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One zone in my house that I cannot get to warm up - Help

Spectrum Member Posts: 1
edited November 2023 in THE MAIN WALL
Hi Guys,

We built a home last year and I put in a slab heat system. Did it myself as there is nobody in my area that is reliable or knowledgeable. (Far north Idaho)

House is about 2700 sq/ft I only split it into 2 zones, one big 6 loop zone for the living area and one 4 loop area for the bedrooms and bath. There is also another 5 zones heating the attached 36 x48 garage.

Used 1/2" pex tied to re-bar on 18" spacing. Loop length varies between 220 to 300 ft.

This is powered by an 225,000 BTU NTI LPG boiler and a Central Boiler classic edge 560 OWB run thru a secondary loop heat exchanger. System pump is an Grundfoss UPS26-99FC three speed pump.

I have a problem where the system will not heat the bedrooms above 65 deg, and it hasn't gotten that cold out yet. (Wife likes the bedroom/bath at 70)

I have my manifold set at 2 GPM per loop and am seeing a delta T of approx 20 deg F

I have checked the flow rates on the bedroom manifold with all of the zones open and also just the bedroom zone open. They do not seem to fluctuate much, other than a few seconds when a zone opens.

This zone is constantly trying to heat, and at night the temp falls off of the 65 down to as low as 61 so far but it has only gotten down into the teens so far. We usually get a cold snap with temps as low as 10 deg below zero for some period of time in January.

I have purged each loop, one at a time, like I've seen in a few YouTube vids.

Am I missing something here? The large area of the house and the garage seem to heat fine. I can't get the bedrooms warm. Obviously, I probably should have spaced my loops closer, but that is water under the bridge at this point.

Any constructive comments are welcome, I am definately willing to make any changes to my plumbing above the floor if it will help. I will post a drawing of the above floor system if I can figure out how to post on this forum.

Thanks from North Idaho!


    HVACNUT Member Posts: 5,761
    You say 20° split but what is the supply water temperature?

    I don't know much about radiant design but 18" spacing seems like a lot. I'm often wrong so who knows? Others here know. 

    Are you using a slab sensor, room sensor, or both?
  • hot_rod
    hot_rod Member Posts: 21,882
    What type of floor covering? What supply temperature?
    Did you run a heat load calc to see how many btu/ hr the rooms need?

    8-12” is a typical tube spacing. How are you reading 2 gpm per loop. That is quite a high flow rate for 1/2” pex loops of that length
    Bob "hot rod" Rohr
    trainer for Caleffi NA
    Living the hydronic dream
  • Why the heat exchanger? The Grundfos 26-99 is too big as is 2 gpm. Should be more like .5/loop, but that’s not your problem. 
    8.33 lbs./gal. x 60 min./hr. x 20°ΔT = 10,000 BTU's/hour

    Two btu per sq ft for degree difference for a slab
  • hot_rod
    hot_rod Member Posts: 21,882
    Why the heat exchanger? The Grundfos 26-99 is too big as is 2 gpm. Should be more like .5/loop, but that’s not your problem. 
    Open type outdoor wood furnace, so a hx for the radiant
    Bob "hot rod" Rohr
    trainer for Caleffi NA
    Living the hydronic dream
    Alan (California Radiant) Forbes
  • Erin Holohan Haskell
    Erin Holohan Haskell Member, Moderator, Administrator Posts: 2,264
    Welcome @Spectrum. Here are some tips for posting photos and using the forum: https://heatinghelp.com/forum-user-manual
  • Paul Pollets
    Paul Pollets Member Posts: 3,656
    Loop spacing should have been 6" or 8", not 18". It's too late now. You'll probably need supplemental radiators to match heat loss loads or you'll be wasting BTU's trying to keep up with a load that can't be met.
  • Sylvain
    Sylvain Member Posts: 58
    have a look at the graph at about 6'15" in this video:
    As you can see, once the spacing is fixed, one has two parameters on which one can act:
    - water temperature and
    - floor finishing.
    Remove floor finishing if it has insulation properties.
    If there is a basement or a crawl space under, try isolating the underside of the slab.

    One will notice that the more the floor finishing provides insulation, the less the spacing change much the heat flux.