High CO2 With Smoke and High Stack Temperature After Cleaning
After cleaning the flue passages of a 25-year-old cast-iron oil boiler, a technician tunes a 1.25 GPH 80°B nozzle at 140 psig. Final readings are: CO2 13.8%, O2 2.4%, smoke 1, gross flue temperature 720°F, ambient 70°F, draft −0.05 in. w.c., and a clean sight glass shows a bright orange-white flame. The boiler’s heat exchanger has heavy scale on the water side; the homeowner has not had it cleaned in years. The technician increases combustion air until smoke reads 0, and CO2 drops to 11.0%. The flue temperature remains 690°F. The boiler is rated 85% AFUE when new. A digital combustion analyzer calculates steady-state efficiency at 78% before air adjustment and 76% after.
1. Why did smoke read 1 at CO2 13.8%, and why did efficiency drop after correcting smoke?
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Smoke 1 at 13.8% CO2 means the burner is operating with too little excess air for complete combustion. CO2 near 13.8% is close to the stoichiometric maximum for No. 2 oil, so oxygen is scarce; some carbon escapes as smoke. The high CO2 number looks efficient on paper, but visible smoke is unburned fuel and is unacceptable.
Opening the air shutter to reach smoke 0 drops CO2 to 11.0%, which increases excess air. More excess air carries additional heat up the stack, so the analyzer’s steady-state efficiency falls from 78% to 76%. The key point is that the real efficiency problem is the 690–720°F flue temperature caused by scale on the water side, not the air adjustment. Scale insulates the heat exchanger, so combustion heat cannot transfer to water.
2. What should the technician recommend, and what final combustion targets should be set?
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Recommend water-side cleaning or descaling of the boiler. No burner adjustment can correct a scaled heat exchanger; the stack temperature will stay high until heat transfer improves. If the boiler is 25 years old and heavily scaled, compare cleaning cost with replacement and mention that AFUE 85% was the new rating, not current performance.
After cleaning, re-tune: target smoke 0 (trace), CO2 11.5–12.5% or O2 4–5%, and a stable flame. Set draft to the manufacturer’s range, typically −0.02 to −0.04 in. w.c. overfire. The flue temperature should drop substantially (often toward 450–550°F on a clean cast-iron boiler), and steady-state efficiency should rise. Verify with a combustion analyzer after the boiler reaches steady conditions.