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  1. #1

    underfloor heating pipe distance

    Hi all, does anyone knows how you determine space between pipes?

  2. #2
    Join Date
    Jun 2008
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    Morgantown, WV
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    129
    a tape measure would be the most accurate and easily method.

  3. #3
    Join Date
    Dec 2009
    Location
    Bristol NH
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    229
    With adetailed heat load report and a system desinged with that information only

  4. #4
    Join Date
    Jun 2010
    Location
    Philadelphia PA
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    2,190

    If your joist are 16" oc

    Quote Originally Posted by anishtain4 View Post
    Hi all, does anyone knows how you determine space between pipes?
    Then you'll want to consider 8" oc for pex and then look up tthe water temp you need to get your BTUs per sq ft
    You have got to learn from other people's mistakes! Because God knows you don't live long enough to make them all yourself !!!!!!!!

  5. #5
    Join Date
    Mar 2001
    Location
    Washington DC
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    73
    There's a somewhat complicated graph called a nomograph that relates the intended room temperature (which relates to the floor temp and the insulation/tightness of the building, the provided water temp, and the r-value of the floor itself, and the spacing of the lines. I can't find it online, so I'll try attaching the one I have. There is a wealth of information on radiant at healthyheating.com, an incredible resource.

    Sorry, I can't attach a pdf here apparently. I can sent it to you if you ask. Check out Healthyheating though, it's great.

    In general, spacing ranges from about 6-12". I generally work in very tight well-insulated houses, so 12" works fine, though it also depends on many other details: slab-edge & under details for concrete, floor & carpet details for wood floors, etc. For floating/underfloor installs (staple-up), use LOTS of heat transfer plates, IMO. Attention to detail is crucial to having radiant work well. Also (as is so often the case) managing customer expectations: it takes time for a radiant system to bring a house to temp. They can't be used with the same night setbacks as, say, a forced-air system. They work well with outdoor resets.

    Good luck!

  6. #6
    Thanks keith, that is the answer, but there should be a formula for that for sure. all of the details of walls, windows is for determining the heat load but floor material is considered for the spacing itself. I will be glad if you send your pdf to a reply of my email sent from "javidrahavard...."

    btw we always use five row of 6" spacing near outer walls, that is kinda company code, do you do something similar? Do you design your work?

  7. #7
    Join Date
    Mar 2001
    Location
    Washington DC
    Posts
    73
    I sent you the nomograph. Yes, I often double-up radiant tubing density near walls, especially with windows and doors. But much more important are slab edge and under insulation details: I put at least 4" of polyiso under slabs, which a lot of contractors think is ridiculous but is a necessary minimum for good operation of a radiant floor slab. The edge detailing is much more complicated, given footings and downturned edges and exterior finishes, etc.

    In my experience the specific spacing of the tubes is not terribly important. Radiant is very forgiving. In a well-insulated space, your floor temp will not be very high, and the distribution of heat in the space will be rather even. Think of it like a giant flat radiator: it's going to heat the space even if you don't put tubes in parts of it.

    I sent you the nomograph, and as you'll see the formula from which it is derived is going to be moderately complicated. Multivariable, obviously, and non-linear. Most people would prefer using the graph, I think. That's enough of a head-spinner.

  8. #8
    Well we put like 3" of polyiso.

    I know but the main company is sooo strict about pipe spacing and I'm after a way to make them sure we are doing it right, else we should send every single map to them and they tell us this number, that is for frustrating.

    Yep I got it but if you know anywhere that I can find the formula I would be glad, I'm a hell of a programmer and can convert it to an excel file very fast, then I can give it to you too.

  9. #9
    Join Date
    Mar 2001
    Location
    Washington DC
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    No I've never seen the formula. It should only take a few steps to derive it from the graph, you just have to determine formulas for the various curves. How's your diff. eq's?

  10. #10
    Join Date
    Mar 2001
    Location
    Washington DC
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    I'm looking a little closer at the nomograph now. First, it's mostly linear (I'd forgotten). That makes things pretty easy. The first thing you need to know is your heat flux, hopefully you already figured that out. That's derived from the heat loss calculation of the house divided by the square feet of radiant floor (which could be different from the total square footage of the house). You can ignore the Surface Temp stuff, it's just FYI. The y-intercept of all the floor panel resistance lines is 0, so using x=y/m you can find x, assigning arbitrary values to the x axis since you only use them to translate between the various steps, and adjusting m for each floor panel resistance (that looks non-linear to me, but you could just use a look-up table to keep it simple).

    The spacing/avg fluid temp piece is a little more complicated, but it also entails design decisions... are you working from a fixed fluid temp? Here's the rub. Let's say you've got an outdoor reset on your boiler, so your fluid temp is varying. That implies, all other things being equal, that you might like different tube spacing at different times... which obviously you can't do. Also, you'll notice that this entire exercise says nothing about increased piping density around doors, windows, or outside walls.

    What I think you'll notice is that while the nomograph is very helpful in raising one's attention and perhaps solving special-case issues, most floor-layout tools don't use it: they spit out 1' spacing in slabs and 8" spacing in underfloor, perhaps (or not) increased at peripheries. At least that's been my experience, when I've gotten things "professionally" laid out.

    I've generally proceeded to ignore such layouts, since they typically lay out in a serpentine pattern, which is simpler technically for several reasons but significantly less effective in assuring a uniform floor temp than a reverse-return pattern. I almost always do that. This all only pertains to slabs, floating or underfloor layouts are more limited by the circumstances.

  11. #11
    Join Date
    Mar 2001
    Location
    Washington DC
    Posts
    73
    Also: most contractors don't use polyiso under slabs. Extruded polystyrene is the more accepted practice, IMO. Polyiso, while having nice high r-values (though changeable with time) absorbs lots of water and degrades in ground contact. There's a thread on it at JLC here:

    http://forums.jlconline.com/forums/s...ad.php?t=52136

    If you look at manufacturer's sites, you'll see extensive mention and details of roof, cavity wall and sheathing applications, and nary a word on sub-slab. Examples here:

    http://www.pima.org/contentpage/Cont...SubModuleID=45

  12. #12
    Well thanks for mentioning that, but this chart is only for indoor temperature of 21 centigrade. also for pipe sizing we use 15,20,25, and 30 centimeters. this chart has 22 centimeters! However I extracted the formula for the top charts, it is x=(heat loss)/(0.517-0.25*Ln(R))
    Floor Panel Resistance means resistance of materials ABOVE the pipes right?
    There should be a clear formulation for this, I keep looking in the books.
    Thanks for the help
    we use counter flow too.
    Ooopss, sry we use polystyrene too

    actually we are using uponor's ufh system, most of our standards are coming from them

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