Combustion Analysis for Furnaces
A gas furnace can look like it's running fine and still be dumping carbon monoxide into a home or wasting a third of the fuel it burns. A combustion analyzer is the only tool that tells you the truth. Here's how to use one to verify safe, efficient burner operation.
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In This Guide
Why Combustion Analysis Matters
Temperature rise and a clean-looking flame tell you almost nothing about combustion quality. A burner can produce a picture-perfect blue flame while generating hundreds of ppm of carbon monoxide, and a furnace can hit its rated temperature rise while running so much excess air that efficiency falls apart. The only way to know what's coming out of the heat exchanger is to sample the flue gas.
Combustion analysis does two jobs at once. First, it's a safety test: it measures carbon monoxide directly, so you know whether the appliance is producing a dangerous level of CO before it ever reaches the occupants. Second, it's a performance test: it calculates real combustion efficiency so you can tune the burner, justify a repair, or document a clean bill of health after a tune-up.
Field reality: Carbon monoxide is odorless and colorless. A cracked heat exchanger, a blocked flue, or over-firing can spike CO with zero warning signs a homeowner would notice. Owning and using an analyzer is what separates a guess from a verified result.
What the Analyzer Actually Measures
A combustion analyzer has two live electrochemical sensors that measure the flue gas directly, plus a thermocouple in the probe. Everything else on the screen is calculated from those inputs. Knowing which numbers are measured versus derived helps you trust the reading.
- Oxygen (O2) — measured. Leftover O2 in the flue tells you how much excess air the burner is pulling.
- Carbon monoxide (CO) — measured, in ppm as-sampled.
- Stack (flue) temperature — measured at the probe tip.
- Carbon dioxide (CO2) — calculated from O2 and the selected fuel type.
- Excess air — calculated from O2.
- CO air-free — calculated: CO corrected to zero excess air (more on this below).
- Combustion efficiency — calculated from net stack temperature and CO2/O2.
Net stack temperature drives the efficiency number. It's the flue temperature minus the combustion-air (ambient) temperature: Net Stack = Flue Temp − Ambient Temp. That's why a good analyzer wants an ambient probe reading — a cold basement changes the math.
Target Numbers for Gas Furnaces
Always defer to the manufacturer's installation instructions first — the nameplate and install manual give the specific input rate, manifold pressure, and temperature rise for that unit. The ranges below are solid field targets for natural-gas residential furnaces when the manual doesn't give a combustion spec.
| Parameter | Typical Target (Nat Gas) | Notes |
|---|---|---|
| O2 (flue) | 6% – 9% | Lower = tighter; too low starves the flame and makes CO |
| CO2 (calculated) | 8.5% – 10% | Stoichiometric max for nat gas is ~11.7% |
| Excess air | ~30% – 70% | Corresponds to the O2 band above |
| CO air-free | < 100 ppm | Goal near 0–50; investigate above 100 |
| Net stack temp, 80% AFUE | ~325 – 500°F | Too low risks flue condensation in a Cat I vent |
| Net stack temp, 90%+ AFUE | ~100 – 130°F | Condensing units intentionally run cool, wet flue |
| Manifold pressure | 3.5 in w.c. | Propane ~10–11 in w.c.; always per nameplate |
Verify input before you trust efficiency
Combustion numbers assume the furnace is firing at its rated input. Clock the gas meter to confirm: Input BTU/hr = (3600 / seconds per revolution) × ft³ per rev × heating value, where natural gas is roughly 1,000–1,050 BTU/ft³. A furnace over-firing 20% will read differently — and often makes more CO.
CO Air-Free: The Number That Matters
Raw CO in ppm is misleading because it changes with dilution. Add more air and the same amount of CO gets spread thinner, so the as-sampled ppm drops even though the burner is making just as much CO. To compare readings fairly and against safety limits, the analyzer corrects CO to an air-free basis — what the CO would read with zero excess air.
CO Air-Free Formula
20.9 is the oxygen content of ambient air (%). As measured O2 in the flue rises, the correction factor grows.
Example: you measure 60 ppm CO with 9% O2 in the flue. COaf = 60 × 20.9 / (20.9 − 9) = 60 × 1.756 = 105 ppm. That "harmless" 60 ppm as-sampled is actually over the 100 ppm air-free threshold once you strip out the dilution.
Acceptable
CO air-free under 100 ppm and stable. A well-tuned furnace often reads well under 50 ppm.
Take Action
Rising, unstable, or above ~100 ppm air-free: investigate. The ANSI Z21 factory limit for gas appliances is 400 ppm air-free — treat anything approaching that as a shutdown.
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Step-by-Step Analyzer Procedure
A repeatable procedure keeps your numbers honest and protects the sensors. Follow the same sequence every time.
1. Zero in fresh air. Power up outdoors or in clean air and let the O2 and CO sensors auto-zero for about 60 seconds. Never zero inside a mechanical room with residual flue gas.
2. Confirm the gas input. Clock the meter and check manifold pressure against the nameplate before you touch the flue. Combustion numbers only mean something if the input rate is correct.
3. Find or drill the sample port. On an atmospheric (Category I) furnace, sample undiluted flue gas upstream of the draft hood. On a condensing unit, use the manufacturer's test ports. Seal any drilled hole afterward.
4. Let it stabilize. Run the burner 3–5 minutes. Stack temperature and O2 keep moving until the heat exchanger warms up.
5. Probe the hottest point. Move the probe until you find the peak stack temperature and highest CO in the stream, then hold it there to record.
6. Record O2, CO air-free, stack temp, draft, and efficiency. Also check draft is negative (typically −0.02 to −0.04 in w.c. over-fire on atmospheric units) and stays negative through the whole cycle.
7. Purge and document. Run the pump in fresh air until CO returns to zero to protect the sensor, then log before/after numbers on the ticket.
Safety: If draft goes positive (spillage), CO air-free climbs and won't settle, or you suspect a cracked heat exchanger, stop and treat it as a potential CO hazard. Red-tag per your company policy and local code — don't leave a producing appliance in service.
Worked Example: Reading an 80% Furnace
The Readings
An 80% AFUE natural-gas furnace, sampled upstream of the draft hood after 5 minutes: O2 = 7.2%, CO = 30 ppm as-sampled, flue temp = 415°F, ambient = 65°F.
Step 1: Net stack temperature
415°F − 65°F = 350°F — within the ~325–500°F band for an 80% unit
Step 2: CO air-free
30 × 20.9 / (20.9 − 7.2) = 30 × 1.526 = 45.8 ppm
Step 3: Excess air check
O2 of 7.2% puts CO2 near 8.8% — comfortably inside the 6–9% O2 target band, not starved, not way over-aired.
Step 4: Efficiency
With a 350°F net stack and healthy CO2, the analyzer calculates combustion efficiency around 81% — consistent with an 80% AFUE nameplate.
Result: PASSES
CO air-free under 100 ppm, O2 in band, net stack high enough to keep the Cat I vent dry, efficiency matching the nameplate. Document it and move on — no adjustment needed.
Adjusting and Interpreting Results
Most residential gas furnaces are not field-adjustable for air the way a power burner is — you dial in the firing rate with manifold pressure, and the appliance is engineered for a fixed combustion-air path. That means your main lever is gas pressure, and it must stay inside the nameplate range. Here's how to read what the analyzer is telling you.
- High O2 / low CO2, low CO: lots of excess air. Efficiency suffers and, on a Cat I vent, net stack may fall low enough to condense. Check for cracked heat exchanger draw-through, oversized flue, or under-firing.
- Low O2 with rising CO: the flame is being starved of air. This is the dangerous direction — back off, verify combustion air and venting, and never chase efficiency by driving O2 too low.
- High CO air-free at normal O2: flame impingement, dirty or misaligned burners, a cracked heat exchanger, or a fouled orifice. Clean and inspect before adjusting anything.
- Net stack too high: over-firing or a dirty heat exchanger not transferring heat. Verify input by clocking the meter.
Don't tune away a mechanical problem. If CO won't come down with correct input, correct pressure, and clean burners, the answer isn't more air — it's inspecting the heat exchanger and venting. A combustion analyzer finds problems; it doesn't fix a cracked exchanger.
Finally, remember the difference between combustion efficiency (what your analyzer shows in real time, based on flue losses at that moment) and AFUE (an annualized lab rating that accounts for cycling and off-cycle losses). Your analyzer reading will usually run a few points above the AFUE number on the nameplate — that's expected, not an error.
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