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Pex run length

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jtpfarm
jtpfarm Member Posts: 46
edited December 2022 in THE MAIN WALL
I'm finally finishing the floor heat under my subfloor in the house. Our house has a 30x40 open room with 16x20 being carpet and the rest hard floor. As I did between our carpeted bedroom and hard floor bathroom, I'd like to have separate runs under the carpet and hard floor to adjust flow differently. 

Here is my issue, if I have a single run under my carpet I'm going to be about 450' including my home runs. I don't have enough room on my manifold to add another run. Is 450' going to be too much with a grundfos 15-58FC pump in my application?

Thanks


Comments

  • Dave Carpentier
    Dave Carpentier Member Posts: 604
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    I push 350-400ft for my garage loop using an Alpha2 on constant pressure mode of 12ft. It flows pretty slow, but it flows.
    Your pump will do nearly 19ft at 0.5 gpm on speed-3, and 17 on speed-2.



    30+ yrs in telecom outside plant.
    Currently in building maintenance.
  • EBEBRATT-Ed
    EBEBRATT-Ed Member Posts: 15,767
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    Why not make two runs and split it at the manifold? Cut the pd in half. 450 sounds like a lot to me. what size pex and brand?
    Zman
  • Zman
    Zman Member Posts: 7,589
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    There are 2 issues with the 450’ idea and they are both working against your goal of more heat under the carpet.
    1. With 450’, the flow will be greatly reduced.
    2. With that much distance (and the reduced flow) your delta t will be greatly reduced and you will have significantly reduced output.

    IMO you don’t need a different flow rate to the carpeted area (higher temp would help but that is more challenging)
    "If you can't explain it simply, you don't understand it well enough"
    Albert Einstein
    GGross
  • Solid_Fuel_Man
    Solid_Fuel_Man Member Posts: 2,646
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    450' with 1/2" pex? 
    Serving Northern Maine HVAC & Controls. I burn wood, it smells good!
  • jtpfarm
    jtpfarm Member Posts: 46
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    Zman said:
    There are 2 issues with the 450’ idea and they are both working against your goal of more heat under the carpet. 1. With 450’, the flow will be greatly reduced. 2. With that much distance (and the reduced flow) your delta t will be greatly reduced and you will have significantly reduced output. IMO you don’t need a different flow rate to the carpeted area (higher temp would help but that is more challenging)
    The reason I am even considering this is because I want the flow slowed down under the carpet. In the other rooms I have the flow slowed WAY down under the carpet. Reason being the delta t is only 2 or 3 degrees on that run because the carpet insulates it. I would say my flow is turned down to 15%-20% under the carpet to get delta to 10 degrees.
  • EBEBRATT-Ed
    EBEBRATT-Ed Member Posts: 15,767
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    The carpet is insulation you would think you need more heat under the carpet not less
  • GGross
    GGross Member Posts: 1,076
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    @jtpfarm

    Slowing down flow will lower overall heat output. Lower flow results in a lower average water temp in a loop which will result in a lower amount of heat being put into the space.

    What tube spacing are you using? at 12" centers I show a 350' loop, or 430' approx at 8" centers for that 16x20 space. Using an estimate 25 BTU /sq ft in that carpeted area, estimated carpet as R2 It looks like you need to move only 0.93 GPM on that long 450' loop but you will have 14' head from the circuit head loss only, add the pressure drop from your manifold (approx 2 ft water) so you need to move about 1 GPM at 16' head loss, and that is only for this loop, and not including the added pressure drop from all of your piping fittings and valves. Now you would need to add the flow rates of all connected circuits, this is likely your largest pressure drop, and see if that would work with your pump. (my guess is no, or it will work until you get to design day)

    I would cut the loop into two loops, without question but that's just me.





  • GGross
    GGross Member Posts: 1,076
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  • jtpfarm
    jtpfarm Member Posts: 46
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    GGross said:
    @jtpfarm Slowing down flow will lower overall heat output. Lower flow results in a lower average water temp in a loop which will result in a lower amount of heat being put into the space. What tube spacing are you using? at 12" centers I show a 350' loop, or 430' approx at 8" centers for that 16x20 space. Using an estimate 25 BTU /sq ft in that carpeted area, estimated carpet as R2 It looks like you need to move only 0.93 GPM on that long 450' loop but you will have 14' head from the circuit head loss only, add the pressure drop from your manifold (approx 2 ft water) so you need to move about 1 GPM at 16' head loss, and that is only for this loop, and not including the added pressure drop from all of your piping fittings and valves. Now you would need to add the flow rates of all connected circuits, this is likely your largest pressure drop, and see if that would work with your pump. (my guess is no, or it will work until you get to design day) I would cut the loop into two loops, without question but that's just me.
    My reasoning for slowing it down was to get the delta t wider and give the run time to transfer its heat into the room. These are running at 160 degrees. But perhaps I'm wrong in that thinking. Maybe I should be slowing down the hard floor instead?

    As I said, this is between floor trusses. So I have a down and back run between each set of trusses. So in that 20' are I have 12 spaces between trusses that have a down/back loop.

    Thanks for your thoughts so far.
  • hot_rod
    hot_rod Member Posts: 22,396
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    jtpfarm said:


    GGross said:

    @jtpfarm

    Slowing down flow will lower overall heat output. Lower flow results in a lower average water temp in a loop which will result in a lower amount of heat being put into the space.

    What tube spacing are you using? at 12" centers I show a 350' loop, or 430' approx at 8" centers for that 16x20 space. Using an estimate 25 BTU /sq ft in that carpeted area, estimated carpet as R2 It looks like you need to move only 0.93 GPM on that long 450' loop but you will have 14' head from the circuit head loss only, add the pressure drop from your manifold (approx 2 ft water) so you need to move about 1 GPM at 16' head loss, and that is only for this loop, and not including the added pressure drop from all of your piping fittings and valves. Now you would need to add the flow rates of all connected circuits, this is likely your largest pressure drop, and see if that would work with your pump. (my guess is no, or it will work until you get to design day)

    I would cut the loop into two loops, without question but that's just me.






    My reasoning for slowing it down was to get the delta t wider and give the run time to transfer its heat into the room. These are running at 160 degrees. But perhaps I'm wrong in that thinking. Maybe I should be slowing down the hard floor instead?

    As I said, this is between floor trusses. So I have a down and back run between each set of trusses. So in that 20' are I have 12 spaces between trusses that have a down/back loop.

    Thanks for your thoughts so far.

    A few things can be learned from this graphic. First is the heat output of the 3 various flow rates. It is the AWT average water temperature in the loop that makes a difference inn the heat output.
    Also notice the temperature at the end of this 300' loop at the slower flow, wider delta.

    You might not want to take advice from anyone that tells you slowing the flow to get more "linger" time in the loop, increases heat output :)

    You'll not get much heat output on the tail end of that longer loop, certainly not at wider delta T operation.

    Tighter delta takes a bit more pumping power, with ECM circs it really not much of an issue, if power consumption is a hangup?
    Bob "hot rod" Rohr
    trainer for Caleffi NA
    Living the hydronic dream
    Solid_Fuel_Man
  • GGross
    GGross Member Posts: 1,076
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    @jtpfarm

    My example is showing a fairly wide delta T for infloor heat at 20 degrees, this is just to show what it would take to heat the space, not built for maximizing comfort. To adjust this for a more usual 10 degree delta T, and take into account that this is joist underfloor heating, with all other estimated parameters the same as my previous post. You would need to pump almost 2 GPM, at over 50' head loss, the 15-58 is too small, but never mind that because the requirements are out of whack. You need to do 2 loops in that space in my opinion


    as for the reasoning behind your flow requirements, it is not the case, faster flow will increase the average water temp, which will result in higher output from your emitters, exception to this possibly being large floor standing cast iron radiators which becomes a hot topic of debate around here every now and then

    defer to what @hot_rod has said regarding this topic