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Thread: Swapping H/W radiators for in-floor!

  1. #1

    Default Swapping H/W radiators for in-floor!

    I currently have old style, cast iron hot water radiators and would like to install PEX, in-floor heating in my kitchen. I have access to the joists from below so installation is easy. In my crawl space there is a main trunk line with copper branch lines feeding the existing rads. Can I simply attach the PEX to these branch lines and run it between the joists? I'm talking about roughly a 350 square foot area. Is there any difference in running the hot water through in-floor PEX pipe as opposed to the cast iron rads?
    I'd appreciate any help.

  2. #2
    Retired Defense Industry Engineer jadnashua's Avatar
    Join Date
    Sep 2004
    New England


    It's not a simple branch and flow job!

    As opposed to the radiator, in-floor radiant needs to be a lot lower in temp, so you'd need a mixing valve to adjust the temp. Then, you'll probably want diffuser plates (usually aluminum) to spread the heat across the area. Then, you need to insulate beneath the pex. Depending on the current circulators and flow, you may need a new zone which could include a circulator.

    A common operating temp for radiators is 180-degree water...you won't want more than probably 120 or so in the floor, maybe less, depending on design and flow. You normally don't want the floor temp higher than around 80-degrees. These numbers are not specific, but generalizations.
    Jim DeBruycker
    Important note - I'm not a pro
    Retired Defense Industry Engineer; Schluter 2.5-day Workshop Completed 2013, 2014


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