In a condensing furnace, the latent heat is extracted by condensing which of the following?

In a condensing furnace, the latent heat is extracted by condensing which of the following?

• A. Fresh air.
• B. Water vapor.
• C. Fuel vapors.
• D. Exhaust (flue) gases.

Answer: (D)

Latent heat in a condensing furnace is recovered when water vapor in the combustion products condenses as the exhaust gases are cooled in the heat exchanger. Burning fuel produces water vapor; as the hot exhaust passes through the condenser and is cooled, that water vapor turns into liquid water. The phase change releases latent heat, which is captured and used to heat the home. Fresh air isn’t condensed for heat recovery, and fuel vapors aren’t condensed to provide this heat. The heat comes from the water vapor already present in the exhaust gases.

Latent heat in a condensing furnace is recovered when water vapor in the combustion products condenses as the exhaust gases are cooled in the heat exchanger. Burning fuel produces water vapor; as the hot exhaust passes through the condenser and is cooled, that water vapor turns into liquid water. The phase change releases latent heat, which is captured and used to heat the home. Fresh air isn’t condensed for heat recovery, and fuel vapors aren’t condensed to provide this heat. The heat comes from the water vapor already present in the exhaust gases.