Welcome! Here are the website rules, as well as some tips for using this forum.
Need to contact us? Visit https://heatinghelp.com/contact-us/.
Click here to Find a Contractor in your area.

I am amazed that nobody has ever had this problem before. Not even mfg.

chingo Member Posts: 22
Watts radiant heating system.
3 zones all exhibiting same issue.
Purged with 1 hp submersible pump.(manifolds done individually, the main loop, then combined)
1 zone 8 loop manifold same elevation as main loop (all runs 250' 1/2" pex)
1 zone 8 loop manifold 6 ft above main loop (all runs 250' 1/2" pex)
1 zone 4 loop manifold 6 ft above main loop (all runs 250' 1/2" pex)
All pumps Grundfos UPS 15-58 set to med.
Calculated head .03 * 250 = 7.5 ft of head
On pump curve that gives a flow rate of approx. 7-8gpm.
Test on Zone same level as main loop. 8runs
ACTUAL test results using 1HP open loop injected between exp tank/bubbler = 3.2gpm total (approx. .4gpm run)
ACTUAL test results using 1HP open loop injected directly into manifold = 4.1gpm total (approx. .5gpm run)
ACTUAL test results using UPS15-58 closed loop 12psi. @manifold .75 gpm toatl (approx. .08gpm run)
Hi speed very little improvement.
Change to UPS 26-99 very little improvement.
Other zones exhibiting same characteristics.

To sum it up i could get my flow if i put a 1 hp circ on each zone.
But that just seem ridiculous.

Thankyou for your time and any help you may be in this issue.

P.S. Nothing helpful to say, why say it.


  • Snowmelt
    Snowmelt Member Posts: 1,334
    most likly not insulated properly and not heating correct area
  • chingo
    chingo Member Posts: 22
    No one has come close to figuring it out and snowmelt comment has nothing to do with flow
  • chingo
    chingo Member Posts: 22
    A good answer might of been mixing valve not functioning since it is common to all 3 zones
  • chingo
    chingo Member Posts: 22
    yes hatterasguy but i am trying to clarify issue, it seems there is a little confusion on it, but your right i have probably exhausted the possibilities on this forum
  • Gordy
    Gordy Member Posts: 9,514
    Where there is a problem there is a solution.

    There is no better place than this site to find the later.

    Your not adding the head loss of the 3/4 supply and return to and from the manifold. How long are those.
  • Zman
    Zman Member Posts: 7,334
    Ok, first off, when you do 4 posts on one subject a whole lot gets lost.
    The system that works (I assumed to be the one you are talking about) has injection mixing with no flow resistance at the point of mixing.

    The one that does not work has a mixing valve. These can add lots of resistance. This is your entire problem (although at .5 gpm it is debatable that you have a problem) What model is the mixer?

    You posted a picture of the pour earlier that showed the tubing at the bottom of the slab, not pulled up at all. You BS you friends and I will BS mine, let's not BS each other...

    Your welcome,
    "If you can't explain it simply, you don't understand it well enough"
    Albert Einstein
  • hot_rod
    hot_rod Member Posts: 16,332
    Disassemble the 3 way mixer, remove the guts and run the system. I'd be willing to bet that is the flow issue.

    You would need to add a flow meter or indicator to know for sure.
    Bob "hot rod" Rohr
    trainer for Caleffi NA
    Living the hydronic dream
  • Ironman
    Ironman Member Posts: 6,877
    Also, a thermostatic (dumb) mixing valve is not the correct component for controlling slab water temp. It will cause the slab to overheat. You need injection mixing or a smart valve with ODR.
    Bob Boan
    You can choose to do what you want, but you cannot choose the consequences.
  • 4Johnpipe
    4Johnpipe Member Posts: 477
    edited October 2015
    It may also be the location of the pump on the mixing valve. Not sure if this was mentioned yet. I would also recommend a smart mixing valve.
    Considerate People, Considerate Service, Consider It Done!
    email: [email protected]
  • KBP&H_3
    KBP&H_3 Member Posts: 67
    What is the return water temperature? , 1- from the slab, 2, to the boiler ( after the mixing valve), a taco I valve will control the return temperature so the boiler can recover and set the max supply temperature. A standard mixing valve doesn't work with high load slabs. The return temp could be low enough that the boiler just cant put out enough to heat the system. There is no mention of any water temperature, just flow. Heating requires moving not only water, but also BTU's.
  • Gordy
    Gordy Member Posts: 9,514
    This issue has been resolved. It was the mixing valve. The OP was using a domestic mixing valve which has a high cv.