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Radiant Heating System Design Help

Whitefaceadk Member Posts: 9
edited April 2020 in THE MAIN WALL

Hi all -

I am designing a radiant heating system for the first floor of a home. The second floor will be conditioned via hydrocoil as well as the basement below the first floor. The application I intend on using is 1/2" uponor tubing stapled down to plywood with 1.5" gypsum overpour.

I intend on having several manifolds, each with 4-12 loops, on actuators for multiple zones per manifold.

I would like to minimize the piping from the boiler, and so am hoping to run one loop around the basement and place closed spaced tees to each manifold; each manifold will have a pump and mixing valve (primary secondary piping). This way, I can run the boiler on outdoor reset for fancoil, while mixing valves will auto adjust radiant tubing temps.

I am going to be running some tubing in a crawl space, and two air handlers with hydrocoil will be in unconditioned space somewhat, and so will plan to put 40% (if thats OK/good for where I live in Northern NJ (design temp 10*F winter)) glycol.

If the manifolds are placed in a crawl space below the first floor, would that pose an issue for air? I see on many manifolds there are air separators on supply/return, which must be at highest point I presume.

Does this plan make sense? Some of the rooms in the home have 12'-15' ceilings; should I be placing my radiant tubing closer together, 6" max, to offset the high ceilings? This is new construction project and will be insulated well. I've attached a sketch of how we would like to do this. I'm not an engineer but I have approximately 7000 ft2; just over 2,000 will be 2nd floor hydrocoil, 4000,5000 radiant, so my primary loop would need to be 1-1/4" I beleive, with closely spaced tees 3/4" for hydrocoils 50k btu each, 1" for indirect water heater, and 3/4 or 1" to radiant manifolds depending on what they call for.

Thanks so much in advance!! Excited to work on our first big radiant project. Open to alternatives as well!!!


  • Whitefaceadk
    Whitefaceadk Member Posts: 9

  • Alan (California Radiant) Forbes

    If the manifolds are placed in a crawl space below the first floor, would that pose an issue for air? I see on many manifolds there are air separators on supply/return, which must be at highest point I presume.

    That shouldn't be a problem. If you set your system up properly, you will be able to power purge for air removal on start-up. And then again, setting your system up properly (pumping away from the expansion tank), you will be able to keep air out of the system.
    As far as tube spacing, the best place to start is with a room-by-room heat loss calculation. Tube spacing is a function of how many BTU's you need to keep the room comfortable at the design temperature of where you live.
    Do you have access to a heat loss calculator? If not, try here:

    8.33 lbs./gal. x 60 min./hr. x 20°ΔT = 10,000 BTU's/hour

    Two btu per sq ft for degree difference for a slab
  • hot_rod
    hot_rod Member Posts: 22,262
    As shown you have a series secondary, so you get a temperature drop at each set of closely spaced tees, the last set will see a much lower temperature. The mixed temperature formula could calculate that.
    Also the secondary circulators need to be on the supply side of the closely spaced tees, looks like you have them on the return branch?

    3 way thermostatic valves for the radiant zones?

    Here is an example of a parallel pipe, all zones seeing the same SWT. Reset and stage the boilers to keep them in condensing mode as much as possible. Boiler goes to high fire for DWH, which might be better on the boiler side of the sep.
    Bob "hot rod" Rohr
    trainer for Caleffi NA
    Living the hydronic dream
  • Zman
    Zman Member Posts: 7,573
    edited April 2020
    The temp drop at each radiant tee is going to be an issue, especially when those BTU hungry slabs at start up. The last slab in line is going to starve until the ones before get to steady state. If you are stuck on the loop design, I would suggest sizing the loop piping very generously and using a robust delta T circ rather than the alpha.

    Why are you putting the fan coils on reset and using fixed temp for the radiant? Reset is super important for high mass radiant to prevent overshoot.

    Some of the pumps in the drawing appear to be backwards. They all need to "pump away" from the expansion tank.
    If this was mine, I would size the fan coils to run at the same temp as the radiant, layout out the tubing to match the heat loss of the space served, and run all the radiant off a single delta p circ https://www.supplyhouse.com/Taco-VR3452-HY1-FC1A00-Viridian-Self-Adjusting-High-Efficiency-Wet-Rotor-Circulator-with-Communication-Module-Less-Flanges-Rotated-115V and put just the zone valves in the remote locations.

    Something like this would work if you want to leave the fancoils and DHW at a higher temp and simplify the radiant side.

    "If you can't explain it simply, you don't understand it well enough"
    Albert Einstein