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What size pex tubing

I am new to using pex in place of copper for baseboard hot water heating. I converted my own house from forced air to baseboard hot water and used three quarter inch Wispro pex. I recently got to see an installation where half inch pex was used. This was in a 1000 sq.ft. addition. Also, the plumber did not install any air elemination device on the system. My question is, what is the proper size pex to use for baseboard hot water systems.


  • Brad White
    Brad White Member Posts: 2,398

    Pex-Al-Pex (PAP) I find to be ideal for a radiator run-out. I prefer PAP for it's higher temperature performance; it does not sag nor appreciably soften when run at higher temperatures (within it's listed rating of course!)

    More capacity performance using 5/8" rather than 1/2" and a lot less tough to wrestle with than 3/4".

    Mind you, this is for a single circuit/element, not the entire building. 5/8" will easily carry 2.0 GPM, enough to heat two decent size rooms. Like anything else, judgment is required.
    "If you do not know the answer, say, "I do not know the answer", and you will be correct!"

    -Ernie White, my Dad
  • Home Depot Employee
    Home Depot Employee Member Posts: 329

    All depends on the lenghts and circuits, radiation, circs.
    Sometimes 1/2, better with 5/8, may need 3/4 and on occasion 1" is required
  • Glenn Sossin_2
    Glenn Sossin_2 Member Posts: 592
    Friction vs Flow - choose the pex size

    First, you need to know how many gpm you are trying to run down the pex loop. Assuming you are using the standard 20F delta T, 1 gpm of water carries 10,000 btu's of heat.

    If you have 40 ft of baseboard on a given loop - that would be 40 x btuh output/ft (avg 580) = 23,200 btu's This would be equal to 2.3 gpm (23,200/10,000).

    Now you look at the friction for the pipe/pex you intend to use and see what the head loss is. Let us assume your heat loop is 100ft long including the baseboard. We need to deliver 2.3 gpm through 100ft of some size tubing. We also need to allow for fittings - adding 50% to your circuit length will usually cover you. Look at the attachment 'pex head loss'. Most of the time, you will want this total head number to be less than 8ft so you can use the common everday circulators like the Taco 007 or the Grundfoss UP15-42 or 58 .

    I plotted out the points on a Taco pump chart and a Grundfoss pump chart for you. In this example, 1/2" pex would have too much head loss, approx 150 x .1415 = 21ft 5/8" pex would be a better choice 150 x .0525 = 7.9ft.

    The space between your plotted point and the pump curve is extra head room. You should have a little cushion figured in there for a few extra ells or a few more feet of baseboard.

    So recap - know your total gpm and total circuit length. Then calculate the head loss - don't forget to allow for fittings. Check that number against your pump curve - leave a little headroom.

    Hope that helps you.

  • hot_rod
    hot_rod Member Posts: 22,158
    20,000 BTU/hr

    is do-able with 1/2 pex. As the "box store" employee :) mentioned length comes into play. Rare to need 1" on a typical residential job, but the numbers tell the true story.

    hot rod
    Bob "hot rod" Rohr
    trainer for Caleffi NA
    Living the hydronic dream
  • Bob Forand
    Bob Forand Member Posts: 305

    Glenn, that is a fantastic explanation. Thanks....
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