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GPM Flow Rate

Hey all, I'm sure there is a million questions that lead up to this being a correct number, so this question might not be the best question to ask but...

What would be the "ideal" GPM of Flow rate per zone in a balanced closed loop, radiant floor heat system? Just trying to get an idea and large overall picture.

I have a Rehau Pro Balance Manifold, and I'm looking at the flow gauges on the top of it. They range from 0-2.0GPM.


  • Gordy
    Gordy Member Posts: 9,546
    edited November 2014
    Depends on the load requirements of that zone.

    Heat loss first
    Then you can determine water temperature, tube spacing, flow rates needed to deliver the required btus for that zone, or multiple loops for a zone. Then you can size a circulator.
    Typically .5-.75 gpm is a ball park estimate. To stay in a reasonable range for an efficient pump.

    So if a manifold supplies 4 loops that require .5 gpm then ypu need to deliver 2 gpm to that manifold. At say 110* SWT. As an example.

    That's the basics everything revolves around the heat loss.

    So whatcha got going? Emitters? Radiant, base board, rads?
  • evanlifetv
    evanlifetv Member Posts: 9
    edited November 2014
    Hey @Gordy, thanks for the tips and info. My system is currently less than .10 GPM when running, so I'm trying to track down some of the problems to make it better/warmer, while I work on trying to get the whole system overhauled. Should PSI be 15-20psi in this style of system too?

    I bought a house with an existing Radiant Floor setup, and have been talking in another topic with some other guys about it. (http://forum.heatinghelp.com/discussion/152002/radiant-floor-heating-2800sqft-house#latest)

    I'm just trying to learn as much as I can at this point, because it appears lots of it is wrong!

    Thanks again for your info.

  • Gordy
    Gordy Member Posts: 9,546
    .10 is pretty low flow. Check supply return temps on each loop for starters should be 10 to 15* for radiant floors. That will be a start in being a detective.

    Sounds like from the other thread you have enough tubing. Just need to dial in flow rates. This could be a poor circulator selection issue.
  • Ironman
    Ironman Member Posts: 7,375
    The problem is the high head loss through the heat exchanger of the tankless. It's choking the flow. That's what we were talking about when I stated it's not designed for space heating. Then, also having it in series with the floor only compounds the problem. The HX is probably designed for about a 10psi pressure drop at 5 gpm. That's no big deal on an open domestic loop that has 60+ psi on one side of it. But on a closed hydronic loop, that's 23 ft. of head which is at or beyond the limit of the circ, not counting the resistance of the In floor tubing.

    Yes, the cold static fill pressure should be 12 - 15 psi.
    Bob Boan
    You can choose to do what you want, but you cannot choose the consequences.
  • Gordy
    Gordy Member Posts: 9,546
    Totally missed that Ironman. try to keep your question in your initial thread. It's beneficial in solving the whole issue
  • Gordy
    Gordy Member Posts: 9,546
    SO you need primary secondary piping. This allows the boiler to maintain its required flow rate while allowing the secondary piping to operate at a different flow rate. In your case a higher flow rate then what your current set up allows. This require a circ for the primary loop, and a circ for the secondary loop. Both have to be properly sized for each primary, and secondary circuit.

    BUT you also need to decide your boiler from your other thread.

    IRONMAN will have all that in his design.