What air ratio or flue gas oxygen should my furnace or boiler run at?
Air ratio is 21 divided by (21 minus the percent oxygen in flue gas). BEE's MSME guidelines target 1.15 to 1.20 for oil-fired and 1.12 to 1.15 for gas-fired furnaces. For boilers the targets are 1.32 to 1.38 on coal and 1.32 to 1.39 on biomass, with carbon monoxide kept within 200 ppm.
Furnace targets
- Forging, heat treatment and glass furnaces: 1.15 to 1.20 on liquid fuel, 1.12 to 1.15 on natural gas
- Aluminium pit furnace: 1.32 to 1.38 on coal, 1.15 to 1.20 on furnace oil
- Galvanizing and lead bath furnaces: 1.18 to 1.24 on liquid fuel
Boilers, thermic fluid heaters and hot air generators
- Coal: standard 1.35 to 1.40, target 1.32 to 1.38
- Biomass such as wood, bagasse and rice husk: standard 1.49 to 1.56, target 1.32 to 1.39
- Liquid fuel: standard 1.26 to 1.33, target 1.18 to 1.24
- Natural gas: standard 1.15 to 1.18, target 1.12 to 1.15
- These apply at 50 to 100% load.
Turning it into an oxygen reading
Rearranged, oxygen % = 21 minus 21 divided by the air ratio. An air ratio of 1.15 is about 2.7% oxygen, 1.20 is 3.5% and 1.38 is about 5.8%. Read oxygen and carbon monoxide together: low oxygen with carbon monoxide above 200 ppm means too little air.
How to hold the target
- Use an online or portable oxygen analyser and adjust air to the reading.
- Fit automatic air to fuel ratio control on the burner.
- Size the blower correctly and place it close to the burner.
- Seal openings so cold air does not leak in.
What Senseible does here
Senseible turns fuel invoices into monthly fuel use and emissions, so a combustion tune-up shows up as less fuel for the same output.
Last reviewed 2026-10-24.