# What air ratio or flue gas oxygen should my furnace or boiler run at?

Canonical: https://senseible.earth/answers/furnace-air-ratio-oxygen-target

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.

- [Boiler excess air fuel saving](https://senseible.earth/answers/boiler-excess-air-fuel-saving)
- [Thermic fluid heater emissions](https://senseible.earth/answers/thermic-fluid-heater-emissions-scope-1)
- [Recuperator fuel savings](https://senseible.earth/answers/recuperator-combustion-air-preheat-fuel-saving)

Sources: [BEE, Energy Conservation Guidelines for MSME Sector (2019): Tables 6.8, 6.22, 6.39, 6.66 and 6.71, with the air ratio formula](https://beeindia.gov.in/WriteReadData/L45218/1734411984.pdf)

Last reviewed 2026-10-24.