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Manifold vs Direct Run

Hello New to the Site!

I am looking to replace (move) existing hot water boiler piping. The current system appears to have been an old gravity system and then a new (in the 60's) gas fired boiler was installed. It is what I would deem a two area direct run system. One side of the house has a direct run and return with 3/4" branches to each radiator, and the other side has similar. I believe the supply side (and possibly the return side) of the direct runs are too large (~3" and 2") respectively. My current system would be a total of 140,00 BTU (I sized using a chart on the actual radiators @ 180F). One side uses ~ 80K-BTU and the balance to other side. Note the actual boiler is "rated" at 240,000 BTUs and is an "R2D2" style.

I have read some notes on using manifolds, but with 20 radiators and the location of the boiler, this comes out to over 1800 lf of piping (in the basement alone assuming I leave the carbon steel in the walls and upper floors.

Therefore, I am considering going back with direct run. My question is do I really need to have that large of "header runs" or can I use smaller. I am looking to replace/move all the basement piping to PEX as well. I've seen some great feedback on considerations for balancing valves, but with the current layout and system, we have had no issues with good heat.

Any other general advice is appreciated. Thanks!


  • Jamie Hall
    Jamie Hall Member Posts: 23,275
    You don't NEED those big pipes, no. However, the head loss through them is extremely small (for example, if you compare the same flow in 3/4 inch vs. 2 inch iron, the little pipe will have about 8 times the head loss). So -- almost all the head loss in your system is in the risers to the radiators, which means that within rather small limits they are most likely all getting about the same flow and are pretty well balanced.

    Now if you go with small diameter -- say 3/4 inch PEX -- as the main lines to the radiator risers, this will no longer be so, and the radiators closest to the source of the mains will get a lot more flow than the ones farther away, and your balance will be off -- possibly quite significantly.

    There are ways to reduce the problem -- reverse return with generous main lines is perhaps the best -- but it is likely that you will also need some form of balancing. If there are valves on the radiators themselves, they may be sufficient. You do not, however, absolutely have to go to manifolds and home runs!
    Br. Jamie, osb
    Building superintendent/caretaker, 7200 sq. ft. historic house museum with dependencies in New England
  • hot_rod
    hot_rod Member Posts: 22,110
    With a home-run manifold system, you don't necessarily need to feed every rad separately. A 3/4 port on a manifold can move 6.5 gpm.

    So if you did a load calc, determined how much each radiator need to put into the room(s) you might series 2 or 3 or more on one run. Possibly cut the number of runs in 1/2?

    Other zoning ideas here, and here.

    Bob "hot rod" Rohr
    trainer for Caleffi NA
    Living the hydronic dream