Key Takeaways
- Test all three states, not just steady state: Combustion readings change at light off, steady state, and shutdown. Measuring only one gives you an incomplete picture
- O2, CO, and CO2 are linked by chemistry: These aren’t random numbers. They connect through excess air, and understanding the link helps you spot real problems
- Stack temp and manifold pressure ground your readings: They act as a reality check on gas readings and reveal mechanical issues your analyzer can’t see alone
- Your analyzer is only as good as its setup: Fuel type selection, calibration, and proper zeroing all affect data accuracy and customer trust
Why Combustion Analysis Matters Beyond the Efficiency Number
Most techs learn that furnace efficiency comes from flue gas O2 and stack temperature. That’s true. But it’s just the tip of the iceberg. What you’re really doing is checking that the equipment burns fuel safely and uses energy well.
A furnace can look “efficient” on your analyzer and still have serious problems. The heat exchanger might be breaking down. A fuel control might be drifting out of spec. A blower issue might be starving the system of combustion air. Your analyzer gives you the data to catch these problems before they become failures.
This isn’t theory. Research from the Midwest Alliance for Clean Energy found that field-installed furnaces run at 6.4% lower steady-state efficiency than the same units tested in a lab.¹ That gap between rated and real-world performance is exactly what combustion analysis closes.
Think of combustion analysis as a conversation with the equipment. The flue gas composition tells you what’s happening inside the combustion chamber. Your job is to listen.
Related podcast episode: Tyler Nelson Part 1 – Combustion fundamentals and analyzer setup with Sauermann Group’s Global Trainer.
The Three Operational States: Why One Test Isn’t Enough
Many techs run a single combustion test during steady operation and call it done. That leaves critical info on the table.
Light Off
Light off is when the burner first ignites. The fuel control valve just opened. The igniter is active. The system is starting the combustion process. Conditions at light off differ from steady state. O2 levels will be different. CO may spike for a moment. The system is transitioning into stable burning.
Steady State
Steady state is where most techs test. The furnace has run for several minutes. The heat exchanger is warm, the system has stabilized, and combustion is consistent. This is useful data. But it’s only one snapshot of what the equipment does during normal operation.
Shutdown
Shutdown is when the burner cuts off. Fuel stops flowing, but residual heat remains. Combustion doesn’t stop instantly. Shutdown data can reveal whether the fuel control seals properly and whether residual burning is happening.
This is also one of the best moments for finding cracked heat exchangers. When the indoor blower pressurizes the heat exchanger after the flame goes out, a crack lets air through. The analyzer shows a sudden spike in O2.
Comparing the Three States
Properly set up equipment should show similar readings across all three states. If light off O2 is way different from steady state O2, something has changed. That could be a mechanical problem or control drift. Without testing all three, you’ll miss it.
A complete combustion analysis report covers all three states. That’s not extra work for the sake of being thorough. It’s the difference between seeing the full picture and guessing.
Reading Combustion Gas Numbers: O2, CO, and CO2
Your analyzer measures three main gases: oxygen, carbon monoxide, and carbon dioxide. These aren’t random numbers. They’re linked by chemistry. Understanding how they connect is how you spot problems that raw numbers might hide.
Oxygen (O2): How Much Air Is Left Over?
O2 tells you how much unburned oxygen exits the flue. In a perfect burn, all fuel would combust and zero oxygen would remain. But that never happens in the real world. You need some excess air to make sure all fuel molecules burn.
The key word is “excess.” How much excess air you need depends on the burner type and furnace design. For modern fan-assisted and power-burner furnaces, expect 3 to 6% O2 at steady state. That works out to roughly 17 to 40% excess air. For older atmospheric or natural draft units with draft hoods, the range is wider: 7 to 9% O2. These systems pull in more dilution air by design.²
Low O2 means less excess air. Combustion is more complete, efficiency is higher, but you’re closer to the edge of safe operation. High O2 means more air coming in. Combustion is less efficient but the equipment runs safer with a bigger margin.
Carbon Monoxide (CO): The Red Flag Gas
CO forms when combustion is incomplete. It means not all the fuel burned fully. Understanding CO action limits and testing protocols is essential for every tech.
Here’s where confusion comes in. The 50 ppm figure you hear thrown around is actually the OSHA Permissible Exposure Limit for 8-hour workplace ambient air. It was never meant as a flue gas standard. For flue gas on vented residential equipment, the National Comfort Institute (NCI) uses 100 ppm as-measured as their best-practice max.³ The ANSI Z21.47 standard allows up to 400 ppm air-free for vented equipment.
A well-tuned furnace should produce well under 50 ppm as-measured in flue gas. Anything above 100 ppm as-measured warrants investigation. Above 400 ppm air-free requires immediate shutdown.
For a deeper dive, check out Carbon Monoxide: The Silent Killer Every Tech Should Know How to Handle.
Carbon Dioxide (CO2): The Complement to O2
CO2 is the byproduct of complete combustion. Together with O2, it tells you about combustion balance. The theoretical max CO2 for natural gas is about 11.7 to 11.9%. You’d only see that at perfect stoichiometric combustion with zero excess air. That’s neither practical nor safe.
In a well-burning furnace on natural gas, expect 8 to 10% CO2 at steady state. If you read above 10%, you’re getting very tight on excess air and approaching unsafe territory.
Here’s the relationship: if O2 is low, CO2 should be higher (more complete burning). If O2 is high, CO2 will be lower (more excess air, less complete burning). This link is predictable and tied to the fuel type and air density. If O2 and CO2 don’t line up the way they should, check for analyzer drift, calibration error, or an atmospheric issue affecting the reading.
Related podcast episode: Tyler Nelson Part 2 – Advanced combustion data interpretation and field diagnostics.
Stack Temperature and Manifold Pressure: The Reality Check
Flue gas readings matter, but they exist in a physical context. Stack temperature and manifold pressure ground those readings in actual equipment behavior.
What Stack Temperature Tells You
Stack temperature measures the gases leaving the furnace. For condensing furnaces (90%+ AFUE), expect exit temps around 100 to 140°F. For non-condensing furnaces (80% AFUE, induced draft), expect 300 to 400°F.² The exact range depends on the design.
Stack temp plus O2 is how efficiency gets calculated. But stack temp also shows whether the heat exchanger is doing its job. If the temp is unusually high, the heat exchanger might be fouled or restricted. If it’s unusually low on a non-condensing furnace, you might have a cracked heat exchanger allowing combustion gases to escape into the home.
Non-condensing furnace flue temps should never drop below 250°F. Below that, you risk acidic condensation that will destroy the flue system.
What Manifold Pressure Reveals
Manifold pressure is the pressure at the burner. On natural gas, the standard is 3.5 inches of water column (nominal). The typical operating range is 3.2 to 3.8″ WC. On propane, expect 10 to 11″ WC.² Two-stage or modulating furnaces will have stage-specific pressures on the rating plate. Always check against the data plate while the unit is firing.
Why does this matter? Manifold pressure and flue gas readings are linked. Low manifold pressure means the burner isn’t getting enough gas. The flame goes lean. O2 rises, CO2 drops, and efficiency falls. Too-high manifold pressure starves the burner of oxygen. CO rises. For more on troubleshooting gas fired ignition problems, including fuel valve and regulator issues, we have you covered.
Without measuring manifold pressure alongside your analyzer, you’re only half-diagnosing. You know something is off, but you might chase symptoms instead of root causes.
Analyzer Selection and Fuel Type: Why Your Tool Matters
Not all combustion analyzers are built the same. The one you choose affects your data quality and the trust customers place in your work.
Some analyzers measure only O2, CO, and stack temp. Others add CO2, manifold pressure, draft, and gas density compensation. More parameters means a more complete diagnostic picture.
Fuel Type Selection Matters More Than You Think
Fuel type selection is critical. Natural gas, propane, and oil all have different combustion profiles. If your analyzer is set to the wrong fuel, the chemistry behind the calculation will be off. Your efficiency number won’t be accurate.
This trips up a lot of techs. They assume the fuel type is set and never verify. Before every test, confirm the correct fuel is selected. It takes five seconds and prevents misleading data.
Keep Your Analyzer Calibrated
Annual calibration is the industry standard. Some manufacturers recommend semi-annual for heavy use. Most analyzers calibrate to room air when you zero them before testing. Some need periodic bump testing or full calibration with reference gases. Know your analyzer’s requirements. Drifted calibration means drifted data. Drifted data means wrong diagnoses.
Using Combustion Data in the Field: Common Patterns
Combustion analysis becomes powerful when you use the data to make decisions. Here are the patterns to watch for:
Normal operation looks like stable O2 and CO2 across all three states. CO under 50 ppm as-measured. Stack temp in the expected range. Manifold pressure on spec. If you see this, the equipment is running as designed.
Restricted air supply shows up as low O2, high CO2, high manifold pressure, and possibly elevated CO. The burner is hungry for air. Check for blocked intake filters, kinked combustion air ductwork, or intake restrictions. This is a safety issue.
Heat exchanger wear appears as high stack temp on a non-condensing furnace. It may rise over repeated measurements. A cracked heat exchanger lets heated flue gas escape before it transfers heat to the air. This means replacement.
Fuel control drift shows O2 and manifold pressure that don’t match the fuel type. Or O2 and CO2 are out of their expected chemical balance. The fuel valve may need adjustment or replacement.
Each scenario needs a different fix. Combustion analysis gives you the data to tell them apart instead of guessing or swapping parts by habit. An ACEEE report found that 70 to 90% of residential HVAC systems have energy-wasting faults tied to improper installation and a lack of proper commissioning.⁴
Commissioning and Quality Control: Build the Baseline
One reason combustion analysis gets skipped is that many techs work on equipment installed without proper baseline data. There’s no record of what the readings should look like when things are running right.
If you’re installing a furnace, take baseline combustion readings at light off, steady state, and shutdown. Save them in your service records. When the customer calls with a problem years later, you have a reference point. You can see what changed instead of diagnosing blind.
A 2024 NREL field study found that contractors using systematic diagnostic workflows saw a 5.4% increase in normalized sensible capacity and an 83% drop in callbacks.⁵ Those numbers speak for themselves.
Baseline data also protects you. If a customer questions your diagnosis, you have documents showing how the equipment has drifted. That builds credibility and trust.
Related Reading: The Truth About Furnace Tune-Ups explores how regular measurement and adjustment keeps equipment running right.
Tyler Nelson’s Combustion Analysis Book
Want to go deeper? Tyler Nelson wrote the book on it. His book, Combustion Analysis: The Essentials, is a field-ready reference covering combustion chemistry, analyzer operation, and data interpretation in plain language. Tyler is the Business Development Manager and Global Trainer at Sauermann Group. His ability to make complex topics practical is exactly why we had him on the podcast. Whether you’re just getting started with your first analyzer or sharpening your diagnostic thinking, this book belongs in your truck. Grab a copy at tyty5.gumroad.com/l/zfsmxo.
Combustion Analysis as a Diagnostic Skill
The difference between a tech who checks boxes and one who solves problems is the ability to think about equipment behavior. Combustion analysis is one of the clearest windows into that behavior.
Your analyzer gives you chemical data. Your job is to turn that chemistry into mechanical and operational reality. Why is O2 this number? What does it mean? What physical condition would cause these readings?
That thinking process separates a parts-swapper from a problem-solver. It’s what keeps customers coming back. They know you understand their equipment at a deeper level than a procedure checklist.
Additional Sources
- Midwest Alliance for Clean Energy, Improving Gas Furnace Performance: A Field and Laboratory Study, mwalliance.org
- TSI Inc., Combustion Analysis Basics, tsi.com; NCI / HVAC Today, How To Determine if Flue Gases Are Sick, hvactoday.com
- NCI / HVAC Today, Air Free CO or As Measured Carbon Monoxide?, hvactoday.com; ANSI Z21.47, Safety Standard for Gas-Fired Central Furnaces
- ACEEE, Increasing Uptake of Residential HVAC Commissioning with Advanced Technologies, aceee.org
- NREL, Optimizing Residential HVAC Systems: Evaluating How the Usage of the MeasureQuick Diagnostic Tool Impacts HVAC System Performance, 2024, nrel.gov
