✅ Combustion Efficiency
⚠ CO Safety Level
Result Breakdown
Full Calculation Table
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How to Use the Combustion Analyzer Calculator
Choose natural gas or propane, since the two fuels have different theoretical air requirements that affect the efficiency calculation.
Enter the O2 percentage, flue gas temperature, combustion air temperature, and air-free CO reading from your combustion analyzer, and the nameplate efficiency if you want to compare against it.
Click Calculate Efficiency to see gross and net efficiency, excess air percentage, and a CO safety check against common air-free CO action levels.
Understanding Combustion Analysis
A combustion analyzer measures the composition of exhaust gas leaving an appliance and turns that into 3 numbers that matter to a technician: how efficiently the fuel is burning, how much extra air is passing through unnecessarily, and whether carbon monoxide levels indicate a safety concern. Reading these numbers together gives a complete picture of appliance performance that no single measurement provides alone.
Steady-State Combustion Efficiency
Combustion efficiency represents how much of the fuel's heating value actually transfers into the building rather than escaping up the flue. This calculator estimates it with a Siegert-style flue loss formula: the difference between flue gas temperature and combustion air temperature is multiplied by a factor that depends on the O2 reading and the fuel, and the result is subtracted from 100 percent. The formula gives efficiency on a net, or lower heating value, basis, which is the European convention. AFUE ratings and nameplates in North America use the higher heating value, so the calculator converts using the ratio of the two, about 0.90 for natural gas and 0.92 for propane, and reports the gross figure with the net figure beside it. It is a steady-state check on a running appliance, so it will not match an AFUE rating, which is a seasonal figure; Natural Resources Canada's energy efficiency regulations for gas furnaces set a minimum of 95 percent AFUE for most new residential gas furnaces. The formula also assumes the flue gas stays above its dew point, so it does not credit the extra heat a condensing appliance recovers from water vapour.
Excess Air and Why It Matters
Complete combustion theoretically needs a precise ratio of air to fuel, but real-world burners need some excess air beyond that theoretical minimum to ensure thorough mixing and complete combustion. Too little excess air risks incomplete combustion, higher CO, and sooting. Too much excess air carries heat straight out the flue without transferring it to the building, lowering efficiency. A properly tuned appliance typically runs a moderate excess air percentage that balances these 2 failure modes, and this calculator treats 20 to 60 percent as a typical well-tuned range. Excess air also depends on the burner getting enough air in the first place, so the combustion air calculator is worth a look when the appliance sits in a tight mechanical room.
Reading O2 vs. CO2
Some analyzers measure O2 directly and calculate CO2, while others do the reverse, but both ultimately describe the same combustion state from different angles. A lower O2 reading, down to a point, generally correlates with less excess air and potentially higher efficiency, while a higher O2 reading indicates more excess air diluting the flue gas. The relationship between O2, CO2, and excess air depends on the specific fuel's chemistry. This calculator takes the O2 reading, works out CO2 from it using each fuel's approximate maximum CO2 (11.7 percent for natural gas and 13.9 for propane), and applies fuel-specific constants in the efficiency formula.
CO Safety Interpretation
Air-free CO readings, meaning CO expressed as if diluting air were removed from the sample, give a standardized way to compare readings across different excess air levels. Common industry guidance is to keep flue CO below 100 ppm air-free, even though the commonly cited maximum allowable in the flue is 400 ppm air-free. This calculator marks readings up to 100 ppm as typical, 100 to 400 ppm as worth investigating, and anything above 400 ppm as needing immediate action. Higher CO alongside acceptable efficiency numbers often points to a heat exchanger crack, burner misalignment, or venting issue rather than a simple air adjustment problem, and Health Canada notes that fuel-burning appliances can release carbon monoxide into a home when they malfunction, are not properly vented, or have blocked flues.
When Combustion Testing Fits Into a Service Call
Combustion analysis belongs in any gas appliance startup, annual maintenance visit, or troubleshooting call involving comfort complaints, unusual odors, or suspected venting issues. Pair it with a gas leak pressure test when opening up gas piping for service, and document results in a commissioning report for new installations. If CO or the flue readings point toward a cracked heat exchanger, the heat exchanger crack test calculator is the logical next step.
Frequently Asked Questions
Excess air is the amount of air supplied to combustion beyond the theoretical minimum needed to burn the fuel completely. Some excess air is necessary to ensure complete combustion in real-world conditions, but too much excess air carries heat out the flue and lowers efficiency, while too little risks incomplete combustion and higher carbon monoxide.
Common industry guidance is to keep flue CO below 100 ppm air-free, and the commonly cited maximum allowable in the flue is 400 ppm air-free, above which the appliance should be repaired or shut down. This calculator marks readings up to 100 ppm as typical, 100 to 400 ppm as higher, and above 400 ppm as high. Exact action levels can vary by code edition, jurisdiction, and appliance type, so always follow the specific guidance that applies to the equipment and region.
Higher flue gas temperature means more heat is escaping up the flue instead of being transferred to the building, which lowers steady-state efficiency. A properly tuned, clean appliance with good heat exchanger contact typically shows a lower flue temperature than one with scaling, poor draft, or a heat exchanger issue.
Yes. Natural gas and propane have different theoretical air requirements per unit of fuel, so this calculator uses the fuel type selection to apply the correct combustion relationship for accurate efficiency and excess air results.
Gross efficiency uses the higher heating value of the fuel, which counts the energy in the water vapour of the flue gas, and it is the basis for AFUE ratings and nameplates in North America. Net efficiency uses the lower heating value, which leaves that energy out, so it reads higher, and it is the European convention. For natural gas the lower heating value is about 90 percent of the higher, so a net result of 94 percent is about 85 percent gross.
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