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    Pex spacing for under floor radiant (joist bay) (25 Posts)

  • chrismcd chrismcd @ 11:17 AM
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    Pex spacing for under floor radiant (joist bay)

    I returned from vacation to find that the HVAC subcontractor had installed 1/2 pex at 16" o.c. (One run per joist bay) for our hydronic radiant system. He claims this is standard but everything I've been able to find suggests 12" max and 8" being typical for this sort of install. We're in Chicago (relatively cold winters).

    Any thoughts?
  • paul paul @ 11:51 AM
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    radiant spacing

    i will be the first to say it. you are screwed.
  • chrismcd chrismcd @ 12:01 PM
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    Radiant spacing

    If I'm screwed, they'll have to redo it...but I need to make my case. Is there something that lays out clear standards for this type of thing (ashrae manual etc)?
  • Paul Pollets Paul Pollets @ 1:28 PM
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    Joist bay heating

    Here's the Uponor manual for Joist Trak heating.

    The correct method usually uses 3/8" barrier pex on 8" centers per joist bay, using aluminum transfer plates. Loop lengths should not exceed 200ft. for 3/8" pex.

    Any contractor who doesn't use the correct method will saddle you with a home that doesn't heat properly and has high fuel costs.

    Caveat Emptor.
    This post was edited by an admin on August 15, 2013 1:28 PM.
  • chrismcd chrismcd @ 2:13 PM
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    Joist bay heating

    Anyone willing to defend the current 16" o.c. install as anything other than corner cutting ?
  • duffy duffy @ 3:36 PM
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    is it staple up system with low temp supply water or is it ultra fin system with higher temp water for supply .one heats by conduction and the other by convection. did contractor do heat loss on area .what kind of boiler is installed
  • Chris Chris @ 7:35 PM
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    Here's Your Case

    May I please see the radiant heat loss and design your prepared prior to the installation in writing unless you wish me to have my lawyer prepare a formal letter requesting it. Until I see and review the heat loss and design you must stop all work.
    "The bitter taste of a poor installation remains much longer than the sweet taste of the lowest price."
  • Zman Zman @ 2:31 PM
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    There is no simple answer.
    The correct way to design a system is:
    1 Do a heat loss calculation on the home.
    2 Determine how many square feet of flooring is available to emit heat.
    3 Size you radiant panel (tubing layout and aluminum plates) to meet your specific heat loss.
    I am guessing your contractor did not do (and does not understand) any of this.
    If you have 16" OC tubing with no plates, you are not going to heat anything.

    You might try the RPA for more info

  • Rich Rich @ 4:10 PM
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    Check out

    this discussion that is underway right now on this very site .
    I have encountered the 16" magic radiant spacing before , and I will tell you that other than in a slab below grade with highly insulated walls and no windows it WILL NOT WORK ! PERIOD . I would imagine you like most people chose radiant for comfort and efficiency , is this correct ?  What 16" spacing in essence does is raise the water temps , cuts your effective floor factor by 50% . 
     This method has become popular with DIYers that believe that we are highly overpaid , quite possibly the JOKER you are dealing with has installed a couple of these homeowner purchased packages and is now impersonating a qualified radiant heating contractor . The only other time I have seen it was in a floor warming scenario where forced air was the primary heat source .  I know of not one manufacturer of a product made for radiant heat that makes a 16" on center panel .
    Warmboard , Sunboard panel , Uponor , Thermal Track , Zurn , and many more .
    No , make him come back and install to an industry standard of 6 , 7 , 8 , 12 " centers .   
    What kind of boiler is going to be used , what is the install method besides spacing ?  You my friend have a fight on your hands .  I cannot mention the name of one of the companies that touts this method but they are in the North East and 3 states in from the ocean.
    You didn't get what you didn't pay for and it will never be what you thought it would
  • chrismcd chrismcd @ 5:05 PM
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    Radiant layout/spacing

    Thanks for all the responses!

    More info:The 1/2 pex is pressed into a metal track. Not sure about the boiler. Insulation will be decent: 3 1/2 inches of open cell foam. Lots of windows. Mostly hardwood floor. One room will have concrete/gypcrete.

    My understanding is you can make the calcs work with any spacing, but the system won't be very efficient or particularly comfortable if the spacing is too wide. Most of the the above seems to support this...argh I guess I do have a fight on my hands. At least it's not to late to rip it out and start over. I should also mention that it's an extensive renovation: I have an architect and g.c. that should be able to help me out with this once I have some solid information.
  • Rich Rich @ 5:37 PM
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    Heat Loss Calcs

    were any done for your house ?  Please tell me that the concrete / gypcrete room has the tubing embedded ! If not this room will never have a chance . 
    You didn't get what you didn't pay for and it will never be what you thought it would
  • Chris Chris @ 7:40 PM
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    You Cannot Get

    More btu/hr per sqft out of 1/2" Pex then you can with 3/8". The only advantage to using 1/2" Pex is a longer loop length of 50' more. If you've ever pulled both you'd know 3/8" is a heck of a lot easier to work with.

    Nobody here can really say if it will work or not without calculating a radiant heat loss and design. I've fix hundreds of jobs by reverse engineering them and let me tell you. The fix some times cost more then the initial installation.
    "The bitter taste of a poor installation remains much longer than the sweet taste of the lowest price."
  • Gordy Gordy @ 8:46 PM
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    My Thoughts are.......

    Better listen to the advice given here.

    Heat loss decides the boiler size, and..

    Room by room heat loss decides, tube spacing, flow rate, and average water temps.

    The most efficient RFH is the one where the tubing is the closest to the top layer of flooring in the detail. The farther the tubing goes down in the floor detail the higher the average water temperature needs to be. Plates help lower the average water temps.

    Beware if you have wood flooring there is a limit to water temps before you start to make the wood floor do adverse things you may not be happy with in the future. So the mind set of broad tube spacing with higher water temps becomes less of an option. Also If you have highr value carpeting for flooring it will diminish output.

    My anolagy to this is you could heat your home with one 14" hot water pipe down the middle of your floor with 210* water.......but would it be comfy, and efficient? NOT
  • chrismcd chrismcd @ 8:58 AM
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    Rfh spacing

    "One 14" pipe..." Yeah, excellent way of putting it.

    Thanks again for all of the help. If anyone knows a good contractor in Chicago who might people able to take a look and offer a second opinion, please let me know...
  • Zman Zman @ 9:38 AM
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    The fact that you have plates is a huge plus.
    I think, rather than starting a war based on "rules of thumb", you should have the proper calcs done, then decide if the system is incorrect.
    I think it is very likely that your problem could be resolved by redoing one or two loops only in the high loss areas.
    I would suggest finding a reputable designer, having the calcs done, then requiring the contractor to make the necessary modifications.
    Would it be fair to offer to split the cost of the design with the contractor? You could strike a deal where whoever is right pays the bill. In some areas this service is provided at no charge by the supply house.
    In the end, you just want the comfortable efficient system you were promised. Proper design is the only way to get there.
  • chrismcd chrismcd @ 10:02 AM
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    Sounds like a decent idea (hiring a designer to check the system) but whatever the calcs I can't seem to find ANY support for 16" o.c. in manufacturers literature/install manuals. 12" is the max I've found...
  • Rich Rich @ 5:05 PM
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    you will find no support for 16" on center from any factory or designer who cares about what we do .  16" may garner some support in a commercial building where many people are likely to be on a daily basis but not a residence . 
    You didn't get what you didn't pay for and it will never be what you thought it would
  • Zman Zman @ 11:32 AM
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    You just have to do the math.
    Until you do a heat loss calc you are just guessing. What you have would work in a super insulated home. There is only one way to know if it will work in your home.
  • Gordy Gordy @ 9:33 PM
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    I agree you need to do the math before shooting this down, but most on here lean towards a radiant floor, wall, or ceiling system that will use the lowest possible water temps to gain the most efficiency from the boiler if it happens to be a modulating condensing boiler.
    This post was edited by an admin on August 16, 2013 9:33 PM.
  • Chris Chris @ 9:50 PM
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    I Don't Know

    If its more about the lowest water temp. I shoot for what fits best to provide even comfortable heat across the floors. I don't care whether I need 120 or 100 as long as my floor surface temps are evenly distributed. Heat loss is a target, my tubing or application is my aiming device.
    "The bitter taste of a poor installation remains much longer than the sweet taste of the lowest price."
  • Gordy Gordy @ 11:09 PM
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    even floor temps

    You state you dont care if you need 120 or 100, but I'm sure you would not approve of 150-160 even if you could satisfy the load with your aiming device unless there was no other way....radiantly speaking of course, and for the sake of fuel efficiency.

    I guess I'm just a believer in extra tubing to even out floor temps coupled with lower water temps is a smart way to a point of course.
    This post was edited by an admin on August 16, 2013 11:11 PM.
  • Zman Zman @ 12:39 AM
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    Warm floors

    I attended one of Siggy's seminars a while back about heating the super insulated house. One problem you run into is that the heat loss is so low that if you run the tubing everywhere at tight spacing, the floors never feel warm.
    The next super insulated house I do (probably my own) , I am going to do more of a strategic layout. Some areas like under furniture will have no heat. Areas with low heat loss and little human traffic will have sparse tubing. Areas under windows and in heavy traffic areas like bathrooms and kitchens will have tight spacing. The design water temp will still be low. I just see a ton of benefits to this approach. After all, you buy radiant heat for the comfort of warm feet.
    I am really not suggesting that the original poster's contractor is doing this, I think he is probably a knucklehead. I am just saying that rather than having the guy tear out the whole job, why not do a careful design and see where you stand.
  • SWEI SWEI @ 1:35 AM
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    Tight houses

    Working on a design for one now.  Design day loss us 18k.  Had to use spacings ranging from 6" to 18" to get room-by-room balance on a single system temp (looking like it's going to be ~85F at design load..)
  • Gordy Gordy @ 5:33 AM
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    Super insulated houses

    Yes Carl, and SWEI,

    Super insulated houses are the exception, and a rarity which should be more of a popularity these days.

    Chances are you'll never have the radiant "warm floor " benefit which really is a sales pitch that needs to be refrained from. Neutral floor temps is more the reality of it.

    In those cases ceilings make more sense, or wall loops around window areas. Probably more so radiant windows. 18k heat loss at design load? Does conventional radiant heat even make sense? Nano mod/con boiler anyone?

    And yes I agree the in place radiant just needs enhancing to the op's situation.
  • SWEI SWEI @ 11:09 AM
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    Its an interesting challenge

    I recommended plates in the ceiling, but they balked at the cost so they're going into the slab.  We're considering an Altherma, but cost is a problem there.  Probably end up with an electric resistance boiler and some solar.

    Did one a couple years ago that came in just under 16,200.  Tubes already in the slab @12" and the owner (an architect) got his "design" (box of stuff) from a certain online plumbing supplier frequently recommended here.  He was about to pump potable water out of a tank water heater using a 007 stainless and a thermostatic (set temperature) mixing valve.  We installed a Thermolec B6-TMB and an ecoricrc e3-4 instead.  Last winter's electric bills were $10-15 higher than the year before when there was no space heat.  Good passive solar orientation/design, of course.
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