Welcome! Here are the website rules, as well as some tips for using this forum.
Need to contact us? Visit https://heatinghelp.com/contact-us/.
Click here to Find a Contractor in your area.


drhvac Member Posts: 190

How critical is sizing pex piping? I have never used it before, and I notice the fittings and the piping itself are alot smaller than copper fittings. I have a buddy who is doing his own baseboard in his own house. I've done the heat load for him, and sized all the baseboard for him, and he is installing alot of it himself. When i went to check up on how he is doing, i find that he used 3/4 in pex on just about everything, including these small brass 90's that have a very small inside diameter. I know he was talking about using the pex, but i told him to sweat a piece of copper with a 90 to get into the basement, then just use the pex to connect to the next baseboard. But like I said he has the small brass 90's on every turn, and I am concerned about noise and flow. When I told him about this, he called a plumber friend of his, and the plumber told him he has never had a problem with the pex, and he uses 1/2" pex. Will this be an issue? I mainly work with duct work, and when sizing that, bigger is better with least amount of resistance. I would think the same be true for piping, and that is what i have read.

Thank you in advance


  • meplumber
    meplumber Member Posts: 678
    Most of the PEX manu's have charts.

    The sizing is very similar to copper.  The instance you describe should be fine.  3/4 PEX can carry a substantial amount of BTU's with a 20 deg. delta T.

    I would have to see a drawing or more info on the layout to be sure, but I have run 5/8" pex between the baseboards for zones of less than 50' of BB with no trouble at all.
  • Ironman
    Ironman Member Posts: 7,378
    edited November 2010
    Pex Sizing

    What your friend is doing is probably not an issue. According to flow charts, Pex and copper have about the same gpm at any reasonable head. What is different is the velocity: the pex has a noticeably higher velocity at the same gpm. You don't want to exceed 4 ft. per second or you will have noise and wear issues. If this were a radiant job where the loops are much longer, the barb fittings would be an issue.

    As a rule of thumb, I normally figure you get 20% less capacity from Pex.

    3/4 Pex can carry about 4 gpm @ 4 ft. per second. Type M 3/4 copper can carry about 6 gpm @ 4 ft. per sec. That's pushing it for either one in a hydronic system.

    You need to determine from your heat loss how many gpm you need in any given loop. A simple formula: at 20deg. delta T, you get 10mbh per gallon. Hence, 4 gpm = 40mbh.

    The universal hydroics formula: BTUh = GPM X 500 X Delta T.

    Make sure your friend is using o2 barrier pex, not the regular kind that is used for plumbing. He'll ruin the boiler with regular pex.
    Bob Boan
    You can choose to do what you want, but you cannot choose the consequences.
  • drhvac
    drhvac Member Posts: 190

    Yes he is using the right pex pipe. My main concern was all those barb fittings. They are so small inside. But thanks for the help, I have a little more confidence now that it will work.
  • bob eck
    bob eck Member Posts: 930

    next time check out Viegs Fostapex pipe. It is pex pipe with a aluminum barrier that is the oxygen barrier and the aluminum keeps the pex pipe from saging and expanding when running hot boiler water thru it. comes in 1/2" - 3/4" - 1" sizes in 20' lengths or 150' rolls. I added three cast iron radiators to my condensing gas triangle tube PE110 gas boiler and ran 1/2" fostapex out and back to each radiator and so far working great.  
  • zacmobile
    zacmobile Member Posts: 211
    1/2" pex

    I have never run larger than 1/2" pex to any radiator and sometimes have used 3/8" when fishing through wall & floor cavities in retrofits. radiators require very little flow to operate (it would be a rare day that you would ever require 4 gpm in one rad more like .5-1gpm) and generally have very little flow resistance, you would more than likely be fine with 1/2".
This discussion has been closed.