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Pipe sizing

TomTA Member Posts: 20
edited November 2019 in Plumbing
I really appreciate any help you guys can give. I'm learning a lot on this site. This stuff is fascinating to me, but much more complex than one would think at first.

I'm "Harry Homeowner". I need to replace my boiler and dhw coil (probably 25+ years old) with a new boiler and indirect. I've had two guys come in to give estimates, and neither inspired much confidence at all. I can sweat pipes as well as the next guy, and I'll get a pro to come in with the proper instruments to actually set the burner, but the rest I'm resigned to doing myself.

In addition to the boiler and indirect, I also need to put some new radiators in (our house is grossly under-emitted, if that's the term...not nearly enough radiators). Plus, my wonderful wife wants radiant floor heating in the kitchen (at some point...not this year).

All this means I'm going to need to run/replace some piping. I've got everything from 1.5" copper near-boiler, to 1.25" and 2" iron going to the radiators; it's all been spliced in over the past 100 years or so. Honestly I'd just as soon take it all out and start fresh. With an 80k BTU boiler, and a 20 degree delta-T, that suggests 1" copper would be adequate (is that right? I see flow rates of 5.5gpm to 10.9gpm at 2 and 4 feet per second).

If I put a 8k radiator in a room, the required flow is less than a gallon per minute assuming 20 degree temp drop, which suggests that even a 1/2" line would be adequate. Is the problem - and the reason I always see bigger pipes at radiators - that the head loss gets to be too great if you go down to a 1/2" or 3/4" pipe? If I keep the head loss and GPM near the pump output curve is that all that matters?

My untrained thought was that 1.25" or 1.5" for the near boiler, and then reducing as the different individual radiator circuits branched off. Is there anything inherently bad or inefficient about using pipe of a "too large" diameter? Does the flow speed get too slow and have some negative effect?

Thanks in advance. I really wish I had looked at this as a career 40 years ago.


  • mattmia2
    mattmia2 Member Posts: 9,941
    Your large piping at the radiators was likely because it was originally a gravity system. The gravity system needed large pipes for circulation. With forced circulation with a pump the piping can be much smaller.

    What you might want to do for aesthetic reasons is bring black iron or even chrome plated brass below the floor then adapt to pex or copper below the floor. (queue electrolysis debate)
    EBEBRATT-Ed Member Posts: 15,829
    what I usually have in my head is 1/2" 1.5 gpm, 3/4" 4 gpm, 1" 8 gpm. You don't need to go any larger with 80k output.

    Oversized pipe may make air removal difficult if it's way oversized air may become trapped....not enough velocity to get it out, other than that oversized is not an issue. You can usually be at least 1 pipe size over "normal" without an issue

    3/4 is common for radiator branches but in many cases 1/2" or eaven 3/8" will carry the enough flow

    Sounds like your on the right track