Combustion Analysis
Flue gas composition analysis including O2, CO, CO2, and stack temperature readings, combustion efficiency calculation methods, and proper use and calibration of electronic combustion analyzers.
- Explain the relationship between O2, CO2, and CO in flue gas and what each reading indicates about combustion quality
- Calculate combustion efficiency using stack temperature, O2, and ambient temperature
- Perform proper combustion analyzer calibration, probe placement, and steady-state testing
- Interpret air-free CO readings and determine when combustion adjustments are required
Leçon 1
Understanding Flue Gas Composition
What Combustion Analysis Reveals
Combustion analysis is the systematic measurement of flue gas composition to evaluate how efficiently and safely a fuel-burning appliance is operating. By measuring the gases produced during combustion, a technician can determine whether the appliance is burning fuel completely, wasting energy up the flue, or producing dangerous levels of carbon monoxide.
Every fuel-burning appliance - whether it runs on natural gas, propane, or oil - combines fuel with oxygen from the air. The ideal chemical reaction for natural gas (methane, CH4) is:
CH4 + 2O2 = CO2 + 2H2O + Heat
In a perfect reaction, all the fuel combines with exactly the right amount of oxygen, producing only carbon dioxide (CO2), water vapor (H2O), and heat. No carbon monoxide, no unburned fuel. But perfect combustion never happens in real-world appliances. There is always excess air, minor impurities in the fuel, and flame impingement issues that produce trace amounts of CO and other incomplete combustion byproducts.
The Four Key Flue Gas Measurements
Oxygen (O2): The oxygen remaining in the flue gas after combustion. Ambient air contains approximately 20.9% oxygen. If the flue gas shows 5% O2, the appliance consumed 15.9% of the available oxygen during combustion. For natural gas appliances, the target O2 range is 5-9% in the flue. Lower O2 means less excess air (more efficient but riskier for CO production). Higher O2 means more excess air is diluting the flue gas (less efficient, more heat lost up the flue).
Carbon Monoxide (CO): CO in flue gas indicates incomplete combustion. A well-tuned natural gas appliance should produce less than 25 ppm air-free CO. Readings between 25 and 100 ppm air-free indicate the appliance needs attention. Readings above 100 ppm air-free indicate a significant combustion problem requiring immediate service. Air-free CO is the CO reading mathematically corrected to remove the dilution effect of excess air, providing a true measure of combustion quality.
Carbon Dioxide (CO2): CO2 is the primary product of complete combustion. For natural gas, the theoretical maximum CO2 (at zero excess air) is 11.7%. For propane it is 13.7%, and for fuel oil it is 15.4%. In practice, CO2 readings of 8-10% for natural gas indicate good combustion with normal excess air levels. Very low CO2 (below 6%) indicates excessive dilution air, while CO2 approaching the theoretical maximum suggests dangerously low excess air.
Stack Temperature: The temperature of the flue gas measured at the breach (flue pipe connection to the appliance). Higher stack temperatures mean more heat is escaping up the flue instead of transferring to the building. For a standard efficiency natural gas furnace (80% AFUE), typical stack temperatures range from 300-500 degrees F. For a condensing furnace (90%+ AFUE), stack temperatures should be 80-130 degrees F because the secondary heat exchanger captures most of the heat.
The O2-CO2 Relationship
O2 and CO2 in flue gas have an inverse relationship. As excess air increases, O2 rises and CO2 falls. As excess air decreases, O2 falls and CO2 rises. This relationship is fixed for any given fuel type and is governed by the fuel's chemical composition.
This inverse relationship is a useful diagnostic check. If both O2 and CO2 are reading low simultaneously, it typically indicates dilution air is entering the flue system (from a draft diverter, a leaking flue connector, or a cracked heat exchanger) rather than passing through the combustion zone. This is a red flag that requires investigation.
Good Combustion (Natural Gas)
O2: 5-9%
CO2: 8-10%
CO (air-free): Less than 25 ppm
Stack temp (std eff): 300-500 F
Problem Indicators
O2 above 12%: Excessive excess air or dilution
CO2 below 6%: Major dilution issue
CO (air-free) above 100 ppm: Unsafe combustion
Both O2 and CO2 low: Flue leak or cracked HX
Air-Free CO Explained
The "as-measured" CO reading from a combustion analyzer includes the dilution effect of excess air in the flue. Air-free CO removes that dilution to show the true CO concentration at the flame. The formula is:
Air-Free CO = Measured CO x (20.9 / (20.9 - Measured O2))
For example, if the analyzer reads 30 ppm CO and 7% O2:
Air-Free CO = 30 x (20.9 / (20.9 - 7)) = 30 x (20.9 / 13.9) = 30 x 1.504 = 45 ppm air-free
Most modern combustion analyzers calculate air-free CO automatically. The air-free number is always higher than the as-measured number because it removes the dilution effect of excess air. Always use the air-free value when evaluating combustion quality and comparing against NCI standards.
The four key flue gas measurements are O2, CO, CO2, and stack temperature. O2 and CO2 have an inverse relationship - if both read low simultaneously, suspect dilution air entering the flue system. Air-free CO is the standard for evaluating combustion quality: under 25 ppm is good, over 100 ppm requires immediate service.