Applicable Systems
Baxi Heating Boiler
Symptoms
The flue gas analysis sensor has detected an abnormal CO2 or CO reading. The boiler enters a safety lockout and displays a flue gas fault.
Possible Causes
- Flue gas heat exchanger blocked by soot or debris from incomplete combustion
- Flue terminal (external wall cap) blocked by bird nest, debris, or ice
- Combustion air intake restricted, causing incomplete combustion and high CO
- Burner gas/air ratio out of adjustment after component replacement
Troubleshooting Steps
- Step 1: Inspect the flue terminal externally for blockage
Clear any debris, nests, or ice before restarting the boiler - Step 2: Check the combustion air intake path for restrictions
Ensure ventilation openings are clear and unobstructed - Step 3: Use a flue gas analyzer to measure CO2 and CO at the flue test port
CO2 should be 8-9% for natural gas; CO should be below 100ppm - Step 4: If readings are out of specification, clean the heat exchanger and adjust the gas valve
Use a manometer to set the correct gas/air ratio per the commissioning table
How to Reset
Reset: After cleaning and adjustment, verify the flue gas readings are within specification, then press the boiler reset button.
Common Replacement Parts
- Flue gas analyzer: Electronic, CO2 and CO measurement (Calibrate before use)
- Heat exchanger cleaning kit: Brush set and solvent (Match to boiler model)
- Gas valve: Modulating type, OEM-specific (Adjust only with a combustion analyzer)
Frequently Asked Questions
- Q: What are safe flue gas readings?
For natural gas: CO2 8-9.5%, CO <100ppm, O2 5-7%. For propane: CO2 10-11.5%, CO <100ppm.
- Q: How often should the heat exchanger be cleaned?
Annually for residential boilers. Condensing boilers need the condensate trap cleaned at the same time.
- Q: What causes high CO?
Incomplete combustion from restricted air supply, dirty burner, or incorrect gas/air ratio. High CO is dangerous and requires immediate attention.
Source: OEM Installation Manual
Category: Home Boiler