Learn & Understand

The Corrosive Threshold: Flue Gas Dew Point

There is a temperature below which flue gas turns destructive — the point at which its water vapor, and any acids it carries, condense into corrosive liquid on metal surfaces. Staying above this dew point, or deliberately going below it with the right materials, is one of the central design decisions in every combustion system.

The Dew Point of Exhaust

Just as humid air forms dew when it cools, flue gas has a dew point — the temperature at which its water vapor begins to condense into liquid. Above this temperature the water stays as harmless vapor; below it, droplets form on ductwork, heat exchangers, and stack walls. The dew point marks the boundary between dry, safe exhaust and wet, potentially damaging condensation.

When Sulfur Makes It Worse

For sulfur-containing fuels the danger is sharper. Sulfur in the exhaust forms sulfuric acid vapor, which condenses at a notably higher temperature than plain water — the “acid dew point.” This means acidic condensation can begin while the gas is still fairly warm, coating surfaces with corrosive acid. The acid dew point, higher and more menacing than the water dew point, is what engineers must respect.

Keeping the Gas Warm

Conventional combustion systems are designed to keep the flue gas above its dew point everywhere along the exhaust path, so nothing condenses to attack the metal. This limits how much heat can be recovered — the gas cannot be cooled too far — but it protects the equipment from corrosion that would otherwise eat through ducts and stacks over time.

Or Embracing Condensation

The alternative is to go the other way. Condensing appliances deliberately cool the flue gas below its dew point to recover the latent heat released as the water condenses, gaining efficiency — but they must be built of corrosion-resistant materials and drain the condensate safely. The dew point thus defines two design philosophies: stay above it and stay dry, or dip below it and reap the extra heat.

Finding the Dew Point

To estimate the flue gas dew point, use the Flue Gas Dew Point Calculator. Check the exit temperature against it with the Flue Gas Temperature Calculator, and the sulfur behind acid condensation with the Flue Gas SO2 Calculator.