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modcon Boiler primary in Pex?

lolo21 Member Posts: 16
any reason why the primary and secondary could not be piped in Pex-Al-Pex? (modcon)

the temp and presure ratings seem to be within specs (50' head of water on top of boiler)

lower thermal mass on the pipes= lower stand by losses,



  • Jean-David Beyer
    Jean-David Beyer Member Posts: 2,666
    Boiler primary in Pex?

    Keep in mind that I am not a heating professional.

    For my mod|con boiler, they require a minimum of one-inch piping, and the contractor said these things work better with one and a quarter inch piping, both in the primary loop and in the rest of the near-boiler piping. The piping in the primary loop was mostly steel until it got close to the closely-spaced Ts, and then switched to copper.

    The length of the primary loop is pretty small, so I do not suppose there would be much saving in cost by switching to Pex. Furthermore, the primary loop must support a circulator, a low-water cutoff, a relief valve, and a FloCheck valve. The near-boiler piping also supports three circulators, air eliminator, purge valves, etc.. I would not wish any of that to be Pex. YMMV.
  • Brad White
    Brad White Member Posts: 2,398
    I agree with JD

    One of those, "just because you can, does not mean that you should", situations.

    Technically, if the application is within specifications, yes, it can be done. JD raises the better question, "why?". Support of the near-boiler devices, circulators, etc. all makes the hard-pipe solution obvious to me.

    Now, if you are talking a "jumper" to a hard-piped manifold or transporting the boiler water through an area with limited access to an accessible secondary distribution center, you may have a case, but I do not see an advantage.

    Obviously flow rates at pressure govern as you state, but can you be assured that a runaway boiler will not exceed that?  (Pex distribution would normally have some mixing device to protect that, but if boiler-side, nothing between plastic and a runaway boiler give me pause.

    Now you also mention 50 feet of static height above this. Is this a commercial installation? Some local codes treat such systems differently than straight residential. Here in Boston they do, but I do not have chapter and verse handy.
    "If you do not know the answer, say, "I do not know the answer", and you will be correct!"

    -Ernie White, my Dad
  • Bob Bona_4
    Bob Bona_4 Member Posts: 2,083
    do it all the time

    I fab up the "business" section-air separator, x tank, iso valves, zone branches, water feed- of the secondary loop with copper or black. I mount this up on Uni strut. PAP from the close T's back to the boiler.

    This is for work that needs 1" primary pipe.
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