The purpose of a condensing boiler?

• A. Increase gas consumption for faster heating
• B. Lower the operating temperature of the boiler only during winter
• C. Improve combustion by filtering exhaust
• D. Extract additional heat from flue gases by condensing water vapor, increasing efficiency

Answer: (D)

The key idea is recovering heat from the exhaust by condensing water vapor in the flue gases. When fuel burns, water vapor is created in the exhaust. A condensing boiler is designed so the return water is cool enough that the exhaust gas cools below its dew point, causing the water vapor to condense into liquid. The condensation releases the latent heat of vaporization, and that heat is transferred back into the heating system. This heat recovery boosts overall efficiency because heat that would have escaped up the chimney is now reused. The condensate is acidic, so the boiler materials and drainage must handle that. This isn’t about burning more gas, nor about filtering the exhaust, and it isn’t limited to winter—it's about capturing heat from moisture in the exhaust whenever conditions allow condensation.