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Boiler piping sizing.

conrajetken Member Posts: 10
I currently have a burnham steel boiler (125,000 BTUH) with three zone circulators. The boiler has 1-1/4” inlet and outlets but was reduced down to 1”. All three circulators are connected byb1” pipe to the 1” header. The house is a 1,500 square foot ranch with a finished basement. The zones are zone 1 goes to attic hot water coil in air handler proving heat to the entire 1,500 sf ground floor, the second zone supplies water to a converted garage which in now a family room (24 ft of baseboard), the third zone supplies baseboard heaters in the finished basement. I acquired a good condition Weil McLain model 68 cast iron boiler which I refurbished. It has a Riello burner and the boiler is rated 150,000 BTUH. The connections are 1-1/2”. My question is, should I build a 1-1/2” supply and return heated feeding the three pumps with 1” pipes from the 1-1/2” header or should I step down to 1-1/4” out of the boiler and build a 1-1/4” supply and return header with 1” taps for pumps? Thanks


  • Jamie Hall
    Jamie Hall Member Posts: 23,286
    You can step down and retain your existing piping. It will put a little extra head on the pumps -- so slightly less flow -- but it shouldn't be enough to be significant.

    If the old boiler was big enough to keep you warm, the new boiler is going to be oversized a good bit -- and probably cycle.
    Br. Jamie, osb
    Building superintendent/caretaker, 7200 sq. ft. historic house museum with dependencies in New England
  • rick in Alaska
    rick in Alaska Member Posts: 1,457
    I can comfortably say your boiler is going to be grossly oversized, unless you don't have any insulation, in which case it is just way oversized. Either way, there is no reason to go more than 1 1/4 header.
    EBEBRATT-Ed Member Posts: 15,524
    FWIW Both your boilers are way oversized for the load as @rick in Alaska pointed out
  • EdTheHeaterMan
    EdTheHeaterMan Member Posts: 7,837
    To better understand pipe sizing you may want to look at the first few pages of this text. http://media.blueridgecompany.com/documents/ZoningMadeEasy.pdf
    This takes it down to basics for you.

    Weil Mclain started making that boiler in the early 1970s and stopped making that boiler in the early 1990s. Are you sure you want to take the time to install that when a more recent WGO or WTGO or SGO series boiler might be available. There will be people with boilers less than 10 years old discarding them left and right as a result of the price of oil. See what becomes available in the next month or so. You may find a 2 section boiler that will more closely fit your size needs.
    Edward F Young. Retired HVAC ContractorSpecialized in Residential Oil Burner and Hydronics
  • hot_rod
    hot_rod Member Posts: 22,143
    If you want to go a step beyond the "padded" rule of thumb pipe sizing method, use the pipe tables. Here is an example for various wall copper tube. You want between 2- 4 fps velocity in hydronic piping. Stay closer to the high end, 4 fps for best air removal. above 5 fps you start to get some flow noise in copper tube and fittings, think an outside faucet flowing 11 gpm :)
    Below 2 gpm the air may not move along with the fluid flow, especially in vertical piping.

    Domestic hot water, especially chlorinated, should stay below 4 fps in copper, even pex for that matter. For cold potable water piping some codes and the copper development tables allow up to 8 fps, assuming it is not a 24/7 condition like hydronics may be. But you will hear that velocity in copper tube.

    Pipe size over 2" is generally sized by pressure drop/ ft not gpm/ velocity.

    This same table shows what flows are needed for above 5 fps power purging.
    Bob "hot rod" Rohr
    trainer for Caleffi NA
    Living the hydronic dream
  • conrajetken
    conrajetken Member Posts: 10
    Thank you!!!