Startup, Combustion Analysis, and Troubleshooting
Domain 6, Startup, Combustion Analysis, and Troubleshooting, tests commissioning, flue-gas interpretation, and a sequence-based diagnostic approach.
Startup checks
Verify gas type, orifice size, manifold pressure with burners firing, venting, combustion air, condensate, temperature rise, and flame appearance. Purge air from gas piping before ignition. Leak-test connections, then check CO, O2, CO2, stack temperature, and draft.
Combustion analysis
O2 shows excess air; CO2 shows how completely the fuel is being used. Air-free CO = measured CO × 20.9/(20.9 − O2). Excess air % = [(20.9/(20.9 − O2)) − 1] × 100. At 6% O2, excess air is about 40% and the air-free multiplier is about 1.40. Many service standards use 400 ppm air-free CO as an action level.
Troubleshooting
Follow the sequence: call for heat, inducer, pressure switch, ignition, flame proof, blower. A pressure switch that will not close suggests a blocked flue, water in the tubing, or a weak inducer. Low flame current points to the sensor. A tripped rollout switch points to blocked flue or a cracked heat exchanger. A tripping limit points to airflow.
Worked example
The analyzer reads 8% O2 and 200 ppm CO. Air-free CO = 200 × 20.9/(20.9 − 8) = 200 × 1.578 = 316 ppm. That is below 400 ppm but close enough to investigate. If CO rises when the blower starts, suspect a heat-exchanger leak or flame impingement.
Traps
Do not tune by CO alone; use O2 and CO2 together. Do not measure manifold pressure before the burners light. Do not reset a rollout switch without finding the cause. Do not mistake a lockout for a failed ignition module.