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Boiler Sizing

I'm looking at 175K (145 out) BTU and 150K (125 out) BTU gas steam boilers. The net IBR is respectively 94K and 109K (Williamson / Weil-McLain). Installed radiators are 95K. The house is old, some insulation is missing in the basement, more may be missing elsewhere. Now running oil underfired from 2.2 to 1.5 Gph without any problems whatsoever. I will cut 15K to 20K BTUs after insulating some pipes, eliminate draft regulator, have the vent damper in, redo the header the proper way. I may get extra BTUs out of the new unit due to 1035 BTU/CF gas I'm getting (this way I'm comparing 129 and 150 output new units). The existing unit is 77% efficient, so the output is around 160K. Do I go with 175K or take some risk with 150? There is also a factor of the thermostat being set to less than 70F resulting in more EDR.


  • Jamie Hall
    Jamie Hall Member Posts: 23,287
    The only factor

    which is relevant in sizing a steam boiler is the total EDR of the system which it is feeding.  You want to match that, or slightly exceed it (you can almost always underfire slightly, as a fine tuning).  And insulate all the steam mains.

    The rest of it -- insulation in the house, draught dampers, etc. is irrelevant.

    So add up the system EDR -- and you can make an allowance for the lower house temp which increases the EDR if you wish -- and go with that.
    Br. Jamie, osb
    Building superintendent/caretaker, 7200 sq. ft. historic house museum with dependencies in New England
  • SteamNuB
    SteamNuB Member Posts: 2
    The only factor

    Thank you for your advice. I will use the larger unit and adjust it lower as needed.

    However, I thought the experience in an area goes beyond the first page of a text book
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