Natural gas needs roughly 10 cubic feet of air per cubic foot of gas for perfect combustion, called the stoichiometric ratio. Real burners add a margin, commonly 15 to 30 percent excess air, read as about 3 to 5 percent oxygen in the flue, so every fuel molecule finds oxygen. Too little creates CO and soot; too much heats extra air that goes straight up the stack.
We're running 9 percent O2, that's around 70 percent excess air, close the air down and you'll see the stack temperature drop.
Every few percent of unnecessary O2 in the flue is fuel spent heating outdoor air.
Also heard as
- EA
Related terms
Combustion analysis
Testing a burner's flue gas for oxygen, CO, CO2 and temperature to set it up safely and efficiently.
O2 trim
A control that measures flue gas oxygen and continuously adjusts the air-fuel ratio to hold it at target.
Running rich
Burning with too much fuel for the air available, so combustion is incomplete.
Stack temperature
The temperature of the flue gas as it leaves the boiler, a direct measure of how much heat is going up the chimney.