What size pex ?

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Heeltee
Heeltee Posts: 20
in Radiant Heating
I am replacing 3/4 copper base borad & copper pipe with cast iron radiators and pex . 5/8 or 3/4 ? I want to say with 3/4. Any suggestions

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  • Gordy
    Gordy Posts: 9,546
    Depends

    On the btus, and flow rates required for the emitters to satisfy the load.





    You changed base board styles. Are you putting back the same amount of output as you removed?



    Was there insulation upgrades to the home since tha original baseboard?







    The bigger the supply pipe the less head, and smaller pump needed.
  • Bobbito
    Bobbito Posts: 9
    3/4

    A smaller pipe = more head (pressure) loss = less pressure to move the water = less heat.  Go with the 3/4 pipe.  If you're using pex be sure to get the pex with the oxygen barrier, slightly more expensive but absolutely worth it.  Just btw, I'm wondering if you're system is hot water or steam and why you decided to switch out the copper baseboards for iron radiators?

    I'm using Slant/Fin Multipak 80, 730BTUh / linear foot @ 180 degree F.
  • Gordy
    Gordy Posts: 9,546
    edited March 2013
    Gotta do the math

    Bobbito,



    You have no idea what btus are needed for the heatloss. How long the loops are etc. So how do you know 3/4" pex will work? Yes he's replacing 3/4" copper.
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