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sizing steam boiler

Brad White_9Brad White_9 Member Posts: 2,440
In that the Burnham Independence IN3 is at 158 and the IN4 is at 271. Seems out of proportion for just one more section...

What about a V8 series with a gas burner if you can get away with that? A V86 with a 1.65 nozzle = 151 SF and the V87 at 2.10 GPH = 189 SF...

Granted the gas burner may not be allowed but the AFUE ratings (FWTAW) are over 85%.

Heck what do I know, I am just an engineer (Thanks Frankie Wrench :)


  • Jim_83Jim_83 Member Posts: 67
    sizing steam boiler

    I am replacing a steam boiler heating a 2 story house with attic. The boiler will be heating the second floor and attic. The first floor is a separate apartment and has its own steam boiler.

    My question concerns the boiler sizing. From reading the posts I know it is very important to size a steam boiler based on the connected load.

    The connected load is 180 square feet steam. There is a boiler rated at 158 square feet steam and the next size is 271 square feet steam. I know the rule is to pick the next size higher, but in this case, is 271 sq ft too big a boiler. Would I be better off with 158 square feet steam boiler? The boiler will be gas fired.

    Some contractors have told me the smaller boiler is OK and the next size up is too big, while others tell me the larger one is the correct boiler size.

    Which boiler is the correct one to use?
  • Garret_4Garret_4 Member Posts: 14

    That's quite a jump.. what make/model is this boiler? Seems like there should be something in between....

    Sizing is a touchy subject. Classic best practice is to size the boiler to the installed radiation (connected load). But there are alternatives.... if your heat loss is significantly less than what you have in radiation, you should consider removing radiators, downsizing (check local preservation yards for cheap rads), or installing "orifice plates." Orifice plates are a recently rediscovered trick that allow you to "downsize" a radiator by throttling the amount of steam that can make it into the radiator.

    Do a careful heat loss calculation for your house, and that should tell you what to do. With a steam system, you'll need some over-capacity.. the big question is, how much over-capacity... twice is probably too much. 5% too little. :) Something in between is healthy.

    What size boiler are you taking out? Has the system run well with it?
  • Brad White_106Brad White_106 Member Posts: 8
    Kind of wondering

    if someone tore a page out of your catalog. That does seem like quite a jump. 158 EDR to 271? I would think at least one intermediate size, incremental to number of sections. I would also ask my self if your piping is well enough insulated so that you could be closer to the net rather than full I=B=R SF rating. Shaving a little but it might make the difference. I will stand by and see what others have to say...
  • I checked

    I checked the Wiel Mclain eg 30 boiler spec.. Net IBR rating..196 sq ft....
  • ConstantinConstantin Member Posts: 3,782
    That's one aspect...

    ... I've also been told that steam system connected load can be modified via orifice plates, TRVs, or other means. In other words, it may also depend on the type of steam system (one or two pipe), the attached emitters, etc.

    We do not have the whole story here, as there is no indication of what the emitter capacity is vs. the actual design-day heat loss. Plus, like you, I find the capacity jumps rather puzzling.

    Steam systems are wonderful creatures...
  • Jim_83Jim_83 Member Posts: 67
    more information

    Thanks everyone for your advice. It’s greatly appreciated.
    Yes Brad, I was referring to the Burnham Independence boilers. It seems like a big jump adding one more section.

    Here is some additional information.

    Current boiler is a Weil-McLain 66 series boiler with four sections. It is about 25 years old and is leaking. It is firing at 0.75 gallons per hour, down fired from original factory spec. of 1.25 gallons per hour. My oil supplier down fired the boiler saying it would save me money. It is a one pipe system.

    Current oil fired boiler takes a long time to get heat from cold start, about 1 hour before radiator start heating up. Once boiler is warm or hot, time to get heat is OK.

    If a three section boiler is used, is there enough water in the boiler so it does not shut down and cycle on low-water cutoff waiting for condensate to return? Would it be advisable to add reservoir piping as describe in Weil-McLain bulletin AE 8403?
  • an hour!?

    Sounds like u have venting plms, not letting air out for steam to get to radaitors.... Some of my installs, start to steam within 7 minutes from cold water start...
  • Jim_83Jim_83 Member Posts: 67
    down firing

    When the boiler was firing at 1.25 gph, I got heat in about 10 minutes. I thought the long time it took to get heat from a cold start was due to the down firing at 0.75gph. I get heat in about 10 minutes when the boiler is warm.

    Am I wrong assuming the long time to get heat is because of the down firing?
  • frank_25frank_25 Member Posts: 202
    I'm just a PLUMBER

    That was a copyright infringement. Expect a letter from my shark. What is your address.
  • Christian Egli_2Christian Egli_2 Member Posts: 812
    Giddy up

    The boiler having been leaking, it would have taken in a lot of fresh water over these last months or seasons. With this, you've probably already built a nice layer of very insulating scale on the hottest parts of the boiler.

    With the insulated heat transfer, it only makes sense that warm up time is real slow. I would suspect this first as the cause of the slow heat. Otherwise, the standard lack of main vents and faulty radiator vents should be rounded up.

    Do you have good main vents?

    The water-line-level accumulation tank is worth it on a system that has slow to return condensate. Your home being three level tall suggests little horizontal travel for the condensate loiter. It probably isn't necessary if your returns work good. Was is standard for your boiler to be overfilled with water after one heating cycle? If not, the level tank would probably be of no value.

    Slow to heat, does not necessarily mean the condensate is slow to return.

    Perhaps you can go with the bigger boiler and have it down fired a bit. Having the bigger boiler will give you a little more of the water content you seem to want (this would do a bit of what the level tank would do).

    Is your pick up in the 180 EDR already? Having to jump over the first floor, your piping might be longer than usual and might call for more pick up factor. Are the risers insulated as the gap the first floor?

  • frank_25frank_25 Member Posts: 202
    from my experience

    I'm more familiar with the Peerless models, but the thought should hold true to all steamers. A WBV can fire at .75 to 1.25, and the ECT goes from 1.25 to 1.75. However there is a limit to the down fire range. Maybe you exceeded it. I remember a gas job that was sold too big. I removed a burner, and 4 years later, the h.o. is still happy.
    (This is for you, BRAD) But what do I know? I'm just a plumber. (In gas for 30 yrs, and oil for 4)
  • Steamhead (in transit)Steamhead (in transit) Member Posts: 6,688
    That 4-66 is way oversized

    its square feet rating is probably about 450. Downfiring it so far is likely the cause of its slow response.

    You shouldn't have trouble with condensate returning on a residential job. Adding a reservoir is overkill.

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  • Ron SchroederRon Schroeder Member Posts: 998

    going from 1.25 to .75 decreases your fire from 175,000 BTU to 105,000 BTU per hour and I would suspect that has much to due with your longer run time unless pump prerssure was increased to make up for some or all of that. Normally it is not sujested that you down fire more than 25 to 20 percent which would be a 1.00 nozzle ie 140,000 BTU, Hey but what do I know I am only an Oil Burner Technician.

  • frank_25frank_25 Member Posts: 202

    Another guy using my "tag".I tell ya, I get no respect. ;~)
This discussion has been closed.


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