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Conflicting Copper Pipe Sizing Info?

StewBrewer Member Posts: 10
I am sizing the Type M copper pipe that will be feeding 3 manifolds from my header pipes.

The individual GPM’s for each of the 3 pipes are 5.3, 4.4 and 4.2 GPM. When I use Siggy’s pipe sizer program or the B&G System Syzer program, it says that ¾” pipe will be fine with corresponding fps values of 3.3, 2.7 and 2.6 fps, which all fall between the recommended 2 to 4 fps.

So, why do I keep coming across various tables and sizing guidelines that state that ¾” pipe should not carry more than 3.5 to 4 GPM?

What Gives?


  • GroundUp
    GroundUp Member Posts: 1,908
    It really depends on the length of the piping to determine whether or not there's a lot of pressure drop in the system which can make circulator sizing a lot tougher. The MechanicalHub guideline is just that- a guideline. I wouldn't be scared of 3.3 FPS under any circumstances, but if your piping is lengthy it may not be very attainable
  • Zman
    Zman Member Posts: 7,569
    edited March 2021
    I think the answer is "It depends".
    For hydronic heat distribution pipes 2-4 ft/sec is a great range. For header and manifold piping, 2-3 ft/sec is better.
    For domestic water with intermittent use, those numbers can be stretched a bit. For DHW recirc running 24/7/365, go as slow as you can < 2ft/sec would be best.

    "If you can't explain it simply, you don't understand it well enough"
    Albert Einstein
    EBEBRATT-Ed Member Posts: 15,534
    The Gpm's show above it what I would use. Excess velocity can cause noise which on some jobs won't matter. But in residential work noise can be an issue. Two much velocity can make air removal difficult as the air can't separate from the water and will wizz by the air vents. Velocity that is too slow will also make air removal difficult as the air can be trapped in the piping and will not be carried back to the air separator
  • hot_rod
    hot_rod Member Posts: 22,157
    Those "rule of thumb" charts are fairly conservative. The Engineers Toolbox has better data for copper tube sizing based on tube size and wall thickness, K,L,M. If you want to maximize. Here is the 3/4 copper tube table. 5 fps and higher is when the noise starts :)

    Up to 2" tube or pipe is sized by gpm flow rate, above that it's typically by pressure drop and you will find higher flow rates used.

    Bob "hot rod" Rohr
    trainer for Caleffi NA
    Living the hydronic dream
  • EdTheHeaterMan
    EdTheHeaterMan Member Posts: 7,844
    It's not rocket surgery. (I heard that from someone herein) As HotRod Bob mentioned those sizing "rule of thumb" guides are conservative. That said if you are looking at shared piping near the boiler, and/or you don't have a very long run, I would use a size larger to be safe. If I were running hundreds of feet of tubing, then the cost of the material becomes a factor. I would go to the "engineering toolbox" to see what is the lowest-cost pipe that will work. No need to spend $$$ on 1-1/4" copper if 3/4" will work. Likewise If 3/4" is right on the edge of the scale, I might opt for 1" to be safe.

    "It Depends," I think I heard that earlier from a very wiZe man!

    Yours Truly,
    Edward F Young. Retired HVAC ContractorSpecialized in Residential Oil Burner and Hydronics
  • hot_rod
    hot_rod Member Posts: 22,157
    Everything in a hydronic circuit should be sized to the gpm required, valves, pumps, pipes, etc. It's never about the size of the hole in the device or pipe when sizing :)
    Don't go overboard on oversizing if flow velocity drops below 2 fps air removal is a bit more challenging. The pipe flow tables keep you on the path to a best design.
    Bob "hot rod" Rohr
    trainer for Caleffi NA
    Living the hydronic dream