Click here to Find a Contractor in your area.
Welcome! Here are the website rules, as well as some tips for using this forum.
Need to contact us? Visit

Residential Steam Boiler Sizing

DonNDonN Member Posts: 4
When sizing a steam boiler is it better to be slightly under-sized or over-sized?

Here's some more detail to understand what I'm really asking. My current steam boiler, a nearly 60 year-old gas fired Peerless, has stopped functioning. As one might imagine it was massively over-sized for my home.

I've had 5 different contractors out to quote installing a replacement boiler and all 5 have come up with different boilers based on capacity. They were nearly all within 20% of each other. I obviously don't want to have an over-sized boiler as I'd really like to be done with $800 monthly gas bills.

I did the EDR calculations for the radiators and came up with 312 sq. ft., which is very close to the number that the two seemingly most knowledgeable contractors that came calculated. The dilemma is one contractor is quoting a unit which is rated 375 sq. ft. and the other a unit which is rated at 281 sq. ft. Both are suggesting a Columbia boiler, just different sizes.

As noted both seem very knowledgeable and both seemed to have compelling arguments as to why they've made their different recommendations, but I as a homeowner, even a well educated one cannot determine is whom is giving the better advice.

I've read about and understand the perils of having an incorrectly sized boiler, but is it better to be under-sized by 10% or oversized by 17%?



  • Jamie HallJamie Hall Member Posts: 14,811
    My inclination...

    but this is just me -- would be to go with the larger size, if the installer agrees that it can be down fired slightly (depends on the burner).  That way, you will be able to trim the output to really match your system.

    On the other hand, if you have very little in the way of mains and risers to heat up, and everything is heavily insulated, you might be able to get away with the smaller one (wince part of what is built into that capacity figure is the pickup factor)..

    But on the whole... I'd go with the bigger one.  And I'll be very interested to see what others have to say!
    Br. Jamie, osb
    Building superintendent/caretaker, 7200 sq. ft. historic house museum with dependencies in New England.
    Hoffman Equipped System (all original except boiler), Weil-Mclain 580, 2.75 gph Carlin, Vapourstat 0.5 -- 6.0 ounces per square inch
  • rcritrcrit Member Posts: 71
    Go big

    I have a Slantfin Galaxy rated at 325 EDR with a load of 334-362 depending on which rads I have the vents closed on.

    Your house may be different from mine. I have a 1922 Dutch Revival with very lousy insulation on the first floor and lots of air infiltration (its a work in progress). The second floor is better and has better windows. For some reason someone thought R-7 was adequate insulation for the 3rd floor.

    The boiler installation is not the best with a copper header, very dirty water and a blocked skim port (by the LWCO). With this system on very cold days where the boiler runs a lot things are generally ok. It generates enough steam to keep the risers warm so heat actually gets to the 2nd and 3rd floors. It's tuned about as well as it can without doing a major overhaul.

    Mild days are another story. It just doesn't run long enough to heat the risers to the upper floor rads despite having the first floor vents nearly completely closed (Hoffman 1A's for the most part set to 1) and the 2nd and 3rd floor vents wide open (set to 5 or 6). I end up doing the reverse once temps are consistently below 30 otherwise we fry on the 2nd floor.

    You might manage with a smaller boiler but you'll probably find yourself running around tuning air vents in the fall and spring.

    Fortunately for me Steamhead is in the process of installing my new boiler this week.
    I'm just a homeowner that has a steam system, take my advice with a few grains of salt.
  • RodRod Posts: 2,067
    Boiler Installation

    You're really lucky to have Steamhead doing your boiler. He's top notch! 
  • Mark NMark N Member Posts: 1,089
    edited November 2011
    Proper Size

    First of all chose the installer that will do the best install. The new boiler needs to be properly piped. Next choose the boiler that is rated as close to the actual amount of radiation you have installed. I have a Burnham IN-4 rated for 271 sqft of EDR. I have 260 sqft of radiators. Last winter I used approx.745 therms of gas from 10/12/10 to 5/13/11 to heat my house. That came out to $857 for the whole heating season. A properly sized and installed boiler is key to efficient operation.
  • DonNDonN Member Posts: 4
    A few more questions...

    Thanks to everyone who replied, I appreciate the extra guidance.

    We've historically had significant problems with water hammers and just lived with it, but having been doing lots of reading here and other places, I now know that the system should operate quietly. While the contractors all posited a few ideas as to why it may be happening, none included any solution I can see in the quotes that would address the water hammers. I got the general impression that they felt it would simply be resolved by replacing the boiler and all of the near boiler piping

    I've been digging through the written quotes and realized none of the contractors even suggested replacing the main line vents or radiator vents. I know none of them have been replaced in the 8 winters I've lived in my home. It seems there's a good chance that some if not all may be the 75 year old originals.

    Worse, when I walked down to just take a look at the main line vents to see if they looked old or new, I noted they're both on tees at the very ends of the main lines.

    Should I insist that relocating and replacing the mainline vents be done as part of the job along with balancing the system and replacing all the radiator vents?

    Also should I be worried about the wet returns? Is there something that should be done there?

    I know the internet can be a dangerous thing, giving a little knowledge when I should be relying on professionals, but the reality is I keep finding things that I think should have been at least discussed, but were not. So now, I have some doubts about the professionals because those conversations did not happen and I know very few homes in the area have steam heating.

    Again any feedback that is going to help me identify the right professional and make decisions that won't come back to haunt me in the coming years will be much appreciated.


  • Mark NMark N Member Posts: 1,089
    Water Hammer

    Go to Systems tab on this page, them click on steam. Then click on Steam heating problems, then click on Problems that plague all steam heating systems. Scroll down to There's water hammer in the system. Give it a read will answer many of your questions.
  • RodRod Posts: 2,067
    Steam Boiler?

    Hi- What models Columbia boiler are they suggesting?  I'm not at all familiar with Columbia Boilers but I was under the impression that in the residential size, they only made Hot Water Boilers.

    - Rod
  • DonNDonN Member Posts: 4
    Columbia Steam Boilers


    The Columbia CEG-C series. I'm not sure why they don't have it anywhere accessible on their website, but if you google it, you'll find a direct link to a PDF on their server.

    I've read pretty good things about them here and other places. They have a local branch up in Baltimore, so it was a common boiler that many of the contractors recommended.


  • DonNDonN Member Posts: 4
    Water hammer


    Thanks! I've been to the systems page, but I some how missed water hammer down there at the bottom.


  • RodRod Posts: 2,067
    Steam Professional

    Hi Don-

       Thanks!  I will check it out. I know Columbia makes good boilers but I hadn't heard that they did residential steam units.  You mentioned Baltimore. If you are located near there I would suggest you contact  Frank "Steamhead " Wilsey.  Ph: 410-321-8116   He and Gordon are top notch steam pros. Here's a link to their page on this site:

    - Rod
  • rcritrcrit Member Posts: 71
    I concur

    I have to agree with Rod, Frank and Gordon are great. I'd suggest you not make any decisions until you talk to them.
    I'm just a homeowner that has a steam system, take my advice with a few grains of salt.
This discussion has been closed.


It looks like you're new here. If you want to get involved, click one of these buttons!