Full Flue Gas Analysis
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How to Use the Combustion Efficiency Calculator
Choose natural gas, propane, or No. 2 fuel oil. This sets the theoretical maximum CO2 percentage and the Siegert loss coefficients used in the calculation, which differ by fuel because of their distinct chemical compositions and stoichiometric air requirements.
Take the gross stack temperature reading with your combustion analyzer probe inserted into the breech downstream of the draft hood or barometric damper, with the boiler at steady-state firing. Enter the boiler room ambient temperature as the inlet temperature. The calculator computes net stack temperature automatically - this is the value used in the efficiency formula.
Most modern combustion analyzers measure O2 directly and calculate CO2 internally. Enter the O2 percentage shown on your analyzer. Always record the air-free CO reading - most analyzers display this automatically as a corrected value alongside the raw measured CO.
As you enter O2 and CO, three live gauges appear showing calculated CO2, excess air percentage, and the CO/CO2 ratio. The CO/CO2 ratio is a key diagnostic - values above 0.02 (2%) suggest incomplete combustion regardless of how the absolute CO number looks, and warrant burner investigation even at moderate CO ppm.
The gauge shows combustion efficiency from the Siegert formula. The breakdown table documents every parameter for your service record. Export the PDF report to attach to the customer's file or your CO inspection documentation, which is good practice for liability protection and required by some provincial gas codes for annual safety inspections.
Field Combustion Analysis for Canadian Gas Technicians
A proper combustion analysis is the single most informative test a technician can run on a gas or oil boiler. It reveals efficiency, safety, and tuning status in one set of readings taken in under five minutes. This calculator implements the standard Siegert formula used throughout the HVAC industry, with fuel-specific coefficients for natural gas, propane, and No. 2 fuel oil.
The Combustion Triangle: O2, CO2, and Excess Air
Complete combustion of any hydrocarbon fuel requires a specific stoichiometric ratio of air to fuel. In practice, burners run with some excess air above stoichiometric to ensure complete combustion under real-world conditions - variable gas pressure, draft fluctuations, and burner wear. The relationship between O2, CO2, and excess air is fixed by the fuel's chemistry: as excess air increases, O2 in the flue gas rises and CO2 falls proportionally, since the extra air dilutes the CO2 concentration. A natural gas boiler tuned to the ideal 2-4% O2 range typically reads 8.5-10.5% CO2, with about 10-25% excess air.
The Siegert Formula
Combustion efficiency = 100 − (K1 × Tnet / CO2 + K2 × Tnet), where Tnet is net stack temperature in °C, CO2 is the percentage in flue gas, and K1, K2 are fuel-specific constants. For natural gas, K1 ≈ 0.38 and K2 ≈ 0.009. The formula captures two loss mechanisms: dry flue gas loss (the K1 term, sensible heat carried away in hot exhaust gas, which gets worse with higher stack temperature and lower CO2/more excess air) and a smaller correction term for moisture and radiation losses (the K2 term). This formula, developed by German engineer Fritz Siegert in the early 20th century, remains the field-standard method because it requires only readings a portable analyzer provides - no laboratory fuel analysis needed. The boiler efficiency calculator applies this same approach with a simplified interface for quick seasonal efficiency estimates.
CO/CO2 Ratio as an Early Warning
Absolute CO ppm matters for safety, but the CO/CO2 ratio is a more sensitive diagnostic for combustion quality trending. A ratio below 0.004 (0.4%) indicates clean combustion. Between 0.004 and 0.02 suggests combustion is starting to deteriorate - worth monitoring at the next service call. Above 0.02 indicates significant incomplete combustion even if absolute CO ppm seems moderate, and warrants burner cleaning, gas pressure verification, or combustion air supply investigation. Tracking this ratio over successive annual inspections on the same appliance reveals gradual burner degradation before it becomes a safety issue. Use the flue sizing calculator to confirm the vent is not contributing to poor draft, which can raise CO even on a well-tuned burner.
CO Safety Thresholds
Air-free CO readings are the safety-critical number. Below 100 ppm air-free is generally acceptable for continued operation with standard maintenance. 100–400 ppm air-free warrants prompt tune-up before the next heating season. Above 400 ppm air-free requires immediate shutdown and service - most provincial gas codes and CSA B149.1 treat this as a critical safety defect. Separately, always check for spillage at the draft hood using a mirror or smoke match, and measure ambient CO in the mechanical room itself; any reading above 9 ppm ambient indicates the appliance may be spilling combustion products into occupied space, an immediate safety concern regardless of what the flue gas analysis shows. See the boiler flue sizing calculator if poor venting is suspected as the root cause.
Stack Temperature and Heat Exchanger Condition
Net stack temperature trending upward over successive annual visits on the same boiler, with O2 and excess air staying constant, points to heat exchanger fouling - scale, soot, or corrosion reducing heat transfer surface effectiveness. Each 10°C rise in net stack temperature above the boiler's baseline costs roughly 1% combustion efficiency. This is a useful diagnostic independent of the absolute efficiency number, since it isolates heat exchanger condition from burner tuning. Cross-check deteriorating stack temperatures against the boiler efficiency calculator, which estimates AFUE from the same field readings and can quantify the seasonal cost of the decline.
Documenting Results for Compliance
Many Canadian provinces require documented annual safety inspections for gas appliances, particularly in rental and commercial properties. A combustion analysis printout or exported PDF report with date, technician information, and all readings provides defensible documentation. Use the PDF export on this calculator to generate a clean record for your files or to leave with the customer alongside your service invoice.
Frequently Asked Questions
Air-free CO readings above 400 ppm indicate a serious combustion problem requiring immediate shutdown and service. Readings between 100–400 ppm air-free warrant prompt investigation before the next heating cycle. CSA B149.1 and most provincial gas codes treat higher CO as a critical safety defect, since CO is colourless, odourless, and can be fatal if it spills into occupied space. Any CO reading combined with spillage at the draft hood, or ambient CO in the mechanical room above 9 ppm, requires immediate corrective action by a licensed gas technician before leaving the appliance in operation. Check the flue sizing calculator if the venting system is suspected as a contributing factor.
Gross stack temperature is the raw flue gas reading from the analyzer probe in the breech. Net stack temperature subtracts the combustion air inlet temperature - typically boiler room ambient - from the gross reading. Efficiency calculations like the Siegert formula use net stack temperature because what matters is the temperature rise added by combustion, not the absolute flue gas temperature. A flue gas reading of 200°C in a 20°C boiler room gives a net stack temperature of 180°C, which is the value this calculator uses for the efficiency result. See the boiler efficiency calculator for a simplified seasonal AFUE estimate from the same readings.