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Delta T and Thermal Mass

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Stokehold
Stokehold Member Posts: 43
I realize that bringing the concrete floor of a basement or garage up to temperature may take some time depending on prevailing conditions and also whether the job was insulated properly.
My question is should the Delta be within design specs immediately or will it take time for return temps to come up? In other words, is Delta T represented by the radiant surface only after it reaches design temperature?
As always, thanks to all who respond!

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  • Zman
    Zman Member Posts: 7,572
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    I have yet to see a concrete slab that exactly matches the design.
    I cold slab will not even be close. You will see a very wide delta until the slab approaches steady state conditions.
    Heating the slab slowly using outdoor reset control is highly recommended.
    "If you can't explain it simply, you don't understand it well enough"
    Albert Einstein
  • Jamie Hall
    Jamie Hall Member Posts: 23,415
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    Zman said:

    ...
    Heating the slab slowly using outdoor reset control is highly recommended.

    In fact, don't even try to bring it up to temperature fast, and certainly not by raising the input temperature. You may crack the slab.

    Br. Jamie, osb
    Building superintendent/caretaker, 7200 sq. ft. historic house museum with dependencies in New England
  • Gordy
    Gordy Member Posts: 9,546
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    Design deltas are at steady state at design oat.

    Concrete slabs are slow to charge, and slow to release. Which with out proper control leads to overshoot of setpoint. Fly wheel effect. As mentioned use proper controls.
  • Stokehold
    Stokehold Member Posts: 43
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    Thanks for the input!
    I had no control over the actual floor install. The home had pex lines sticking out of the floor when I purchased it.
    I simply did a pressure check and then calculated length of pex by calculating volume. That was tedious in itself because I had to fill each loop and then carefully drain the water and measure.
    It worked out to five loops at 290 ft. per. I built everything else from manifold back to boiler.
    Anyway, to make a long story short, after supplying 110° for 18 hours, the return temps were still lagging by 10°. The return was sitting at 80°. The design was a 40,000 BTU load, 4 GPM with a 20°Δ.
    According to my flow meters, I can't seem to push more than 3 GPM. Head loss calculations fell within the parameters of the pump but who knows how that tubing was installed (i.e. kinks or too close bend radius) that may be affecting flow. Half inch pex has a 300 ft. max as it is.
    Anyway, the basement does get warm and I should mention that 1260 square ft. of heated slab is also heating another 680 sq. ft. of unheated basement.
    So, finally, should I be happy or go with a higher head pump and get that flow up to an easy 4 GPM?

    Zman
  • Mark Eatherton
    Mark Eatherton Member Posts: 5,853
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    I think you're doing fine. Once the mass is completely charged, and we're not just talking about the slab, the DT will probably drop to less than the 20 you designed for. Kudos on your effort to confirm tubing length. Not many DIYer's would have the wherewithal to think of doing it that way. Tedious, but extremely accurate.

    ME

    There was an error rendering this rich post.

    ZmanCanucker
  • Stokehold
    Stokehold Member Posts: 43
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    Thank you for your vote of confidence, Mark.
    You guys at the Wall should be commended for you input.
    Believe me, it is really appreciated!
    Mark EathertonratioMilanD
  • Gordy
    Gordy Member Posts: 9,546
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    18 hours from a cold start is darn good. You should be happy with you success!