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Is this do-able?

Kestrel Member Posts: 102
Trying to figure out if retrofit, joist-space radiant floor installation will meet heat needs.

Huge help from Brad White last week on heat losses.  The question concerns a large, mostly open first floor, to which I have access to joist space in context of basement remodel (building 'man-cave' for teenager/friends/guitars/XBox/etc)

So, about 700 ft sq main floor, with about 579 ft sq available for under-floor install (reduced by cabinetry, up and down stairs, etc).  Heat loss for total area 18500 BTUH at design temp of 20'F here in Seattle, or 32 BTUH/ft sq.  Is this viable?

More typical 40'F conditions here require 11100 BTUH, or 19 BTUH/ft sq - which seems more reasonable and within reach.


  • NRT_Rob
    NRT_Rob Member Posts: 1,013
    edited May 2011
    what is the floor finish

    carpet, for sound absorption? If so, probably not.

    with wood, you will technically exceed warranty specs on most wood floors. On a floating floor I wouldn't care about that as long as you used water temperature reset. High temp though.

    with tile, no problem.
    Rob Brown
    Designer for Rockport Mechanical
    in beautiful Rockport Maine.
  • Ironman
    Ironman Member Posts: 7,224
    Heat Loss...

    Your heat loss actually comes to a little over 26 btu's pr. sq. ft. With tubing on 8" centers, you would have 870' of tubing (580 x 1.5). Allowing 20 btu's pr. lineal foot output would give you 17,400 btu's. This is some what conservative, but would also depend on floor coverings, wood thickness, etc. So it should be enough, depending on these variables. 1000btu's isn't enough to make a difference at this size and you could run the water temp slightly higher to compensate.
    Bob Boan
    You can choose to do what you want, but you cannot choose the consequences.
  • Kestrel
    Kestrel Member Posts: 102

    ...are mostly hardwood.  Sub is 0.5 inch shiplap fir (100yr old), wood is 0.75" Jatoba.  Kitchen is cork tile, bath ceramic tile.  Vast majority of the area is the jatoba.

    Is there a BTUH/FtSq limit for hardwood?  Or is it based on water temp?
  • NRT_Rob
    NRT_Rob Member Posts: 1,013
    most MFGs

    won't warrantee over 85 degree surface temp which is about 30 BTUs/sq ft.

    however in your case if warranty is not a concern you'd probably be fine with the thinner wood (including subfloor).
    Rob Brown
    Designer for Rockport Mechanical
    in beautiful Rockport Maine.
  • Kestrel
    Kestrel Member Posts: 102

    Yup, around 30 BTUH/FtSq is what I was aiming at, at design temp, and usually much less.
  • Kestrel
    Kestrel Member Posts: 102

    I'll go back and check my numbers later this evening
  • Kestrel
    Kestrel Member Posts: 102
    Are these calculations correct?

    I'm trying to figure out the input water temp, the delta T, and the flow rate, using the formulas from Modern Hydronic Heating.

    If I need to deliver 18000 BTUH and have 580 ft s to do it, that's an upward heat flux of 31 BTUH/ft sq.

    Water Temp = Air Temp + q(up) * (Rff + Rair)

    If Air Temp=67




    Water Temp = 127.5 (average)


    For flow rate,

    BTUH = GPM * delta T * 500

    18000 = GPM *10 * 500

    GPM = 3.6 (total throughout 4 loops)


    Where I'm getting confused...

    if q(up) is 31, and q(down) is 3.8,  a=0.566

    That gives...

    b = (a * L) / (500 * F)

      =  (.566 * 870) / (500 * 3.6)

      = 0.274

    Outflow Temp is then = Room Temp + (Input Temp - Room Temp)*e^-b

    If Room Temp is 67,

    Input Temp is 130 (Ave Temp + 0.5*delta T)


    Outflow Temp = 115

    This is what doesn't add up for me:

    Ave Temp is 125

    Delta T is 10,

    but outflow temp is 115 - more consistent with delta T=20 (Is that too high?) and input temp is 135 (is that too high?)

    Thanks for everyone's help and patience!!
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