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Steam boiler sizing

Ken_8 Member Posts: 1,640
is written anywhere except in the mind of common sense.

1) Larger than required steam boilers cost more than properly sized boilers.

2) The labor and materials to properly install an oversized boiler adds unecessarily to the installation cost- both from a labor and materials stand point.

3) Oversizing the boiler will make it short cycle. Short cycling wastes fuel.

4) Oversizing cycles switched components more than properly sized boilers. If a boiler is bigger than needed, the number of on/off cycles will be greater. All switched componenbts only have a limited number of cycles by design. The life of "over-cycled" components will be seriously compromised if not matched to the load (EDR).

5) This may not be written in any book. It is burned into the minds however of every ethical and knowledgible contractor on the planet. "Oversizing a boiler is as bad as undersizing it.!

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  • Patrick_28
    Patrick_28 Member Posts: 8
    Downside to oversizing a steam boiler?

    I'm scanning my Lost Art book trying to find specific paragraphs that explains the downside to over-sizing a steam boiler. I'd like to find this in print, and make copies to hand out to prospective customers. I hope Dan doesn't mind! Does anyone know exactly what pages I might find this info on? Oh yea.....I have read chapter 6! I'm either blind or it's somwhere else in the book. Yes I do know how to properly size! Just need the specifics (In Print) on why not to oversize. Thank You, Patrick

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  • Patrick_28
    Patrick_28 Member Posts: 8
    oversizing steam boilers

    Thanks Ken. Your un-ethical, stupid contractor buddy!

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  • michael_15
    michael_15 Member Posts: 231
    just a note

    On the wasting fuel due to short-cycling. . .

    Do the "startup" and "shutoff" moments in a boiler's lifespan consume undue amounts of fuel? (Sort of like the idea of idling your car instead of turning it off, I guess.) Outside of that, I can't see why short-cycling in theory would use any more fuel than, heck, an undersized boiler.

  • t. tekushan
    t. tekushan Member Posts: 141
    matter of \"degree\"

    Having experience with both over and under sized boilers, the oversize has wasted less fuel. After taking into account EDR both existing and missing(if anythings been removed) and a realistic accounting for piping a properly sized boiler should be chosen. BUT. From this optimum point, moving towards undersized, there is a sharp drop off of efficiency as you downsize. The pick-up factor is sacrificed first, meaning that heat delivery is seriously delayed, and since little or no heating is occuring during this period it is all waste. There is a point at which an undersized boiler will deliver no heat while running continuously. Thats zero efficiency.
    Oversizing wastes fuel but in a less radical way. It is more linear in its losses as you increase the boiler size from optimum. The big problem I have observed is overheating of the rooms in mild outdoor temperatures. The large boiler will pressurize the system quickly, overpowering the venting capacity, it quickly fills the radiators with more steam than you really need, then shuts off on pressure, but meanwhile the venting is still releasing air and allowing the steam to fill the radiator anyway. Presto! Too much heat! No matter how you slice it, indoor Bermuda short weather is expensive. You can add to this extra draft losses with an oversize in addition to higher water content that has to be brought to a boil (all of which stays in the basement after shut down).

    I use an old-fashioned method of sizing. It was based on old boiler heating surface-to-EDR ratios.

    EDR/100 = Horsepower; HP*33,472 = gross boiler output

    For example, EDR=400 is 4HP. 4*33,472= 133,888 gross output.

    The typical way today is 400EDR*240btu=96,000 btu net output. 96,000*1.33=127,680 gross output.

    The old and newer (less old?) methods give you similar numbers, with a little more pick-up wiggle room with the older method. When you choose the boiler, pick the one with a higher but close output rating. Usually both methods give you the same boiler choice.
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