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High delta T

My shop is 32x60 with 18' walls. Walls are 4" of rigid foam. Floor is 6" slab with vapor barrier and 4" of foam. Have 8 300' 1/2 per loops in slab. Using a toyotomi OM-128 oil tankless boiler. Have 2 zones in system with 15-58 grundfos circ pumps. One zone has 5 loops the other 3. Thermostat are set at 54. Delta runs between 50-70 no matter what I have done. On the 5 loop zone manifold reads .5gpm per loop. If I close 4 it will go to 1gpm. Any input would be great.


  • GroundUp
    GroundUp Member Posts: 1,496
    That's 100-140,000 BTU. Are you sure those are accurate temps and flow rates? What is the supply water temp? Where is the tubing placed in the slab? Is this a continuous issue throughout the heat call or just at startup? How warm are you keeping the shop? Just speculation, but I'm going to step out on a limb and guess your supply temp is far too high and the tubing is at the bottom, causing the huge delta as the slab is only half the temperature of the supply water. Just for grins I'll guess a 150 SWT and a 65 degree ambient air temp. Am I close?
  • What's your tube spacing? What's your question?
    8.33 lbs./gal. x 60 min./hr. x 20°ΔT = 10,000 BTU's/hourTwo btu/ per sq ft for degree difference for a slab
  • SuperJ
    SuperJ Member Posts: 605
    Tankless heaters are made for high Delta's which normally makes them not so great for space heating. Hopefully your piping strategy is hydraulically separating the high pressure drop tankless from your load circuits.
    What's the Delta for your circuits in and out of the slab?
  • Voyager
    Voyager Member Posts: 378
    Pictures are still worth a 1,000 words. Is high delta T your only concern? Is the shop heating OK? Is fuel consumption high?
  • eyakoutlet
    eyakoutlet Member Posts: 4
    The tube space 12" and is at bottom of slab right on foam. I have used a couple different thermometer and all say the same delta of 50-60. Supply temp is 130. Shop thermostat is 52-54. I will take some pictures of distribution layout. Shop heats fine but seems a little heavy on fuel consumption. I'm concerned about the short cycling on heater.
  • Voyager
    Voyager Member Posts: 378
    edited December 2018
    That is unfortunate. Having the tubing under 5”+ of concrete means that you are going to have very slow response and you will put a lot of heat into the insulation and then the ground than you need to. Are the slab edges insulated? If not, you will also lose a surprising amount of heat through the slab edges, particularly given the low tubing placement.

    The tubing ideally would have been placed in the center or even slightly above center of the slab.

    I am still surprised that your delta T stays that high. Mine ran about 25 degrees when I was initially heating the slab, but now that it is at steady state I run 15 degrees and sometimes less. I have 3/4” pex in roughly the center of a 5” slab with 12” spacing and loop lengths of about 450’ give or take. I run 75 - 80 SWT and RWT runs 60-65 in moderate weather (20s outside). I suspect it will be a little higher once it gets cold out.
  • Gordy
    Gordy Member Posts: 9,537
    edited December 2018
    Tubing at the bottom is not ideal at all. That’s a lot of mass for 12” center tubing to manage efficiently at the bottom of 6” of concrete.
    You do have good under slab insulation however.

    Is the perimeter of the slab insulated?. That can account for huge edge losses.

    When you are talking about the delta t. Are you referring to the difference in room temp, and supply water temp, or the difference between supply, and return water temps? I ask because a return water temp is not mentioned.

    What are the return water temps?

    This unit is not a tankless water heater. It is stated as usable for heating applications.

    It is not a modulating unit there for it will short cycle since it is probably not seeing any where near it’s full output for the load.

    What is the differential dip switches set at? You have a choice from 3.6 degrees or 36 degrees. The higher delta will help reduce short cycling.
    Flow rates through that unit should be 6 gallons per minute. This will narrow the delta. Can go down to a little over 4 gpm which would result in a wider delta. Which is about what you are running if your flow meters are correct.
  • eyakoutlet
    eyakoutlet Member Posts: 4
    I am wondering if I have enough pump to move that much fluid. When I convert .5gpm to fps for 1/2 pex i get .87fps. Everything I have read says that's not fast enough to get to turbulent flow. Supply water temp is 130 and return is 70-80. The closest delta I have seen was 120 and 90 with only 1 loop out of 8 open.
  • Gordy
    Gordy Member Posts: 9,537
    edited December 2018
    Yes that’s what I was alluding to.

    High delta is usually an indicator of low flow, or high load.

    If this has been running for a while, and I mean a couple weeks with all that,mass then you have a low flow issue.

    The HX is probably high head, and should be piped primary secondary.

    Really that is a oil fired hybrid instantaneous water heater you are using. It has a 5 gallon tank. It alludes to hot water heating using “hydronic” in its advertising. It’s also rated for space heating, but on the web site in their sales video nothing is mentioned about using it for space heating.

    Since it’s 128k output with out modulating abilities it will short cycle. I highly doubt your heatloss is close to that.
  • Zman
    Zman Member Posts: 7,435
    How is it piped? Pictures would help.
    Your delta and flow rates indicate that it is running flat out and not cycling at all. Are you seeing short cycles?
    I am not convinced that you have a laminar vs turbulent issue with the flow rates. You are obviously unloading BTU's. The only issue with a delta that high is that it will not heat the floors very evenly.
    "If you can't explain it simply, you don't understand it well enough"
    Albert Einstein
  • eyakoutlet
    eyakoutlet Member Posts: 4
    System has been running for a couple of years but I know it's not ideal and want to add DHW to the building so Now is time to address plumbing issues and wonder if install an indirect water heater and use it as a buffer tank as well.