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does downfiring save money?

Jim Davis
Jim Davis Member Posts: 305
Ditto's, Kudo's and !!!!! Effective heat transfer radically falls off as firing rate drops. Every 1% additional O2(Oxygen)in the flue gas causes a 2% minimum loss of efficiency! Never rely on an analyzer for efficiency reading. 90% are eroneous the other 10% are off 10-15%.


  • does downfiring a steam boiler

    save money? providing that all the radiators are getting steam and everything works fine at the lower firing rate? i'm told no more down than 20% but have never been told why.
  • scrook_2
    scrook_2 Member Posts: 610
    don't over underfire

    ...is that like "I'm over Under, and he's under Dunn"?

    Anyway, you do not want to underfire a boiler too much or the stack temps will get too low an you'll get condensation in the chimney, conversely you don't want to oversize the boiler too much at install either, but what is done is done.
  • Steamhead
    Steamhead Member Posts: 16,719
    If you downfire

    test the resulting configuration with your digital combustion analyzer. Make sure the stack temp isn't so low that you get condensation in the chimney.

    If the combustion parameters are OK, and all the rads fill completely with steam, you'll get longer, more efficient burns.

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  • GW
    GW Member Posts: 4,676
    check the O2

    Downfiring will increase O2, resulting in less heat. This may be kinda obvious, but a wimpy flame with lots of air can't hold a candle next to a bigger and "hotter" flame.

    Don't touch the thing unless you have a digital combustion analyzer, and you have some clue as to what you're doing!


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  • rudy_2
    rudy_2 Member Posts: 135
    Gerry has a combustion analyzer and \"knows how to use it\"!!

    Also keep in mind that as the O2 goes down 1% (for example) the flame temp drops dramatically, around 300 degrees. This reduces the delta T between the flame and water as well as tremendously reducing the amount of 'radiant' heat transfer which accounts for about 3/4 of the heat transfer in a boiler.

    As flame temp is reduced, flue gas temp comes down which lowers the delta T in the heat exchanger (and increases the afore mentioned likelyhood of condensation).

    Reducing the volume of flue gases also reduces the 'scrubbing action' of the flue gases against the heat exchanger surface which lowers conductive heat transfer.

    In all likelyhood you will get a higher SSE reading on your analyzer but this can be misleading as SSE does not necessarily reflect these sorts of effects.

    As Jim D explained to me years ago, as a technician, don't pay much attention to SSE, it can be misleading. There are times when you increase SSE (ie underfiring equipment), however, fuel usage will increase....

    As a tech, all I pay atttention to is O2, stack temp and (of course) CO. If those are all in line there's not much else to be concerned with.

    If the equipment is way oversized, you are between a rock and a hard spot - cycling every couple of minutes is not an efficient way to operate either. The bottom line is that equipment is designed to operate most efficiently at it's full rated capacity...
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