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Sizing Steam Boiler

Steve_193 Member Posts: 4
Weil McLain just got back to me and the EDR for the baseboard is actually 47 sqft. So gives me a total EDR of 411. Still right in the neighood of your calculations. Great example!

Thanks for the great explaination of everyhing.


  • Steve_193
    Steve_193 Member Posts: 4
    Optimum Sizing

    I have a two family rental that has steam heat, in which the boiler cycles about every minute or so once reaching pressure. The house uses convectors similar to the old Trane Heat Cabinets (which I obtained from the libray on this site). I am using the Trane calculations for BTU/hr for each cabinet plus adding a Weil McClain CIBB in the back porch. I used 1.33 times the calculation to allow for the system piping and I come up with around 127,500 BTU/hr for the boiler. Even this figure may be a little high, as I did not measure each cabinet in the house but used an average sized cabinet value. I know some are smaller and some are larger so should be pretty close.

    The current Boiler is rated at 250,000 BTU/hr. Obviously grossly oversized. My question is, with optimum boiler sizing would the boiler fire continuously from the time there is a call for heat until the call is satisfied? Is that the goal? What do you think my fuel savings would be if I resize the boiler to match the load in this case?
  • Brad White_203
    Brad White_203 Member Posts: 506
    Sizing by Radiation

    Hi Steve

    Actually, your net radiation square footage (EDR) is what you need to size that. Your boiler size includes the 1.33 factor already.

    So if you have a total Equivalent Direct Radiation of say, 400 SF (= 96,000 BTUH), that 400 SF is the Net I=B=R SF of Steam rating you are trying to match.

    Say you find a hypothetical selection rated at 425 EDR (102,000 BTUH, the next smallest boiler being too small so you go with this one).

    You will find that the "Gross Output" is really equivalent to 135,660 BTUH and the fuel input at say 82% would be 165,440 BTUH.

    Take a look at a real selection -or several- and see what I mean.

    Are you planning on replacing the boiler anyway? Or just because it is over-sized? Curious.

    The ideal, in my estimation, is to fire the boiler (and vent the system) such that it fills quickly with steam and continues to do so, cycling as needed until the space temperature setpoint is met. That is not asking too much.

    With an over-sized boiler, you will certainly achieve that, but if it produced twice the steam you need, you will short cycle. How this affects efficiency is hard to predict. Loss of "ramp-up" effort (shutting down just as the combustion products become stable) is one loss that cannot be recovered.
  • Steve_193
    Steve_193 Member Posts: 4
    Boiler Replacement

    I am trying to bring down my energy cost in this building. I have been hitting as high as $600 for a single month of natural gas. It is a two unit building but fed by one boiler and I am eating the cost of the heat. Great for a write off, but still hurts the wallet.

    OK just downloaded th IO&M from the Peerless site. The current boiler is a Peerless 61-07 with and input MBH or 240, and NET I=B=R is 604 SF. I am waiting on Weil McLain to get the EDR rating on the CIBB.

    I have calculated 364 ft EDR for the convectors. The CIBB is 14Ft long. I wouldn't expect it to be more than 40 ft EDR. So that would put me around 400 ft EDR. So fits into your example calculations above. So still looks significantly oversized.

    The boiler was in the house when I purchased it two years ago. So if reducing the boiler size will majorly reduce my heating costs I will be looking to replace.
  • Charlie from wmass
    Charlie from wmass Member Posts: 4,314

    We always recommend seperating out heat for rental units as when they have to foot the gas bill they put on a sweater when cool not open the window. Also boilers can be smaller and you are less likely to have no heat in the structure if one goes down. Just food for thought.
    Cost is what you spend , value is what you get.

    cell # 413-841-6726
  • Steve_193
    Steve_193 Member Posts: 4
    Separating Heat

    Unfortunately it was originally single family home and would require major repiping. Could get very costly.
  • Brad White_203
    Brad White_203 Member Posts: 506

    A Burnham IN-6 (450 SF) might make a good selection. Peerless 63-04 (458 SF) is another. H.B. Smith G-8 #8-S-4L at 413 SF might be tight but the #8-S-4H at 467 SF covers you.

    There are others and I will leave it to others in this forum to comment. Everyone has their favorites and some have their experience-based concerns. I have no dog in this hunt, so just tossing out some comparable size selections.

  • mel rowe
    mel rowe Member Posts: 324

    Just a homeowner here who likes to lurk around this site and pick up good info. I just wondered how good the insulation was for the bldg. Sometimes it is more fuel/cost efficient to upgrade insulation than to change to a right-sized boiler, unless the old boiler needs changing for other reasons. Also, I was wondering about what kind of rad vents you had. I have seen results from studies of large apt. bldgs. with many overheated areas where the tenants tended to control temperature with the old "double hung" vents. By replacing the old vents with thermostatic rad valves (TRV's) they were able to achieve a 15% savings. FWIW
  • frankiewrench
    frankiewrench Member Posts: 15

    Here is my 2 cents worth..................after reading these blogs, I feel it is my right to ask how are you planing to install the CIBB??? Do you know that baseboard is usually used with forced hot water systems and not steam?
    There are 3/4" tappings throughout the CIBB which allows for very poor condensate flow. Therefore you need to install the CIBB with a return back to the boiler or into a wet return, or with a sream trap. And don't forget to find the appropiate place to vent the baseboard. Do not use any of the baseboard tappings to vent otherwise you'll be mopping the floor whenever the boiler makes steam.
  • Brad White_191
    Brad White_191 Member Posts: 252
    Fair point to make, Frankie

    Most CIBB that I have seen has a 9-foot limitation when used with steam and if not pitched properly, even less will not matter.
  • Steve_198
    Steve_198 Member Posts: 1
    CIBB Installation

    This is a two pipe system. I was planning on using steam trap and piping condensate to the wet return. I realize CIBB is normally used with hot water, but is also listed for use with steam. Weil McLain has steam ratings for up to 25ft in length, so I'm sure somebody uses them in this application. Only issue I may have is trying to get enough pitch to allow condensate to flow properly. Might be a little trial and error on this part, as I don't want the pitch to be too noticable for visual appearance. But, might not be able to avoid that.
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