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What is a condensing boiler?


A condensing boiler is a boiler designed to recover waste heat, and particularly latent heat from water vapour produced during the combustion of fossil fuels.

The condensing boilers include two stages of heat collection, as one can see in the following picture.

Legend:
  1. Flue almost dry at the chimney exit
  2. Sealed and corrosion resistant chimney
  3. Insulation
  4. "Primary" heat exchanger
  5. Return water
  6. Flow water
  7. "Secondary" heat exchanger area
  8. Fan
  9. Trap
Example of a sectional scheme of a condensing boiler.

In the first stage, the primary heat exchanger collects the heat directly from the hot gasses, as it occurs in a traditional boiler.

In the second stage, which contribute to the high efficiency of these boilers, the secondary heat exchanger collects the latent heat, which is gained by condensing the water vapour from the flue gases.

Some condensing boilers integrate these two stages of heat collection in the same heat exchanger.

Source:
ADENE, BRECSU (Building Research Establishment Garston, UK)