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FHW Boiler supply and return size

bill Mc
bill Mc Member Posts: 1
I have a question about the supply and return piping being too small.

The job has a Utica FHW oil fired boiler with 5 zones one to a baseboard loop three to air hanlder units and one to an inderect fired HW heater which the manufacturer recommends a one inch supply and retun. The current supply/ return is being run in one inch copper.

I say that it is too small and will restrict the flow to the zones if they are all/or just 4 zones are calling on the coldest days.


  • kevin coppinger_4
    kevin coppinger_4 Member Posts: 2,124
    what is the load?

    1" can generally handle 80K @a 20f delta T...1.25 generally 140K....kpc

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  • Brad White_9
    Brad White_9 Member Posts: 2,440
    I do not disagree with Kevin

    but would also add that the header flow load in GPM may differ from what your BTU load would otherwise indicate.

    My point is, your zone loads may differ in delta-T and therefore flow. AHU coils may take a 30 degree drop, radiation 20 and the indirect whatever it wants.

    I would size the header for the sum flow rate of all zone circulators if calling all at once. What is the harm in a header at 1.25" or 1.5" size?

    I would suspect the indirect would take the lion's share when calling, so I would make that the first zone, even direct off the outlet of the boiler while the building heating goes straight ahead to the heating zone header. You can see if you keep the 1" common size you will have some fighting in the back seat. (Settle down, kids.)

    Bottom line: Size the header for connected flow rate, not BTU's per hour. Be generous such that the pressure drop in the header is minimal.

    My $0.02

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