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Splitting heating loop - what size pex to use?

weekendguy Member Posts: 181
I have an overly long heating zone (250 ft) of 3/4" copper. I have a way to split it about half way and send a common return back to the boiler. I would then feed both ends of the 3/4" from the boiler. I would love to use orange pex for this. Normally I would conclude that 1" copper should be sufficient to return (2) 3/4" but with pex having a smaller ID I am hesitating. The return will be almost a straight shot with one sweeping 90 about half way. I have read both sides of this argument. I really don't want to work with pex any larger than 1" so if 1" is not sufficient I can run (2) 3/4" or (2) 1" side by side. 2 pipes is not preferred since it increases heat loss due to larger surface to volume ratio. Any advice on this quandry? I'd really prefer a single 1" if that will carry the load.


  • Big Ed_4
    Big Ed_4 Member Posts: 2,243
    I believe 65' of baseboard is the max for one loop ...Use A-pex and you will never go back
    I have enough experience to know , that I dont know it all
  • Zman
    Zman Member Posts: 7,447
    It really comes down to how you are pumping it and how many BTU's you are trying to get out of your emitters.
    If you do a sketch showing your configuration it would help.
    "If you can't explain it simply, you don't understand it well enough"
    Albert Einstein
  • Paul48
    Paul48 Member Posts: 4,469
    I believe from his other post, that he's splitting 69000 btus. So 1" should be fine, but you'll need a way to balance the split loop.
  • NY_Rob
    NY_Rob Member Posts: 1,370
    Remember to use oxygen barier pex....
  • weekendguy
    weekendguy Member Posts: 181
    @Hatterasguy @paul48 Nice to have you both weigh in on this question as well! Thanks for the reminder of the glove valves. Will absolutely add them. Oxygen barrier pex of course.
  • weekendguy
    weekendguy Member Posts: 181
    @Hatterasguy @Paul48 Putting together a parts list. Will I need to replace the 3/4" zone valve with a 1" since I am splitting its output to (2) 3/4" runs? Or is that few feet of pipe and valve short enough as to not create too much friction (and therefore the water can speed up)?
  • bob eck
    bob eck Member Posts: 930
    Look at using Viega Fostapex 1" tubing. 1" pex with aluminum as the oxygen barrier. This tubing will not grow in length and does not droop when running hot boiler water through it. When using regular radiant pex it does grow in length and you will need to support it a lot more than the Viega fostapex tubing.
  • weekendguy
    weekendguy Member Posts: 181
    @Hatterasguy @paul48 Sorry about all these questions. I would prefer to use sweat globe valves - I always have more problems installing when I adapt threaded to copper (threads leaking). I see that 90% of all brass globe valves are threaded. Is there a reason? I expect that these valves will be left alone once set. Any reason not to use sweat? This is one I have identified:


  • Paul48
    Paul48 Member Posts: 4,469
    They're threaded for ease of replacement.
  • weekendguy
    weekendguy Member Posts: 181
    edited November 2016
    @Hatterasguy That's the only valve I could find in sweat that wasn't at least $60 or even $200. I think I will be ok on clearance. Also after talking the job over with my son we decided to go with (2) runs of 3/4 pex considering the difficulty of the route we need to take and the stiffness of 1". I will add rubatex to both. Thanks again.