Of the readings a combustion analyzer gives, O2 says the most about how a burner is set. The oxygen left in the flue gas is air the burner didn't use, so it shows whether the burner is running lean, rich, or close to right.
High O2 means more air is going through the burner than the fuel needs. That air is heated and sent up the stack, which lowers efficiency. Low O2 is the opposite problem. With too little air, some of the fuel can't burn completely. CO rises, the flame can become unstable, and oil burners start to soot.
The target is the lowest O2 at which CO stays low and the flame is stable. No single number fits every burner. It depends on the burner design, the fuel, and the firing rate, and many burners need more excess air at low fire than at high fire. That's why readings are taken at several points across the firing range.
NOx adds another constraint. Most NOx forms in the hottest parts of the flame, and excess air affects flame temperature. Low-NOx burners use design features such as flue gas recirculation or staged firing to control it. On those burners we tune to the manufacturer's O2 and NOx targets rather than to the lowest possible O2.
Settings don't stay put. Changes in combustion air temperature, fuel properties, and wear in linkages or actuators move the fuel-air ratio over time. An annual tune-up resets it. An O2 trim system measures flue gas O2 continuously and adjusts the air to hold the setpoint between tune-ups.
Related reading: Combustion Tuning: What It Is and How Often, O2 Trim Systems and Boiler and Burner Formula Cheat Sheet. See all of our services on the What We Do page.
Want to know where your burner sits? Request service or email service@combustionspecialists.com.
