High CO at High Fire After Burner Reassembly

Read the pattern, not the single number.

• CO is low at low fire (60 ppm) and very high at high fire (1,800 ppm) with O2 still around 3%. That is a mixing or fuel-delivery problem that only shows up when gas flow is high - not a global lack of air.
• O2 of 3.2% at high fire is not by itself alarming for gas, but O2 is an average. A long, lazy, yellow-tipped flame with good average O2 is the classic picture of a flame that is rich in places and impinging on a relatively cool surface before it finishes burning.
• Supply pressure of 6.0 in. w.c. against a 7.0 to 11.0 in. w.c. requirement is out of spec. The burner cannot reliably deliver its rated input, and a regulator or orifice problem at reassembly is a strong candidate - wrong orifice, diffuser or baffle installed backwards, primary air shutter moved, or a gas cock left partly throttled.

Can it stay on line? No. 1,800 ppm CO means substantial incomplete combustion. It is a sooting and CO-exposure risk, and running overnight simply bakes the fault into the tubes. Shut down, or hold at low fire only with someone actively troubleshooting - the contractor's 'normal for a cold boiler' is not a defensible answer.

Order of work:

1. Fix the fuel supply first. Bring gas pressure at the train into the 7.0 to 11.0 in. w.c. range and verify manifold pressure against the commissioning curve at low, mid, and high fire. A burner cannot be tuned around a low gas supply.
2. Inspect the reassembly against the manufacturer's drawing: orifice size and orientation, diffuser/baffle position, air shutter, flame scanner sighting, and whether anything was left out or reversed.
3. Then adjust combustion air. Set high fire for roughly 3% O2 on gas, then mid and low fire, checking CO at each point. Target well under 400 ppm air-free - a clean gas flame runs near 0 to 100 ppm; 1,800 is not a tuning nuance.
4. Verify draft and the flue path - check for anything left in the flue or breeching and confirm negative pressure at the breach.
5. Recheck stack temperature after the fix; it should be consistent with the corrected excess air.

What not to do:

• Do not respond to high CO by simply opening the air shutter. That can cool the flame, hurt low-fire stability, and raise stack losses while leaving a mixing defect in place.
• Do not drill or swap orifices without the manufacturer's data.
• Do not accept a single-point test at one firing rate on a modulating burner.
• Do not sign off on a combustion test run with an analyzer that has not been zeroed and calibrated.

Lock the burner out and escalate.

• The boiler is not fit to run. Persistently elevated CO with corrected fuel supply and reasonable excess air points to a burner/combustion-chamber problem: wrong or damaged diffuser, flame impingement from a mislocated flame, a tube or baffle disturbing the flame pattern, or a combustion volume that no longer matches the firing rate (an overfired 600 hp boiler behaves this way).
• Confirm the analyzer with a second reading at the same port and a fresh zero - a bad sensor can manufacture a fault, and you do not want to pull a boiler apart on a bad cell. Repeat the measurement at low, mid, and high fire and record it.
• Get the burner manufacturer or a qualified combustion service on site with the original commissioning data. This is no longer an operator tuning task.
• Document everything - readings, times, the contractor's involvement, and the decision to shut down. If the contractor disagrees, the logged data and the shutdown decision are the operator's protection.
• On restart, require a full combustion test across the firing range with CO, O2, and stack temperature recorded before the boiler is returned to normal service.