not effect the floor at all. A common misconception is that a concrete floor expands. Concrete just like everything else shrinks when it dries. Cracks most often form from the concrete being stressed because of a inside corner or different consistences in the mix.
    The most important thing in a pex pour is be careful. Lay plywood over the pex to run wheelbarrows on so not to cut the tubing. Be extra careful when you dump the wheelbarrows as the cleat in the front can cut the tubing with 150lbs of concrete in it. Make sure the tubing is pressurized to around 100 psi so if you do cut it there is an audible sound. Also a pressure gauge is a good idea. Check it throughout the pour. If you cut a tube and it has to be fixed after the floor is dried it can be pricey. Much easier to fix it and then go on.
  21. barnartist

    barnartist Minister of Fire

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    GKG is right. Do all of what he said. Spend the money on the insulation. Just do it. I also learned the hard way.

    A side note here. Any of you watch the planet green channel? I watched a show the other day, they were rebuilding a John Deere dealer in Greensboro- the town that was hit by the big tornado.

    Any way, I CANT BELEIVE i saw them-and filmed this, but they were installing the radiant heat, and actually showed the contractors only using the bubble wrap for insulation. This was one huge building. They are using corn for the heat sourse. I can't imagine how much more corn they will need to heat it now...
  22. yooper rich

    yooper rich New Member

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    mhvfd, how is the Pex attached to the blueboard?
  23. Deering

    Deering Member

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    Here in the SE Alaska rainforest we have lots of wood, but getting it to dry can be a challenge. I'm looking at installing a boiler in an attached shed (yet to be built) along with wood storage. I'm starting to think that insulating the shed, installing a HRV, and heating it off of the boiler it is the way to go.

    Pex in the slab where the wood will be stored seems like one promising option, but I may not want to keep the shed heated once the wood is dry. Can pex in the slab with water in it be allowed to freeze? Will the tubing fail or the concrete crack? Or do I have to blow it out or go with glycol?
  24. heaterman

    heaterman Minister of Fire

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    There's a LOT of ignorance out there, even amongst supposed professionals and it makes me madder than a wet hen. They don't seem to understand the basic premise of radiant heat is that it travels in ALL directions from the source. The myth commonly quoted is "heat rises" This is not so. Hot air rises but radiant energy travels 360* from the source until it strikes an object that it can dissipate its energy on. Think about the heat from the sun, traveling 93,000,000 miles through space at a temp near absolute zero. How the blazes does it make you feel warm when it gets here? Duh! It's not hot air, it's radiant energy. Once you understand that, common sense dictates that you need something to block the heat flow into the earth and direct it toward the area you want to heat. Radiant heat done poorly will suck mind boggling amounts of input.....as the guys in that JD dealership will soon find out. Green my butt.

    I gave a quote to a local business that was putting up a 10,000 sq ft machinery repair shop. I was too high mainly because I quoted 1-1/2" blue board for the entire floor while my "competition" insisted that double bubble was all they needed. The customers urge to "save money" led them to select the other contractors proposal. They burned up a large HeatSource, a 6048 CB and now have a huge OWB that they load wood into with a forklift on pallets. I don't know the brand. They can't get the building above 50* when the OD air temp hits 0, they burn over 120 pulp cords of wood per season and they can grow tulips around the building in February. Now they are considering abandonment of the infloor and installing overhead fan forced heaters. In their case it's probably the best option they have. Truly sad!
  25. BrownianHeatingTech

    BrownianHeatingTech Minister of Fire

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    You will need to blow it out, or use glycol. To avoid filling the whole system with glycol, you can separate that zone with a heat exchanger, if necessary.

    Joe
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