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How to size a steam boiler

when it comes to sizing the boiler for the job , because it's already figured out for me before the install . But we do run into situations where theres a steam boiler with a zone of baseboard heat off the internal coil . What is the proper way to size a new boiler for this situation ?

I know you need the square foot of radiation for the cast iron rads . But how much of a pickup factor do you need for the mains and other variables ? Is the pickup factor already in the SQ FT number in the new boiler specs ? And how do you factor in a baseboard loop ? Thank you .


  • Yes

    The pickup factor is already figured in, the sq ft rating is the radiator sq ft, with nothing added.

    Take a third of that number and that is what you can use for a baseboard zone.

    For a water heater, you can use all of it, if you use a priority relay for the DHW.

  • So , if I measure

    all the rads and come up with 400 sq. ft. , I can for example , safely use the Slant Fin LD 30 which has a capacity of 421 sq. ft. ? I mean just for the steam part ?

    And if I take 1/3 of that number , around 133 sq. ft. , how would I go about converting that number to see if it will handle the connected baseboard loop ? Thank you Noel for the info .
  • 240 BTU's per Sq Ft Steam

    Sorry, I left that out.

    Yes, you have the rest correct, too. Right off the rating plate, the number is the max amount of radiation you would connect it to.

  • Aha -

    thats the conversion I was looking for , thanks again , Noel .

    Oops , one last question - for a baseboard zone off the steam coil , it's sized by a typical heatloss of the room the baseboard is in ?
  • GREAT question

    I need to find out the exacts, but here is the gist of it.

    A coil is rated at how many GPMs it will deliver at a certain temperature rise. That itself is based on intermittant draw (meaning it assumes that the coil starts out at boiler temperature, not at street temp.) so it is inflated a little.

    Assuming it figures a 100 degree F. rise, you could multiply it by 5 for a 20 degree rise and be OK. Now the intermittant draw number is a bit optomistic.

    On a 100,000 boiler, the tankless rating is about 2 GPM, so that would be 10 GPM at a 20° F. delta T, or over 70 feet of 30 baseboard, if the coil rating were constant. It isn't, so I need to learn more and get back tomorrow to you.

    Great question.

  • The math is way beyond me

    I'm glad theres guys like you who are around to figure it out , hopefully without getting a headache like I do with math . The situation does not come up alot , but when I'm on a job , I take a quick look at the connected load and the baseboard and make sure the boiler is not grossly oversized . But the baseboard loop always throws me off .
  • Baseboard from the boiler water is the easy formula

    Direct math.

    I'll be back. I need to find out the fudge factor on the coils.

  • Hi Ron,

    It turns out simpler than I thought. The coil is 1/2", so the max flow rate is about 1.5 GPM. That's enough for about 25' of Fineline 30 or 15.

    I'm glad you asked.

This discussion has been closed.