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Btu/FT from pex

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Johnny6
Johnny6 Member Posts: 7
in THE MAIN WALL
I was wondering if there is a pex chart for btus/ft emitted, similar to those for baseboard.

I have been using a heatloss program that does not calculate the supply and return values of leaders from the loop in a room to a manifold.

I am hoping to find a number for btu/ft provide by 1/2" pex in concrete at 100 degree water.

If there is formula or a chart or direction in in how to figure it out the help would be greatly appreciated.

John 

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  • Zman
    Zman Member Posts: 7,569
    edited May 2012
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    More info

    There are a bunch more variables you need to figure this out.

    GPM

    Length of loop

    Tube spacing

    Thickness of slab

    Position of tubes within slabs (usually incorrectly at the bottom)

    Under slab insulation

    I think that is it. Seigenthaler's "radiant precision" has all the formula's
    "If you can't explain it simply, you don't understand it well enough"
    Albert Einstein
  • Johnny6
    Johnny6 Member Posts: 7
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    ?

    I figured I would have to go through it all but was wondering if there was a 'safe' number for a standard 12" spacing 250' loop in a basement that would cover worst case scenario.
  • hot_rod
    hot_rod Member Posts: 22,139
    edited June 2012
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    RadPad

    is a handy tool for quick "ballparking". I think they are still for sale at the RadiantProfessionalAssociation website.



    In a concrete slab figure about 20- 22 btu/ sq. ft. with a 250 foot loop flowing . 5 gpm with 12" on center tube layout. Slab surface temperature 83- 85F air temperature 70F.



    The slab output has a lot to do with the delta T between the surface temperature of the slab and the ambient room temperature.



    hr
    Bob "hot rod" Rohr
    trainer for Caleffi NA
    Living the hydronic dream
  • Johnny6
    Johnny6 Member Posts: 7
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    Thank you

    This what I was looking for. A 20 degree delta T seems to the usual design for us.

    Thank you
  • Mark Eatherton
    Mark Eatherton Member Posts: 5,853
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    20 degrees of separation...

    Looks good on paper, but rarely happens in the real world, except when accelerating a slab from a cold start. In reality, the DT is more like 7 to 12 degrees F



    Adjust accordingly.



    ME
    It's not so much a case of "You got what you paid for", as it is a matter of "You DIDN'T get what you DIDN'T pay for, and you're NOT going to get what you thought you were in the way of comfort". Borrowed from Heatboy.
  • bill_105
    bill_105 Member Posts: 429
    edited June 2012
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    Speaking of DT

    Well here is a good a chance as ever to ask.

    Can one of you smart guys carry on a sermon on delta T?  Okay, 20 with baseboard and 10 with radiant, fine. I'm aware of the GPM  requirement, But who came up with these magic numbers? Why are they so significant. What is trying to be accomplished or avoided?

    With someones help I'd be better at carrying on like I know what I'm talking about ")

    Thanks in advance.
  • Gordy
    Gordy Member Posts: 9,546
    edited June 2012
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    Delta t

    10,15,20,30 delta ts are used to calculate design requirements of the heating system. Like Mark says rarely will these delta ts be those numbers when a system is warmed up, and running. You will probably only see those design numbers from cold start say unoccupied home comming on line from deep setback. Otherwise that delta t is a narrower band.





    Delta ts are a good trouble shooting tool also.

    Gordy
  • hot_rod
    hot_rod Member Posts: 22,139
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    delta T in radiant loops

    If the delta T between the supply and return is too wide the last part of the loop would not have much heat output, and you would experience cool spots in the floor.



    OIn a commercial slab, or snowmelt the wider delta would not be so noticeable. same with a commercial radiant slab where bare-foot comfort was not a concern.



    When you start a slab on a cold winter day, for the first time the delta T will be huge, and it may take days to see it drop down to 20 or even 10. Low mass emitters like fin tube baseboard will accelerate much more quickly.



    I agree the delta T is a moving function based on load and conditions. which makes me wonder why a delta T pump would be desireable, trying to always nail the delta T at any given point in the systems operation? Don't you want, and need that large delta T on start up to move the most amount of energy?



    Mod con boilers like low return water, so starting a cold, cold slab makes them happy, if a delta t pump limited that cold return, the boiler efficiency would suffer and the slab response would be longer.



    hr

    hr
    Bob "hot rod" Rohr
    trainer for Caleffi NA
    Living the hydronic dream
  • Zman
    Zman Member Posts: 7,569
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    I agree

    I agree that Delta T circs are not appropriate for radiant (or any other "parallel" application). A delta P would be far better.

    They do have a place in a low mass series installation, like so many of the hot water baseboard retrofits we are seeing. They also have great potential as a zone circ for air handlers and for mixing assembly's.Many have built in outdoor reset capabilities.

    Carl
    "If you can't explain it simply, you don't understand it well enough"
    Albert Einstein
  • hot_rod
    hot_rod Member Posts: 22,139
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    Why wouldn't

    the same concept apply for low mass or air coils. If the room is cold on first start up the fluid would have a high delta T then close up as the room warms up. How does locking the pump to a specific delta T provide an advantage.



    Maybe for injection mixing or some process that has a temperature tolerance requirement?



    hr
    Bob "hot rod" Rohr
    trainer for Caleffi NA
    Living the hydronic dream
  • NRT_Rob
    NRT_Rob Member Posts: 1,013
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    I"m not typically into delta-T pumping

    but it sounds suspiciously like you're saying you'd get more heat out of a loop with less flow there Hot Rod. Raising the flow would lower delta-T but still increase potential heat output.



    if you've already maxed your heat source, so be it, but in our very low flow system designs (20 loop DT standard design) under startup you could be in a condition where your larger initial DT still doesn't fully capitalize on the heat source's ability to generate heat.



    otherwise, I could see single zone/constant circ dt pumping as a good way to reduce pump energy usage. but you can get the same savings just going ECM. ECM DT pumping would be king in that case. and of course as soon as you add zoning I'd go to DP.
    Rob Brown
    Designer for Rockport Mechanical
    in beautiful Rockport Maine.
  • Zman
    Zman Member Posts: 7,569
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    Low mass

    A low mass system will reach a "steady state" operating condition much quicker than high mass. I guess my point is if you are designing based on a delta T. Why not stick to the design and use as little electricity as possible. A properly sized series baseboard zone will run very comfortably and efficiently on delta T. I do have reservations about delta t on parallel and zoned applications.

    Bill,

    Next time ask something less controversial like: What is the best truck or what beer tastes better!

    Hah.



    Carl
    "If you can't explain it simply, you don't understand it well enough"
    Albert Einstein
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