Flue Gas Dew Point Calculator

Why dew point matters for flue gas systems

If flue gas cools below its water vapor dew point anywhere along the exhaust path, moisture condenses on ductwork, heat exchangers, and stack surfaces - potentially causing corrosion, especially where fuel sulfur content also creates a more aggressive, lower acid dew point.

Worked example

For a water vapor partial pressure of 7.0 kPa:

Td = (243.5 x ln(70/6.112)) / (17.67 - ln(70/6.112)) = 38.98 C

Frequently asked questions

Is this the same as acid dew point? No - this calculates the water vapor dew point only. When fuels contain sulfur, sulfuric acid vapor forms in the flue gas and condenses at a meaningfully higher temperature than pure water vapor alone, requiring separate consideration in equipment and stack design.

Why does this matter for boiler design? Condensing boilers are deliberately designed to operate below this dew point to recover additional latent heat, while conventional boilers and their flue systems are designed to stay above it everywhere to avoid corrosion damage from condensation.