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Between joist floor heating design question - max loop runs

Hi all,

First post after quite a bit of reading for the past year.

We finally are doing a DIY floor heat install with sort of a consulting contractor who has some, but not a lot, of experience in floor heating (he is installing our AC)

Background: our house is around 2500 sq ft. Room by room heat loss calc puts me around 60,000 btu/h loss. It's an older home but we and previous owners have done quite a bit of envelope enhancements so it's ok insulated generally, good in the attic and between the basement and crawl. We're Zone 5. All finish floors are 2 1/4 oak with mostly 1x6 plank subfloor. Two bathrooms have tile floors.

We are keeping the existing cast iron baseboard system in the house. A porch room had fintube baseboard and an addition room over it had the same. We are replacing those with only floor heat (though leaving the supply and return pipes in case I want to add panels later). We are installing the floor heat for comfort and to perhaps run cooler water throughout both systems. The way oversized boiler will power both systems, which will be on different subsystems.

I'm using 1/2inch pex - stapled up with the thin plates (I know the group here prefers thick plates, but that's not in the budget for the whole house, I may use them in the one room, the porch room, if I need the extra heat as that room has a lot of loss because of lots of windows - but that's the last one of the install).

The contractor I'm working with suggested max loop length of around 150ft for each loop. That seemed low to me and I was going to shoot for around 300ft. I did my first run and it came back longer than I had hoped. It came to 339ft total of actual in the floor pipe. I can easily cut 13' off of that to bring it down to 326' and replace that 13' with 3/4inch that'll be using to run the vertical length and back to the main manifold (8ft ceilings and about 20ft to the maifold. so 2x for supply and return).

So before I go further with other loops, want to get some thoughts. I can cut that in half and turn that into 2 loops and just put a couple fittings in the ceiling, which I don't like, but there are some there anyway for various other plumbing and one in the loop from a helpful but careless wife who aggressively pulled some pipe to create an unremediable twist.

I heard that it's important to keep each loop sized balanced so that'd change my future runs to about 150-175ft loops instead of 300 to 325 loops. Thanks for the help!


  • Ironman
    Ironman Member Posts: 6,876
    edited September 2019
    We try to keep ours around 250' with 1/2" tubing. Two reasons for this:
    1. We use 1000' rolls, so 4 X 250 = 1000.
    2. This length keeps the head down where a small circulator (ups15-58) can be used.

    You can go as long as 300', but you'll have issues to address if you go more.

    I'd cut that present loop in half and then keep the rest around 250'. The flow setters on your manifold can be adjusted to compensate for the shorter loops.

    I hope you're using quality extruded aluminum heat transfer plates?
    Bob Boan
    You can choose to do what you want, but you cannot choose the consequences.