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System Resistance Curve

Forum Friends,

I'm working at understanding the system (resistance) curve for a Radiant Floor Heating System.  Clearly, fittings, valves, etc. add to the resistance of the flow in the system, but it seems that the actual radiant-floor loop is treated simply as length of pipe.  I've not seen it noted anywhere that the bends in the floor tubing increase the resistance.  I would think that a bend in the tubing would act something like an elbow.  Is my thinking off track in this matter?


Martín Romero


  • Mark Eatherton
    Mark Eatherton Member Posts: 5,840
    Not a big difference...

    It would be virtually impossible for the manufacturers to tell you what your pressure drop would be, short of knowing exactly what your design was. The return bends are very smooth, and their contribution to the overall pressure drop through a given circuit is not worth worrying about, especially at lower flow rates typically seen in RFH circuitry. The fittings are another thing...

    Trust me, you have a WHOLE lot of other things to worry about that will affect your delivery of comfort that you have absolutely NO control over. Like infiltration.

    And likewise, there are many things contributing heat to your delivery "system" that you are NOT taking into consideration, like solar gain, internal machine gains and body gains. What happens on paper is completely different than what happens in the real time world.

    If you do the system by the numbers, and you have not missed some obviously glaring issue, like a misplaced R value in a given wall (R 7 where R 19 is shown), your system will be 2 times bigger than it will ever need to be. That is my personal experience, and has been echoed by others who pay attention to things like that.

    But like I said, pay attention to the details that you have no control over, and be certain to specify them in your contract (ACH) and get it acknowledged (initials here) so that if it does raise its ugly head later, your butt is covered in paper.

    It's not so much a case of "You got what you paid for", as it is a matter of "You DIDN'T get what you DIDN'T pay for, and you're NOT going to get what you thought you were in the way of comfort". Borrowed from Heatboy.
  • Martin Romero
    Martin Romero Member Posts: 40
    Tubing Bends


    Thanks for the tip.  No wonder nothing turned up in my research with regard to pressure drop through the bends.

    I'll continue to be mindful of the factors that you mention in your message.  No need to be wasteful.  I imagine that there are some regional scale factors that are used once the numbers have been run by the book.  I'll continue to snoop around.

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