Combustion analysis is the systematic measurement of flue gas products from gas-fired appliances to determine operating efficiency, verify safe operation, and diagnose performance problems. The tool at the center of this process — the combustion analyzer — measures oxygen (O2) and carbon monoxide (CO) directly, calculates carbon dioxide (CO2) and combustion efficiency, and displays stack temperature and draft pressure. According to TSI, "combustion analysis begins with the measurement of flue gas concentrations and gas temperature, and may include the measurement of draft pressure and soot level" (TSI, Combustion Analysis Basics).
What the analyzer measures
Combustion is the reaction of fossil fuels with oxygen in air to produce heat. The principal products of complete combustion are carbon dioxide (CO2) and water vapor. When combustion is incomplete — too little air, poor mixing, or a cracked heat exchanger — carbon monoxide forms instead. A combustion analyzer measures what the human eye and nose cannot detect.
| Parameter | How Measured | Good Range | Action Level |
|---|---|---|---|
| Oxygen (O2) | Direct (electrochemical sensor) | 5-12% | 0% or >15% |
| Carbon monoxide (CO) | Direct (electrochemical sensor) | 0-80 ppm | >200 ppm or >400 ppm air-free |
| Carbon dioxide (CO2) | Calculated from O2 | 5-10% (natural gas) | >12% |
| Stack temperature | Thermocouple | 330-400°F (condensing) | >500°F (80% AFUE) |
| Draft pressure | Differential pressure sensor | -0.02 to -0.04 in. w.c. | Positive draft or no draft |
| Combustion efficiency | Calculated | 78-82% (80 AFUE), 90-98% (condensing) | Below rated AFUE |
CO2 is not measured directly by most analyzers — it is calculated from the measured O2 concentration. Air contains 20.9% oxygen; the oxygen remaining in the flue gas indicates how much excess air was supplied beyond what was theoretically required for complete combustion (TSI, Combustion Analysis Basics).
The CO safety threshold
Carbon monoxide is the critical safety measurement. The ANSI Z21.47 standard for gas-fired central furnaces specifies that CO in an air-free flue gas sample should not exceed 400 ppm under normal operating conditions. The National Comfort Institute uses a more conservative best-practice maximum of 100 ppm as-measured. A well-tuned furnace should produce well under 50 ppm as-measured in flue gas. The DOE requires that if ambient CO in the living space exceeds 9 ppm, the area must be ventilated before further testing. During worst-case draft testing, if ambient CO in the combustion appliance zone exceeds 20 ppm, the draft test must be stopped immediately (DOE, Combustion Appliance Safety).
CO air-free vs. as-measured.The analyzer's "CO air-free" reading mathematically removes the dilution effect of excess air from the CO measurement, giving the CO level as if zero excess oxygen were present. This is the value ANSI Z21.47 uses for the 400 ppm limit. To comply with BPI-1200 and AHRI 1260 standards, the analyzer's O2 reference must be set to 0% for correct air-free CO readings (Fieldpiece, CAT85 Combustion Analyzer).
Efficiency gains from proper tuning
The EPA's Combustion Efficiency Optimization Manual provides a clear rule of thumb: boiler efficiency increases by 1% for each 1.3% reduction in flue gas oxygen — equivalent to a 15% reduction in excess air. Efficiency also improves by 1% for each 40°F reduction in stack gas temperature (EPA, Combustion Efficiency Optimization).
The EPA provides a concrete example: reducing flue gas oxygen from 7.4% to 3.0% and stack temperature from 475°F to 440°F improved combustion efficiency from 82.3% to 85.7% — a 3.4% efficiency improvement and approximately 4% fuel savings (EPA, Combustion Efficiency Optimization).
Combustion efficiency is calculated as:
\[ \text{Efficiency \%} = 100\% - \left( \frac{\text{stack heat losses}}{\text{fuel heating value}} \times 100 \right) \]
The two largest heat losses are sensible heat in the stack gas (carried out by high exhaust temperatures) and latent heat from water vapor. Reducing excess air reduces the volume of gas that absorbs heat and carries it up the flue. Reducing stack temperature means more heat was transferred to the conditioned space instead of being wasted (TSI, Combustion Analysis Basics).
Too little air is dangerous.Reducing excess oxygen improves efficiency — but only to a point. The EPA warns that "further reduction of excess oxygen beyond the minimum condition results in very rapid increases in CO and other combustibles, flame instability, furnace pulsation, and boiler explosions". The minimum oxygen level is found by reducing excess air until CO begins to rise, then backing off slightly. This is why a combustion analyzer is essential — you cannot safely tune a burner by guesswork (EPA, Combustion Efficiency Optimization).
Proper testing procedure
The DOE and industry standards specify a clear testing protocol:
- Pre-test: Calibrate the analyzer in fresh outdoor air, away from exhaust. Verify ambient CO is below 9 ppm.
- Stabilize: Run the furnace for at least 10-15 minutes to reach steady-state operation. All access panels must be in place.
- Probe placement: Insert the probe into the flue downstream of the last heat exchanger but upstream of any draft diverter or barometric damper. The tip should be centered in the gas stream.
- Record: Wait 60-90 seconds for readings to stabilize. Record O2, CO, CO2, stack temperature, draft, and calculated efficiency.
- Purge: After testing, keep the pump running in clean air until CO returns to 0-1 ppm. This extends sensor life.
The probe must be positioned before any draft diverter because air entering through the draft hood dilutes the flue gas sample with ambient air, producing falsely low CO and O2 readings (DOE, Combustion Appliance Safety).
Combustion air requirements
For atmospheric (non-sealed) combustion appliances, the DOE requires that the space containing the equipment provide 50 cubic feet per 1,000 Btu/h of combined input rating for all combustion appliances in the space. In confined spaces, vents to outdoors must provide at least 2 square inches per 1,000 Btu/h of free area, with half the grille area within 1 foot of the ceiling and half within 1 foot of the floor. Minimum net free area for each vent is 100 square inches (DOE, Combustion Appliance Safety).
Standards and compliance
Combustion testing touches multiple standards: ANSI Z21.47 sets the 400 ppm CO air-free limit for gas furnaces, BPI-1200 and AHRI 1260 define testing protocols for building performance professionals, and NFPA 54 (National Fuel Gas Code) governs vent and chimney installation. The EPA's optimization manual notes that manufacturers' recommendations and local and national safety codes must be followed at all times during tuning (EPA, Combustion Efficiency Optimization; DOE, Combustion Appliance Safety).
Naming a product or manufacturer in this article is for identification purposes only and does not constitute an endorsement by HVAC Zone Inc. We are a multi-brand dealer and remain neutral on brand preference.
Is your furnace running at peak efficiency? Request a consultation — we'll perform a full combustion analysis, check CO levels, measure stack temperature and draft, and tune your equipment to manufacturer specifications for maximum efficiency and safety.