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Boiler sizing how much btus

I have 10 radiators
2 cast iron 4 ft with 142 rings/fins
3 38 height 4 tubes by 9 sections/length 10 inch wide
2 25 height 3 tubes by 6 sections/length 8 inch wide
3 slim style 27 height 4 tubes. One 21 sections one 16 sections and one 12 sections 4.5 inch wide.
I have a 3 story about 2500 sq ft home.
Just trying to find a ballpark number for the steam boiler I would need. Currently there is a 299.000 which needs to be replaced. From what I read that is sized incorrectly. Any help would be appreciated. Also. First floor 4 radiators 10 ft ceiling other two floors 3 each 8ft ceiling.


    EBEBRATT-Ed Member Posts: 15,510
    Go on line and look for the "Burnham Heating Helper" You can download it and calculate your radiation load yourself. After that pick a boiler that will heat that radiation load in sq feet of steam that you calculated
  • Jamie Hall
    Jamie Hall Member Posts: 23,270
    You left out a critical bit of information: is this steam or hot water? The two are sized very differently.

    If it is steam, figure the "EDR" of the radiators -- there are tables for that -- and add it up. The total is the EDR rating (not BTUh) of the boiler which you need. It is OK to go a little undersized -- up to perhaps 10 percent -- or a little oversized -- 5 % or so, but the closer you can get the better.

    On the other hand, if this is hot water, the place to start is to calculate the heat loss of the house. This isn't that hard, and there are on-line calculators to make it even easier. Slant/Fin has one I like, but there are others. The result will be the net BTUh rating of the boiler you need. Again, try to get close. Undersized is not so good -- but significantly oversize is a problem too.
    Br. Jamie, osb
    Building superintendent/caretaker, 7200 sq. ft. historic house museum with dependencies in New England
  • kcyrus
    kcyrus Member Posts: 2
    Steam heat @Jamie Hall
  • Jamie Hall
    Jamie Hall Member Posts: 23,270
    OK -- then look up those radiators -- there is a book "Every Darn Radiator" available here: https://heatinghelp.com/store/detail/e-d-r-ratings-for-every-darn-radiator-and-convector-youll-probably-ever-see
    and add them up, then match that to the boiler EDR -- and there is your boiler.
    Br. Jamie, osb
    Building superintendent/caretaker, 7200 sq. ft. historic house museum with dependencies in New England
  • mikeg2015
    mikeg2015 Member Posts: 1,194
    Total Ballpark, it’s a round 500EDR. Which seems about right for a 3 story that size. But you’ll want to look them all up. Plus you’ll have pickup.

    But I’d guess a 175 or 200k will get you there. But again you’ll need ot look all of those up, I took a few WAG on sizing for my ballpark.

    No way those radiators get you to a 300k boiler. typical gross oversizing we see here as boilers were replaced going from coal to oil conversion, to gas conversion to modern gas boilers. I think what often happens is the oil ot gas conversion they look at the max oil burner capacity not the installed nozzle size. Then they fail to realize that hte gas gun might be 60% efficiency on a good day and don’t factor that in. Each time they throw on a little “padding”. You know... just to be safe.
  • pecmsg
    pecmsg Member Posts: 4,836
    First you need to find the heat load loss for the envelope then calculate what size radiator(s) for each room THEN the size boiler to match the Total EDR.