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steam burner combustion efficiency

Joe Brix
Joe Brix Member Posts: 626
that high efficencies come from running at the level of condensation in the furnace or boiler. Great from radiant systems and well insulated FHA homes since the temp rise in the air supply will be low.

With steam it's the laws of physics: you gotta bring warer up to 212° to make steam. Todays steam boilers have less mass and hold less water so your system efficency will increase a great deal.

But this high operation temp will hinder any AFUE test.


  • Dan_29
    Dan_29 Member Posts: 111
    steam burner combustion efficiency

    I have been looking for a replacement gas steam boiler lately and have been told that 80-82% combustion efficiency is the best I can hope for. Which means that 18-20% of my burned gas is going up the chimney. Why is this the case?

    Is the problem with steam versus forced hot air (fhw) or forced hot water (fhw) one of economics:there are less steam systems than fhw or fha, hence the industry ignores the problem, or a problem of engineering difficulty peculiar to steam (with no solution)?


    PS. I am aware that fha systems can have 95% efficiency and fhw can have 85% plus effiency.
  • Steamhead (in transit)
    Think \"system efficiency\"

    scorched-air distribution systems lose at least 20% between the furnasty and the rooms. Hot water is much better than that, but can freeze in an extended power or fuel failure. Steam has the advantages of hot-water without the freezing danger, since the system drains dry when shut down except for the boiler itself and some pipes in the basement.

    Assuming both systems are in good condition and the boilers are similar, the overall efficiencies of steam and hot-water systems aren't far apart at all.
  • Brad White_38
    Brad White_38 Member Posts: 40
    Nothing against steam

    but the efficiencies between steam and hot water are comparable when you are talking conventional garden variety boilers. Both are in the AFUE range in the low-mid 80's. Poorly sized? It can drop off quite a bit with any boiler.

    But go to condensing and your flue gasses can leave not much warmer than the water temperature. Sometimes it can get close to the return water temperature due to counter-flow.

    With steam it is hard to get the flue gasses low and never lower than the leaving steam certainly. The hottest medium, the steam is leaving at the coolest flue gasses. In a condensing boiler, the coldest incoming water sees the coolest exiting gasses and finishes them off.

    The notion that FHA can retain 95% over sustained periods is a bit dubious IMHO. Combustion efficiency, sure. But in general you do not modulate your air temperature much. When it is cold out you have X temperature air supplied. When it is milder out you very well may have the same temperature supplied (true of most FHA systems). Hence the furnace has to cycle... with Mod Con boilers (or any hot water boiler with proper controls) you can almost infinitely vary the water supplied to heating elements, meeting the load quite accurately.
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