Yellow Flame and Rising CO at Low Fire on a Gas-Fired Boiler

Immediate action: shut the burner down and take the boiler off line. 700 ppm CO in the flue gas is unburned fuel leaving the furnace. That is a fuel-rich condition with two consequences: potential puffback or furnace explosion on the next light-off, and carbon monoxide migrating into the boiler room and the building if the breeching or casing leaks.

Do not attempt to correct this by turning screws until you know which side of the problem you are on. Keep the boiler on standby only under a proper purge, ventilate the room, and notify the building engineer.

What the symptoms indicate together:

• Yellow-tipped flame plus high CO plus falling flame signal = fuel-rich mixture or poor fuel/air mixing at low fire.
• Rising stack temperature = the flame is longer or burning late in the furnace, or soot is forming on the tubes. Either raises the gas temperature leaving the boiler.
• Unchanged draft rules out a stack or breeching draft problem and points downstream at the combustion air supply and the burner's own air path.

The single most useful next reading is O2 and CO2 at the same low-fire point. Low O2 with high CO confirms air deficiency; normal or high O2 with high CO points to a mixing or burner-internals problem instead.

Work from the outside in, and record a baseline at low, mid, and high fire at each step.

1. Restore combustion air openings. Two louvers closed is a real air-supply reduction. Open them, confirm the openings are sized for the total connected input and are unobstructed, and confirm no storage has been stacked against them.
2. Check the burner linkage against its commissioning marks. A set screw replaced "by feel" can leave the low-fire air damper nearly closed while high fire still looks right, which matches the reported pattern exactly. Compare linkage position and paint marks, stroke, and damper angle at each firing rate.
3. Draw flue gas at the same sample point and compare O2, CO, CO2, and stack temperature across the firing range. If low-fire O2 is well below the 6 percent originally recorded, the air-side explanation is confirmed.
4. Inspect the burner head, diffuser, gas orifices, and scanner sight tube. A cracked diffuser, plugged orifices, or a dirty sight glass will produce a weak flame signal and poor mixing even with correct air.
5. Check manifold gas pressure at low fire. Low fire is often set by low manifold pressure; a drifting regulator can over-fire the low-fire position and produce a rich flame.

If all of that is clean and the CO persists, call a qualified combustion technician with a calibrated analyzer and re-tune before the boiler goes back on automatic.

Return to service is a documented event, not a judgement call made at the panel.

• Re-tune by a qualified technician using a calibrated analyzer, covering low, mid, and high fire, with the combustion air openings in their permanent condition.
• Record O2, CO, CO2, stack temperature, and manifold pressure at each firing rate, and compare to the original commissioning data. Acceptable targets are typically around 3 percent O2 at high fire and higher excess air (roughly 5 to 7 percent O2) at low fire, but the manufacturer's data governs.
• Verify flame signal is above the manufacturer's minimum at every firing rate, not just high fire, and confirm the scanner sighting is clean.
• Prove the flame safeguard by a documented flame-failure test at low fire, confirming the main fuel valve closes and the burner locks out.
• Log the cause, the correction, and the names of everyone who worked on the burner, and note in writing that louver positions are not to be changed without an engineering review.

The boiler does not go back to automatic firing just because the flame is no longer yellow. It goes back when the numbers across the whole firing range say it is safe.