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High level information and resources for slab radiant overpour

rossn Member Posts: 76
edited February 2022 in Radiant Heating
In a related thread you all convinced me that I should take a close look at radiant floor above my uninsulated slab.

If someone can provide insight or point me to where I might find this information, that would be awesome.

- Can I practically get 25 BTUs/sf on an overpour floor radiant system? Assume uninsulated slab (below grade) with R5 rigid insulation laid down on slab, 1/2 or 3/8 tubing topped with an overpour at a minimum thickness (Gypsum, leveling cement, or concrete ). Final flooring is vinyl plank (low r-value).

- For the above described flooring build-up, what is the maximum Average Water Temp typically prescribed to keep the flooring temperatures within the acceptable range (I believe that is below 85F)?

In my case, I already have a 130AWT system and am looking to see how far off I would be from compatibility (without modification). Note that I would have the system setup on TRVs.


  • hot_rod
    hot_rod Member Posts: 22,120
    25 is about the high side for a comfortable floor. 82F is about as warm as you want the surface to be comfortable, especially on hard surface flooring. Above 82 or so folks tend to get sweaty feet :) The less R-value over the actual tube the better, of course. You have a good selection for that.

    Here is the rule of thumb for radiant output. You get 2 btu for every degree difference between the room ambient and the floor surface. So in a 70F space with 82F surface
    82-70=12X2= 24 BTU/ sq ft output.

    Radiant ceilings and floors give a higher output as you can run them much warmer. Unless you stand on your ceilings or walls :) Sheetrock can run up to 120F from what I'm told.

    The SWT to get to 82F at the surface depends on how much of the heat energy is traveling downward, this is why the insulation is key to running low SWT. 110- 120F should be plenty. Try the 130F see it it works to your liking.

    You want to match the SWT to the load as closely as possible, avoid over shooting temperatures. You are building a low mass system, it should heat quickly, not flywheel away like slabs can.

    An ideal system would allow changing SWT based on changing loads. ODR helps, indoor reset helps. It really comes down to how much technology and $$ you want to throw at it in quest of the perfect radiant. certainly you can upgrade controls as you go.

    Web have 31 issues of Idronics available for free at www.caleffi.com

    These two speak to your questions best.
    Bob "hot rod" Rohr
    trainer for Caleffi NA
    Living the hydronic dream
  • rossn
    rossn Member Posts: 76
    edited February 2022
    hot rod - first off, I want to say thank you so much!

    I will have to dig in more. I'm currently scrambling ordering all materials for my project, given pricing instability, so time to investigate is thin.

    Yes, I've seen Siegenthaler reference 120F as the drywall limit. USG & American Gypsum say to keep their drywall at 125F or below. I think above that there are some fire safety concerns, and the joint compound is a separate topic. According to warmboard, at 140F water temp, the ceiling temp is 115F.

    In my situation, I have other hydronic heating in the structure that is already designed around 130 AWT @ design condition. The system is on an outdoor reset already, and everything will be on TRVs. I believe the TRVs may help temper the floor temps a little at steady state, but can't quantify that.

    I think it will add a lot of complexity or re-work of the distrubion if I have to get two manifolds (of 5) to lower temps, given those are already all installed.

    If I could run at 125 or 120AWT at design condition, perhaps I can rely on the over design of the rest of the system to carry OK at those temps. I looked at Uponor's ADS software, and don't think it really lets me put enough variables in to predict floor temp reliably. I would like to think there must be some variable about the mass of the heating surface that the PEX is imbedded, which would affect the max temperature the water could run at for 82F surface temp.

    I just don't have a sense of how warm SWT can typically be with overpours with R5 under-insulation, without exceeding that 82F. If it is typically 125-130, then I may be OK. If 110, then it's clear I have to figure out the multi-temp aspects.

    I'm trying to get a hold of a local radiant professional I have worked with, but haven't heard back yet in the last week, and unfortunately many other materials will be dependent upon if we shift radiant to the floors (think interior door height, stairs, etc).
  • hot_rod
    hot_rod Member Posts: 22,120
    It's always nice to have the entire system at that same, one temperature.

    I've always heard over 12-15° difference between zones or circuits is when you should consider another mixing station. I suppose it depends on how sensitive the lower temperature zone is to SWT over-temperature conditions. Most areas see design condition about 2% of the heating season.

    I think with the "air space" system you have now, 130F is not a problem. If you increase the transfer to a more conductive transfer, you may need to back down to 110?

    Two options I see, crunch a lot of numbers with data or assumptions. Or trial and error. 130F won't burn down your home :)

    Bob "hot rod" Rohr
    trainer for Caleffi NA
    Living the hydronic dream
  • rossn
    rossn Member Posts: 76
    edited March 2022
    Thanks, Hot Rod. Unfortunately, it is sounding like it will be both. Upstairs will definitely be radiant ceiling... downstairs might flip the other way if I can get the temps to work out. Is there a practical and relatively simple way to adjust the temperature at the manifold, such as with a manual mixing valve (while still having the whole system on the outdoor reset)?

    As long as I know that a mixing valve could be added local to the manifold after the floor is installed, then I'm in better shape for trial-and-error approach. If I had to run another 3/4 from the mechanical room, that would be problematic. In my build, there are additional downstream manifolds from those locations that need the warmer temps.