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sizing pex home runs to radiator

Excellent idea Scott on adding a few plates here and there on the returns. I would imagine a few chosen spots such as in front of sinks, outside entrances are appreciated.


  • Mike F
    Mike F Member Posts: 23
    sizing pex home runs to radiators

    I would like to run 3/8 pex home runs to a mix of ci and panel rads. the largest of which is 7500 btu's . Am I making a mistake not running 1/2.
  • Timco
    Timco Member Posts: 3,040

    I run 2-pipe zones all in 3/4 to CI rads.

    Just a guy running some pipes.
  • Home Depot Employee
    Home Depot Employee Member Posts: 329

    Unless it is a rediculous long run, I don't see why 3/8" wouldn't work in your application as the rad requires less than 1 GPM.

    Save your money on the larger PEX
  • Brad White_200
    Brad White_200 Member Posts: 148
    I use 5/8\" Pex-Al-Pex

    and not just to be different...

    The OD is the same as 3/4" copper so is easy to support with clips. Holds it's shape and the flow characteristics are wide enough to handle the smallest to the largest radiators. I even have served two large radiators via a 1.25" old gravity riser from a single 5/8" circuit.

    Everyone has choices.
  • hot_rod
    hot_rod Member Posts: 21,853
    3/ 8 or 1/2

    would carry ,75 gpm easily. 100 feet of 3/8 flowing .75 gpm of 160F water would have a pressure drop of 8.5 feet of head. easily within reach of a typical 80 w circ.

    At a 35 degree delta T in the circuit, common in Europe, you could move 15,000 BTU/ hr @ 10.7 feet of head pressure drop.

    Consider TRV's and a delta P circ for the best efficiency on the distribution side.
    Bob "hot rod" Rohr
    trainer for Caleffi NA
    Living the hydronic dream

  • and PEX-AL-PEX to reduce noise issues!

    It's pretty rare to need more than 1/2" pipe for any single, dedicated radiator.
  • scott markle_2
    scott markle_2 Member Posts: 611

    Nothing pulls or handles quite as nice a 3/8. Two pipes pull smoothly through a 1.125 inch hole.

    Noise is not much of an issue in constant circulation systems. Temperature changes are gradual and don't cause the rapid expansion noises that can occur in on/off regulated systems. Full reset constant circ. also makes me more comfortable with the possibility of long term pex chaffing issues.

    I'm doing a 3/8" home run panel radiator job right now. On some of my shorter runs I'm extending my return leg a bit and doing extruded plates for a little bonus radiant in selected areas.
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