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.

Garage slab - Flow drops off to below heater threshold

g1teg Member Posts: 3
I installed this last spring as I was finishing up my garage build, the temp came up outside, and I put the floor heat to the back of my mind... its getting cold again.

I have 2 loops. 1/2" 02 pex, both under 250' in length.
DHC-E 12 electric heater
2 UP15-42F Grundfos pumps
Taco control (single zone)

The issue is that it will pump/heat for about 8 minutes. During this time, I show about 0.5 gpm on each loop. Slowly the flow drops, until it is below the flow that the heater needs, and it will shut off but keep pumping, very slowly.

I feel like I have purged all of the air. I've gone back and forth, upgraded my purge pump, reversed the flow. I've tried with just one of my 2 circulator pumps, tried in primary/secondary loop setup, and single loop.
I need ideas of why my flow would be great for 5 minutes or so, then slowly drop off?


  • Jamie Hall
    Jamie Hall Member Posts: 21,544
    Do you have access to a pressure gauge, preferably two? I'm thinking that somewhere -- hard to say where -- there is a loop or bend which is getting kinked as the PEX expands (unless it's PEX-AL-PEX, which doesn't expand as much) and gradually shuts off the flow. You could check if you had two gauges by looking at the pressure drop through a loop.
    Br. Jamie, osb
    Building superintendent/caretaker, 7200 sq. ft. historic house museum with dependencies in New England
  • g1teg
    g1teg Member Posts: 3
    Ok that makes a bit of sense. but if that were the case, I shouldnt lose flow through both loops equally right? if anything one would drop flow, and the other would raise?
  • Jamie Hall
    Jamie Hall Member Posts: 21,544
    One would think so. But... the flow may not increase all that much, depending very much on the pump curve of the pump. Also, one gremlin like that might imply two... or more... or on a common pipe...
    Br. Jamie, osb
    Building superintendent/caretaker, 7200 sq. ft. historic house museum with dependencies in New England
  • hot_rod
    hot_rod Member Posts: 19,926
    s plugged strainer perhaps?
    Bob "hot rod" Rohr
    trainer for Caleffi NA
    Living the hydronic dream
  • g1teg
    g1teg Member Posts: 3
    The more reading I do, the more I think its a result of my primary/secondary tees being too far apart.

    I made my own Tees with a ball valve between, and I think the spacing is causing issue. From what I'm reading, this can heavily restrict flow through the heater.

    Will replace with this and see if it helps. https://www.menards.com/main/storeitem.html?iid=1531290472853
  • GroundUp
    GroundUp Member Posts: 1,654
    Can you provide a photo please? Regardless of tee spacing, the flow should not be dropping over time. If anything, it should be rising. A photo would help get a visual and may point out something obvious to one of us
  • Zman
    Zman Member Posts: 7,468
    Is there glycol installed?
    What type of air eliminator do you have?
    "If you can't explain it simply, you don't understand it well enough"
    Albert Einstein