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Sizing a steam boiler replacement

BigErl Member Posts: 38
Hello everyone. I’m looking for some help/advice on sizing a steam boiler replacement. This is for a 4 story building with 2 apartments per floor. The boiler I’m replacing is a WM EGH -115. I measured all the radiators and got 665.16 for my EDR. Using a pickup factor of 1.5 and converting to MBH gives me 239,458. According to Weil Mclain’s chart an EG-75 is big enough with a DOE rating of 247. Does this sound right? I’m a little nervous about replacing a 500,000 btu boiler with a 299,000. Are there additional things I should be considering because this is an apartment building and not a house? Should I go up a size to an EGH-85?


  • mikeg2015
    mikeg2015 Member Posts: 1,194
    1.5 is a pretty big pickup factor. You’d only use that for a uninsulated vapor system in a semi conditioned space, or uninsualted pipes located inside the walls. 1.33 is more common. 1.20-1.25 if you know its well insulated. 2 pipe or 1 pipe? 2 pipe needs a little more pickup I think with the condensate returns needing to be heated.

    I come up with 159k with the 240BTU/EDR. This assumes 1psi. A PEG 65 I think is perfect assuming your main is at least somewhat insulated or in conditioned space, risers are in conditioned space and not inside exterior walls. IF so, then 75 would be great.
  • Jamie Hall
    Jamie Hall Member Posts: 23,415
    Even better. Don't bother to convert your EDR to BTUh -- just look at the boiler rating for the steam square footage it's rated for. According to Weil-McClain, the EG-75 is rated at 750 square feet. The EG-65 is 654. Assuming that your pipes are insulated, or will be, in my opinion would be better suited with the -65.
    Br. Jamie, osb
    Building superintendent/caretaker, 7200 sq. ft. historic house museum with dependencies in New England