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Pipe size to cast iron radiators

Kendo Member Posts: 3
Hey everyone! I have a job that i looked at today and the cast iron radiators are piped in 1/2 pex. Its piped in series from one radiator to another. No venturi tees. Does anyone think this system will work? And if not what is the best way to pipe it? Thanks in advance. Kenny


  • Alan (California Radiant) Forbes
    You don't need

    venturi's on a series system.

    It may work as long as the radiators on the 1/2" PEX loop are less than 15,000 BTU output (total).

    Typically, the best way I like to pipe radiators is 2-pipe, indirect return.
    8.33 lbs./gal. x 60 min./hr. x 20°ΔT = 10,000 BTU's/hour

    Two btu per sq ft for degree difference for a slab
  • kcopp
    kcopp Member Posts: 4,443
    edited May 2014
    I am pretty....

    sure that will not work right.

    I looked at a similar system years ago and they were tring to do that w/ 3/4" pex....didn't work either.

    I ended up installing a set of manifolds (s&r) w/ 6 ports (for 6 rads)  and ran 1" to the manifold. 1/2" pex to each rad. I set the new boiler up w/ a outdoor reset control and the system worked very well.
  • icesailor
    icesailor Member Posts: 7,265
    Will it work?

    IMO, it will never work properly if it is as you describe. You can only run so many Gallons Per Hour of hot water through the radiators, the first radiator will get hot and the last one may never get hot. If it is piped as a series loop, like 3/4" copper baseboard. It was obviously installed by someone who had absolutely no understanding of how and why fluids travel through a hydronic system.
  • icesailor
    icesailor Member Posts: 7,265
    In & Out:

    Did you mean a two pipe reverse return?
  • Kendo
    Kendo Member Posts: 3
    Thanks for the input

    Thanks for the input.  I figured i was going to have to repipe it.  Just wanted to make sure.  Thanks alot