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Use PEX for zone heating

billchat Member Posts: 11
Hi forum
I was hoping to get some input . I currently have and want to keep my large cast iron radiators in the tudor I recently brought in long Island new york . I currently only have one zone hydronic for the whole house with only one circulator. I have a two inch supply and a two inch return . Every radiator is branched off the supply independently and returns independently to the two inch return.

I would like to add heat to the basement and also break it into zones.. I was debating having a manifold installed and break the room into zones.

Do you recommend pex with the oxygen barrier to supply the cast iron radiator (each ranging from about 3-6 foot) . Each radiator could have a supply and return to the manifold.

One of my concern do I have to worry about expansion within the system because of the size radiators. I know I will have to strap the pex alot because of expansion. But will the expansion tank be able to hold do I need another or lager or will it be more efficient. I think I would break the house into 4 - 6 zones (1)attic, (2)second floor, (3)half first floor because of a wood insert, (4) other half of first floor, (5)office stay warm because of insert (6) basement.

Other concern will I loose to much heat from the pex.

Any thoughts or recommendations are appreciated


  • vvzz
    vvzz Member Posts: 39
    If anything, your current expansion tank will be more appropriate since the system will have less volume if those big supply/return pipes go away.

    I have used Pex-Al-Pex for my system(restored cast iron radiators) and I was impressed with the lack of expansion problems. Regular o2-barrier pex just droops no matter how much you staple it.

    Are you replacing the boiler too? Take a look into constant circulaiton with TRVs on each radiator. I think that is much better approach than 6 zones.
    Bob Bona_4
  • Bob Bona_4
    Bob Bona_4 Member Posts: 2,083
    Pex-al-pex is the only product I'd recommend for high temp heating.
  • BenWoj
    BenWoj Member Posts: 33
    Pex with oxygen barrier is fine. My geo installer questioned the same thing about high temps when I was running a boiler. He did some research for his sake and found out the high temps weren't a problem.

    If your only stretching 3-6 feet you won't loose much heat.
  • 4Johnpipe
    4Johnpipe Member Posts: 480
    We just finished a job using HePex and a 10 port manifold. The customer optioned not to zone it. We have 1/2" running to each radiator drop and are able to have flows in the 1GPM or less range. We also have a much lower AWT (150 degrees) running a 30 degree Delta T. To new to publish fuel cost yet.
    You may find that controlling the flow to comfort will provide the same result as breaking the house up into zones.
    Considerate People, Considerate Service, Consider It Done!
    email: [email protected]
  • SWEI
    SWEI Member Posts: 7,356
    Same experience here. When we properly size the emitters and the distribution piping, we get fantastic results.
  • billchat
    billchat Member Posts: 11
    Looking for input and would you recommend using 1/2 or 3/4 PEX to supply existing 3/4 feed to old cast iron radiators. Is there any problem with temperature.
  • billchat
    billchat Member Posts: 11
    i thought I had read that the pex rating is to low to provide boiler supply temperature to old cast iron radiators
  • Canucker
    Canucker Member Posts: 722
    billchat said:

    i thought I had read that the pex rating is to low to provide boiler supply temperature to old cast iron radiators

    Not true, oxygen barrier pex fed from a manifold is exactly how I feed my rads. Hottest they get are 150F on the coldest day of the year.
    You can have it good, fast or cheap. Pick two
  • Tinman
    Tinman Member Posts: 2,808
    Another thing to consider is local code. They're always going to have the last say. Most of the municipalities near me only allow PAP for home runs to high temp emitters.
    Steve Minnich
  • bmwpowere36m3
    bmwpowere36m3 Member Posts: 512
    Having worked with PEX-A/B/C and PAP… I'd take PAP any day. Unless you need to "snake" it thru closed walls or very tight spaces. I used Roth PAP and working with it was a pleasure. Unwinding, "straightening" and snaking it took some effort, but once in place and bent, it stays put. I secured it every 24", but honestly could have done 36" and it would have stayed put.

    Whereas PEX-A/B/C… you unwind it and it wants to always roll itself back (what a pain in the rear). On top of that, its almost impossible to get a "straight" run… looks like a "snake". That's the only regret I have for running PEX for potable water… it was much cheaper than copper and install went very quickly (not a single leak either), but the lines look like someone just threw them in place.

    Given my OCD'ness, I've contemplated redoing the main "trunk" line in the basement with copper. Then do a modified home-run setup from the trunk to various fixtures.