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Why The Close Tee's

I've been doing a bunch of reading on radiant systems with primary and secondary loops. In nearly every discussion or example it comes up that the secondary supply and return lines from the primary loop should be close together. Most say within 4x the primary pipe size? Some also say the primary piping should be straight for 8x the primary pipe size on either side of the pair of Tee's. It's not a problem to comply with these instructions but I'd love to hear the physics of why this matters.


  • hot_rod
    hot_rod Member Posts: 16,327
    Bob "hot rod" Rohr
    trainer for Caleffi NA
    Living the hydronic dream
  • JohnNY
    JohnNY Member Posts: 2,917
    It has to do with the restriction that develops through the length of the pipe: more as it gets longer. You want the water to "want" to move straight through the pipe and tees (called the *run* of the tee) and not be influenced to move through the tees' branches (called the *bull* of the tee). The longer the pipe between the tees, the more likely water will be influenced to gravitate through the branches.
    Contact John "JohnNY" Cataneo, Master Plumber for Consulting Work
    Or for plumbing in NYC or in NJ.

    Or take his class.
  • winesalot
    winesalot Member Posts: 32
    Thanks for the great answers.

    Why is my favorite question. If you don’t understand why then you don’t understand.
  • Steve Minnich
    Steve Minnich Member Posts: 2,758
    Another way to put it - the closer the tees in the primary loop, the less pressure drop between the tees, the less pressure drop between the tees, the less chance of inducing flow in the secondary loops.
    Steve Minnich
    Minnich Hydronic Consulting & Design, LLC
    [email protected]