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Consensing Boilers

Vinny71 Member Posts: 1
What makes a condensing boiler so efficient? Is it becasue there is less water in the boiler to heat and is it because there is a lower stack temperature in the chimney? I am trying to research this please let me know if you have any knowledge of this, thank you.


  • Jamie Hall
    Jamie Hall Member Posts: 23,430
    Presuming that you are looking at hot water

    boilers here.  And the basic answer is not that there is less water in the boiler (although there may be) but that the design is such that more of the heat is extracted from the flame.  Thus the stack temperature is low enough to condense the water of combustion; ideally this will occur somewhere where that latent heat will transfer to the water in the boiler return.

    Clearly the boiler return water must be well below 212 for this to work.

    It is possible to do this with steam boilers, too, with what are called economizers (which pass the stack gas through a heat exchanger, heating the return water).

    As has been noted in previous threads on condensing boilers, the condensate from the stack gas is quite acid and remarkably corrosive.
    Br. Jamie, osb
    Building superintendent/caretaker, 7200 sq. ft. historic house museum with dependencies in New England
  • CMadatMe
    CMadatMe Member Posts: 3,086

    The ability to extract flue gas condensation and turn it into btu's (1 gallon of condensate = roughly 8,000 btu's) is one part. The other being the ability to lower it's firing rate and water temperature to provide the heat needed for a particular moment based on a heat loss is the other.

    That is why you here many of the guys here talk about "heat loss." It is critical that a heat loss is done. It is needed by the installer to set up the right heating curve on the boiler to maximizes system and boiler effficiency. On retro-fit jobs you also have to measure the footage/capability of existing heat emmitters and then compare them to the heat loss to calculate the best heating curve for the application.

    A condensing boiler's true AFUE ranges from 87%-95% depending on set-up. I see more running more in the 87% to 90% range than the 90% to 95% range. Why? No heat loss..The installer just sets the boiler to 180 degrees for his design temp and walks away.

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