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On the right track? (Calculations)

Im fairly new to this so I'm learning as I go. I have a radiant heat system upstairs thats using 1/2" pex at 8" apart. I've calculated my heat loss to be around 5500 BTU/hr.

GPM = 5500/10,000 = .55

Head loss = 200ft loop...2*.51 = 1.02

Does this seam right so far?

If I had 3 200ft loops in a zone, would I multiply the head loss by each loop?

How do I know what my supply temperature should be?


  • Zman
    Zman Member Posts: 7,472
    edited September 2013


    Is your tubing in a slab?

    Where did you get your constants?

    For multiple loops, your head will remain the same. The GPM 's get added together.

    I would suggest reading Modern Hydronic Heating by Siegenthaler.

    You will like it, a lot!

    "If you can't explain it simply, you don't understand it well enough"
    Albert Einstein
  • Gordy
    Gordy Member Posts: 9,537
    edited September 2013

    I see you are using .51 feet of head per 100' of 1/2" pex at .5 gpm flow rate.

    Head is the friction of the fluid moving through the pipe the circulator has to over come at a given flow rate. So the loop, or circuit with the highest head is all you need to worry about.

    So in your case IF all three loops require .55 gpm your circulator would need to be sized to move 1.65 gpm (.55 gpm x 3 loops) at 1.02 feet of head at the distribution manifold for the three loops.

    Now with all this in mind you also have to look at the total system. Your referring to part of it.
  • Zman
    Zman Member Posts: 7,472


    I see where the 10,000 came from, 500*20. The problem with that approach is that your radiant panel (tubing assembly) has to be able to give off that much energy.

    I would start out by deciding what the optimal system  max water temp should be on your coldest design day. With a condensing boiler you are best off running a temp less than 130.

    Then, I would determine what panel design will give me the correct amount of BTU's  at that water temp on that day.

    I would size the circulator to provide a 10 degree delta t by design .

    I like the loop lengths around 200 feet because an inexpensive (to buy and operate) circulator will push around .75 to 1 gpm giving nice even distribution.

    Outdoor reset is a must to prevent overshooting room temps and will provide maximum comfort.

    "If you can't explain it simply, you don't understand it well enough"
    Albert Einstein
  • KatieY
    KatieY Member Posts: 5
    Thanks for the book reference

    I've been trying to find a good book to read about radiant, but a lot of the books that show up on a search have horrible reviews. Ill look for that book you suggested tonight, thx!

    This would be an underfloor (wood) heating process and looked up my head loss using the pex chart provided on a website.
  • KatieY
    KatieY Member Posts: 5
    That makes sense

    That's for that explanation Gordy, that makes me feel a little more comfortable knowing I'm at least somewhere in the ballpark of understanding.

    Thanks for all the replies. Looks like I have more reading to do.
  • Jean-David Beyer
    Jean-David Beyer Member Posts: 2,666
    Ill look for that book you suggested tonight, thx!

    You will not have to look very far:


    There is some risk of sticker-shock, so take a deep breath before and after looking at the price. But it an extremely good book and well worth the money.
  • Zman
    Zman Member Posts: 7,472
    Another option...

    Modern Hydronic Heating is by far the most comprehensive book out there. It has a ton of formulas, and will give some folks an incredible head ache. You seemed eager to dig into the math so I figured it was the book for you.

    If you want to spend less money and want less math, I would recommend this one http://www.heatinghelp.com/products/Hot-Water-Heating-Books/26/90/Hydronic-Radiant-Heating-A-Practical-Guide-for-the-Nonengineer-Installer

    It will give a good overview of radiant heating, it will make you laugh a bit and no head ache.

    "If you can't explain it simply, you don't understand it well enough"
    Albert Einstein
  • Robert_H
    Robert_H Member Posts: 135
    The Library might work

    I was able to get a copy of Modern Hydronic Heating by Siegenthaler for a month at my local through the state (CT) inter-library loan.  I bought a copy of Hydronic Radiant Heating - A Practical Guide for the Nonengineer Installer which has a lot of good practical information as well.
  • Dave H_2
    Dave H_2 Member Posts: 527
    Flow Calcs


    Your flowrate of .55 gpm is for the entire room. If you have three loops, flow in each loop would .55/3 = 0.18 gpm per loop.

    But as discussed earlier, I would use a 10 degree temp drop for radiant rather than 20. It will give the room a more consistent temperature.

    So 5,500/(10*500)=1.1gpm and with three loops = 0.37gpm per loop.

    When calculating the head loss, it depends upon the water temperature, the hotter the water, the easier it is flow so the viscosity changes.

    Take a look here to figure out the water temperature needed based upon your heatloss. http://viewer.zmags.com/publication/9c73a879#/9c73a879/1


    Dave H.
    Dave H
  • KatieY
    KatieY Member Posts: 5
    Got my book today

    Since I love having my information at my tips I ended up purchasing the book. Lots of information, but it's nice having one source I can reference for now. That link is also useful, just need the time to read it all!!
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