Flue Gas Calculators
Chimney Draft Calculator
Calculate residential or small commercial chimney draft in mm water column from chimney height and temperature difference.
Dry Flue Gas Calculator
Convert wet flue gas volume to dry basis by removing the water vapor fraction.
Flue Gas CO2 Calculator
Estimate flue gas CO2 percentage (dry basis) from a fuel's theoretical CO2 max and the excess air percentage.
Flue Gas Density Calculator
Calculate flue gas density from temperature, pressure, and average molar mass using the ideal gas law.
Flue Gas Flow Calculator
Calculate flue gas flow rate in Nm3/hr from fuel flow rate and gas volume per kg of fuel, for stack and fan sizing.
Flue Gas Nitrogen Calculator
Calculate flue gas nitrogen percentage (dry basis) by difference from measured O2 and CO2 percentages.
Flue Gas Oxygen Calculator
Estimate flue gas oxygen percentage (dry basis) from the excess air percentage supplied to combustion.
Flue Gas Temperature Calculator
Calculate flue gas exit temperature after heat is absorbed by a heat exchanger, economizer, or boiler.
Flue Gas Volume Calculator
Calculate total flue gas volume produced from fuel mass burned and gas volume per kg of fuel.
Flue Gas Water Vapor Calculator
Calculate the mass of water vapor produced in flue gas from a fuel's hydrogen content and mass burned.
Stack Draft Calculator
Calculate natural stack draft in Pascals from stack height, ambient temperature, and flue gas temperature.
Stack Gas Velocity Calculator
Calculate flue gas velocity in a stack or exhaust duct from volumetric flow rate and cross-sectional area.
Wet Flue Gas Calculator
Calculate total wet flue gas volume by adding water vapor volume to dry flue gas volume.
What Goes Up the Stack, Measured Precisely
Flue gas carries away the byproducts of combustion — nitrogen, carbon dioxide, water vapor, excess oxygen — and its volume, temperature, and composition determine everything from stack draft to emissions compliance. Thirteen calculators here cover flue gas volume, flow, draft, and gas composition on both a wet and dry basis.
Popular Flue Gas Calculators
Highlights across volume, draft, and composition:
- Flue Gas Volume Calculator — calculates total flue gas volume produced per unit of fuel burned.
- Stack Draft Calculator — calculates the natural draft a chimney produces from stack height and temperature difference.
- Flue Gas Oxygen Calculator — estimates excess oxygen remaining in flue gas, a key combustion tuning indicator.
- Dry Flue Gas Calculator — calculates flue gas composition with water vapor removed, the basis most gas analyzers report on.
- Flue Gas Density Calculator — calculates flue gas density from temperature and composition for draft and fan calculations.
Why Flue Gas Analyzers Report on a "Dry" Basis
Most portable flue gas analyzers remove water vapor from the sample before measuring oxygen, carbon dioxide, and other components, because water vapor condenses inconsistently in sampling lines and would introduce measurement error — this is why combustion tuning numbers (like "3% O2") are almost always dry-basis figures, not the true wet flue gas composition. Converting between wet and dry basis matters whenever a dry-basis reading needs to be compared against a wet-basis mass or volume balance for the same combustion process.
Frequently Asked Questions
Why does stack draft depend on temperature difference?
Draft is created by the density difference between hot flue gas inside the stack and cooler ambient air outside, so a bigger temperature difference (or taller stack) produces stronger natural draft.
Explore More
Need the emissions side of flue gas too? See the Emissions Calculators, or check combustion air on the intake side in the Air & Fuel Calculators.