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Gas Boiler Distribution Pipe Sizing

daveb781 Member Posts: 3
Weil McClain Gas Boiler CGM 140,000 btu

Existing system has 2 zones each with circulators

I have temporarily added a radiant zone which is added on the end of the 1st floor loop (as an initial - worked out nicely as far as transferring warm thru the floor, it’s a staple up app application).

I want to make the radiant lop a separate loop w/ circ pump.

My Question related to the primary piping out of the boiler.

Existing condition: is 1-1/4” immmediatly reduced down to 1” black iron pipe to the air scoop. 1” inch copper to the 1st branch (which is ¾” – baseboards), then ¾” copper to the second branch (which is also ¾” – baseboards).

The radiant loop is 5/8” pex. Right now its only 1 loop. But I plan to add a manifold to feed 2 – 5/8” loops.

I wondering if the initial piping out of the boiler should be upgraded to 1-1/4 pipe to feed1st and 2nd branches, then use 1” to feed radiant (which will ultimately feed 2 -5/8 pex loops – maybe even more)

I realize this thinking is more along the lines of how a domestic water system would be piped which is supplied under a constant pressure.

I understand that pipe size in a closed loop hydronic heating system is sized based on principals of thermodynamics and heat transfer.

The heat loss analysis for the house = 80,000 btus (at 20 deg drop, flow rate =8-9 gpm

1st Floor (which includes all radiant loops) = 50,000 btus (at 20 drop, flow rate = 5-6 gpm)

If the pipes are based on flow velocities (2 to 4 ft per second)

Main pipe supply from boiler could be 1” for 9gpm

1st floor supply could be ¾” for 6 gpm

According to this the existing system OKAY to support the additional radiant loops

I’m just wondering if there are any other considerations pro/cons to changing the pipe sizes.



  • Paul48
    Paul48 Member Posts: 4,469

    Ten degree delta-t for radiant and 20 for the baseboard.
  • daveb781
    daveb781 Member Posts: 3
    Gas Boiler Distribution Pipe Sizing

    Good suggestion - will incorporate.

    Any benefit by upgrading to 1-1/4 out of boiler?
  • daveb781
    daveb781 Member Posts: 3
    Gas Boiler Distribution Pipe Sizing

    The issue that I'm concerned about is that: the main pipe from the boiler is sized based on heat transfer, the branch loop pipe sizes are sized based on heat transfer.

    It seems to me that their needs to be a double check (purely based on flow capacity) that the main boiler piping (from where the individual branches return and join together at the boiler to the point where the individual branches separate to supply each of the respective branches) can provide the required flow.

    If the:

    1st floor baseboard loop is pumping at 6 gpm.

    1st floor radiant loop is pumping at 3 gpm and

    the 2nd floor baseboard is pumping at 5 gpm

    The main distribution piping needs to be able to handle 14 gpm

    Is the assumption, that the individual circulator pumps will combine forces to increase the velocity in the main distribution pipe to provide the required flow? (closed loop system)

    At 14 gpm the flow in 1" main pipe out of the boiler is 5.7 ft/sec.

    Even though a 1" pipe can transmitt the required 80.000 btu's for the whole house, should the 1" pipe be upgraded to 1-1/4" keep the velocity in the main pipe less than 4 ft/sec??

    Or is there a different design methodology for the main distribution piping???

    Thanks for any assistance

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