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boiler size

jim127 Member Posts: 3
I have a six unit apartment building heated with steam. Two pipe system. One bedroom units with dining room. Four rooms with bath. Each apartment has four radiators. Here is my question. Radiators are two foot high boxes installed in walls, opened at top and bottom for air flow. Heat exchanger is 3/4 inch fin tube. Steam trap on each radiator. There is a total of 66 feet of fin tube on 24 radiators. Boiler is a crown 241000btu unit. It was sized on building square footing.
Bad sections has to be replaced. Its to big, When steam builds up it cycles every three, four minuets. How do I find the right size boiler for this small amount of fin tube. Mains 4 inches. Risers 1 1/2 inch. Building 37 by 41 ft. 8 foot ceilings.
Thanks for any help.


  • Steamhead
    Steamhead Member Posts: 15,425
    How long does it take for steam to build up?

    Do all of your convectors (that's what these things are called) heat up quickly and evenly?

    What steam pressure are you running? This system sounds like Vapor, which tops out at about 8 ounces or so.

    Have you checked the actual EDR rating of each convector to determine if and by how much the boiler is oversized?
    All Steamed Up, Inc.
    Towson, MD, USA
    Steam, Vapor & Hot-Water Heating Specialists
    Oil & Gas Burner Service
  • Jamie Hall
    Jamie Hall Member Posts: 19,139
    edited July 2016
    If you can't find the EDR for those convectors, it's not really that hard to calculate. First, determine the area of each fin, taking into account both sides. So that would be the height times the width, minus the area of the 3/4 inch pipe, doubled. Then count the number of fins per foot. Multiply that by the area of each fin, and divide by 144 to get the area in square feet -- and that is your EDR per foot of convector, or close enough. The fact that they are in cabinets of a sort will affect that, but not as much as one might think.

    Then size your boiler based on that. You should be close.

    Example: say the fins are 4 inches on a side, and suppose that they are a quarter inch apart. Thus there are 48 fins per foot. Each fin has a total area of about 31 square inches, for a total area of about 10 square feet per foot.

    For 66 feet of convector, in that example, your total EDR would be 660. For that, I get about 220,000 BTU, including a generous pickup factor.

    But -- don't go by that. measure and count the fins yourself and do the math!
    Br. Jamie, osb
    Building superintendent/caretaker, 7200 sq. ft. historic house museum with dependencies in New England
  • nicholas bonham-carter
    nicholas bonham-carter Member Posts: 8,564
    there are a number of ways a steam boiler can short cycle:
    1. Over sized and cycling on pressure.
    2. Properly sized, and many of the steam traps are nonfunctional, making the boiler twice as big as needed.
    3. Improperly set up non steam suitable thermostat can short cycle the call for heat.
    figure out your EDR, and choose a boiler which matches that value, expressed in square feet of steam on the rating plate. Make sure the piping instructions are followed as a minimum, and insulate all the supplies. Install a vaporstat to keep the pressure down in the ounces range, and choose a thermostat like the Honeywell Visionpro series which can have a remote sensor. This enables you to keep the control for the temperature in a secure place away from the tenants. Do not use any temperature setback.
    Post some pictures of the present boiler, and piping.--NBC
  • jim127
    jim127 Member Posts: 3
    Thank you for your help. I found the EDR value of the convectors.With this I will get the right size boilers Again thank you Jim127