Gas Conversion Leaves High CO and an Unstable Flame
A 300-hp, 150-psig firetube boiler was converted from No. 2 fuel oil to natural gas. After the contractor's tune-up, the burner runs steadily at high fire, but the portable analyzer shows 1.5% O2 and 600 ppm CO; stack temperature is 480°F and the flame scanner signal fluctuates between 2 and 4 volts instead of holding near 8. The gas manifold pressure is 7 inches water column, within the burner nameplate range of 5 to 14 inches. The draft at the breach is -0.05 inch water column. The boiler room has a CO monitor that reads 35 ppm near the burner inspection door. The operator wants to open the forced-draft damper manually to bring CO down. The building engineer says the burner was 'factory set' and should not be touched. The boiler is carrying 8,000 lb/hr and the stack is not smoking.
1. Interpret the O2, CO, scanner, and draft readings. What does the combination suggest, and why is opening the damper manually not a sound first step?
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1.5% O2 is very little excess air; 600 ppm CO confirms incomplete combustion. The fluctuating scanner signal says the flame is not stable, so the problem is not simply 'add air.' A manual damper opening can change the draft, lift the flame, alter the scanner signal, and create an unsafe condition before the real cause is found. The 35 ppm CO near the inspection door also means combustion products are entering the room; ventilate and find the leak. Possible causes include a gas manifold pressure that is too high at the burner for the orifice, a mis-set air linkage, a dirty or restricted burner head, a fan or air-path problem, or a flue restriction. The 7-inch manifold pressure is within nameplate range but does not prove the burner is set correctly.
2. List the diagnostic and adjustment steps you would take before returning the burner to automatic, including what would make you shut down immediately.
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First, take the burner off automatic and shut it down if CO is rising, the scanner is unstable, or flame is visibly unsafe. Ventilate the room and investigate the CO source. Then check combustion-air openings, fan belts and dampers, flue draft, gas train for leaks, gas pressure at the manifold while firing, and burner head and orifice condition. Use a calibrated analyzer at low fire first, then high fire; adjust the air/fuel linkage to the burner manufacturer's curve, not to a guessed O2 number. Verify the scanner sees a stable flame at both ends of the firing range. Return to automatic only after repeatable readings and a stable scanner. If CO stays high, or the flame cannot be stabilized, keep the boiler out of service and call a qualified burner technician.
3. How would you answer the building engineer's claim that a factory-set burner should not be touched?
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Factory settings are a baseline, not a field guarantee. After a fuel conversion, the burner, orifice, linkage, and air path must be commissioned in the actual boiler with the actual venting and draft. A factory setting cannot account for a dirty blower, a changed gas heating value, or a restricted flue. The operator should not make random adjustments, but a qualified technician should verify and document combustion across the firing range. The claim is therefore not a reason to leave a flame with 600 ppm CO and an unstable scanner in service.