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Calculating BTU output of a radiant floor

achromy Member Posts: 2
Hello, long time lurker here. You guys rock.

I am building a radiant floor system, and I could use some help figuring out the BTU output of a radiant circuit. I have Siegenthaler's book, but I'm still struggling with this.

I'll lay out some facts and assumptions, and let's go from there.
First, this is a 1/2 pex tube and aluminum transfer plate system built above the subfloor on 3/4 sleepers. Engineered flooring (click-lock) directly on top of transfer plates. This space is a 2nd floor, above heated space.

Let's assume 100F SWT, 0.5GPM, 300ft circuit, 6" spacing, 4" wide extruded transfer plates, and floor covering of R=0.15.

Also, is there a calculator or spreadsheet where I can plug in this info and play with the variables?



  • hot_rod
    hot_rod Member Posts: 21,835
    As a a general rule of thumb mid 20's btu/ sq foot out of a comfortable radiant floor. The output is based on the actual floor surface temperature, stay below 83- 85°. vs the ambient air temperature.

    So a surface of 83-70= 13X2 would be 26 btu/sq. ft. (2 btu/sq ft/ degree difference)

    If you can find one, the RadPad slide rule calculators were great for quick calcs. There may be an online version at drakeip.com

    So basically your output will depend on how warm you run the floors and the ambient temperature in the space.
    Bob "hot rod" Rohr
    trainer for Caleffi NA
    Living the hydronic dream
  • achromy
    achromy Member Posts: 2
    Thanks for the replies.

    I checked out the slide rule. Very cool tool. It's taken me some time to process all of the things it does. I'll definitely be coming back to it.

    So, I can know the btu output of my floor based on the surface temperature, but it seems that knowing what water temp will get the floor to any given temp is the hard part. The slide rule can't know if I am using heat transfer plates or if I do, whether they are heavy gauge, extruded, or thin, like aluminum foil. I suppose these are some of the harder variables to deal with. I found this chart which is product specific. I bet I will be able to run slightly lower water temps since my spacing can/will be tighter and I'll likely use heavier gauge heat transfer plates. This will get me close enough... like I can basically calculate worst case performance with it, which will meet my needs.

    Thanks everyone.