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New boiler, how to resize a low mass boiler to a high efficiency boiler

John66 Member Posts: 3
A customer of mine has a weil-mclain 245,000 input,with a water output of 176,000 btu,propane. Getting to many confusing answers from different people on how to size this boiler,
They have 6- hydro air units, 2- baseboard zones in basement,1- 150 gallon water tank,can I get some direction on how to resize,


  • JStar
    JStar Member Posts: 2,752
    Perform a proper heat loss of the building. There's no other option.
  • John66
    John66 Member Posts: 3
    @jstar , am trying to avoid that,since it’s a side job. Is there any online resource I can look up

  • Jamie Hall
    Jamie Hall Member Posts: 23,661
    John66 said:

    @jstar , am trying to avoid that,since it’s a side job. Is there any online resource I can look up

    There is no other option, assuming you don't want to do a hack job. Use the Slant/Fin calculator: https://www.slantfin.com/slantfin-heat-loss-calculator/
    Br. Jamie, osb
    Building superintendent/caretaker, 7200 sq. ft. historic house museum with dependencies in New England
  • hot_rod
    hot_rod Member Posts: 22,491
    A heat load is the best step. Or add up all the heat emitters, no reason to install more boiler then the connected heat emitters can move

    Assuming they were sized properly
    Bob "hot rod" Rohr
    trainer for Caleffi NA
    Living the hydronic dream
  • motoguy128
    motoguy128 Member Posts: 393
    Use their previous gas bills and work backward on to it.

    Every $100 of natural gas is probably around 30-35k of peak output capacity required. Subtract DHW use first. So take the coldest month and use that. You can take heating degree days and get more precise.

    Otherwise add up all emitters and the design water temp.

    Can round down for a higher mass historic home and round up for lower mass new construction.