Combustion Analyzer Overview

A combustion analyzer is the instrument that lets you look at what’s coming out the flue and tell whether the fire inside the appliance is any good. Everything else in your diagnostic kit tells you what the electrical controls are doing. The analyzer tells you what combustion is doing. They answer different questions, and on a surprising number of calls, the electrical side is fine and the combustion side is the real problem.

This chapter is not a commissioning guide — that’s its own subject, and commissioning an appliance with a combustion analyzer deserves a week of hands-on training, not a chapter in a diagnostic book. The goal here is more modest: understand what the four primary readings mean, understand how they relate to each other, and recognize the patterns that tell you “this is a combustion problem” rather than “this is a controls problem.” That recognition alone will save you hours on calls you’d otherwise spend chasing the wrong fault.

The instrument

Any residential-class analyzer — Fieldpiece SOX3, Testo 320 or 330, UEi C155, Bacharach Fyrite — works the same way. You turn it on in fresh air, which lets the O₂ sensor calibrate itself to atmospheric 20.9% oxygen. You insert the probe into the flue through an existing test port (some manufacturers build one in) or a hole you drill and later plug. You let the appliance run to steady state — typically three to five minutes of burner-on — and then you read the display.

Under the hood, the analyzer has a small electrochemical cell that measures oxygen content, a separate cell that measures carbon monoxide, and a fine-wire thermocouple on the probe tip that reads stack temperature. Everything else — CO₂ percentage, excess air, CO air-free, combustion efficiency — is calculated from those measurements together with the fuel type you tell the meter to assume (natural gas vs. propane, plus variants on some units).

What each reading tells you

There are really only four numbers you need to read fluently. Everything else is context.

O₂ (oxygen in flue gas) — how much unreacted oxygen is passing through the appliance. Lower O₂ means combustion consumed more of the oxygen, which usually means less excess air. Higher O₂ means more excess air diluting the combustion products. Typical normal range for residential NG is 4 to 9 percent. Below 2 percent you’re starving combustion and risking CO production. Above 10 or 11 percent you’re running too much air and paying for it in efficiency.

CO (carbon monoxide) — how much unburned fuel is leaking through. CO means combustion was incomplete — fuel that should have burned to CO₂ stopped at CO. Clean residential combustion produces very little: under 100 ppm air-free is the shop standard most of the industry uses, under 30 ppm is excellent. Above 400 ppm air-free the appliance is unsafe, by ANSI Z21 residential standards.

Stack temperature — how hot the flue gas is when it leaves the appliance. Heat going up the flue is heat not going into the home, so stack temperature is the single biggest efficiency driver. Atmospheric and 80% forced-air appliances typically run 300–500°F stack. Condensing 90%+ equipment runs 100–180°F. Way higher than expected means heat isn’t transferring into the load — scaled heat exchanger, dirty surfaces, over-firing. Way lower than expected on non-condensing equipment means flue gas is condensing inside the vent, which will destroy the vent over time.

Combustion efficiency — a single calculated number that represents how much of the fuel’s input energy is going into the load rather than up the flue. The analyzer computes this from stack temperature minus ambient temperature and the O₂ reading, using a form of the Siegert equation. Atmospheric equipment at 75–82% steady state is normal. Modern 80% forced-air is supposed to hit 78–82%. Modulating condensing equipment runs 90–96% steady state.

Reading them together — the interactive

Use the model below to get a feel for how the numbers relate. Pick a scenario at the bottom to load a realistic reading set for a specific fault pattern, then start moving the sliders to see what combinations indicate what. The interpretation panel tells you what the current combination of readings is trying to tell you.

Combustion interpreter — live model

Natural gas80% forced-air82.5% efficiency

Fuel

Natural gasLP / Propane

Equipment type

Atmospheric80% forced90%+ condensing

Measured readings

O₂ in flue

7.0%

CO measured

35ppm

Stack temperature

380°F

Ambient (combustion air)

68°F

Derived values

CO₂

7.8%

Excess air

50%

CO air-free

53ppm

Combustion efficiency

82.5%

Interpretation

Combustion looks clean

All readings within normal bands for the selected fuel and equipment type. Record the values on the invoice as a commissioning baseline.

Scenarios

Healthy 80% NG furnaceHealthy modconHealthy atmospheric boilerSooting burnerOver-fired / dirty HXExcess-air atmosphericDangerous COLP modcon (healthy)

A few patterns worth recognizing by feel:

Sooting burner signature: low O₂ (2–3%), high CO (400+ ppm), moderately high stack. The combustion is starved for air, which produces soot and CO. Dirty burners, closed air shutters, or blocked combustion-air openings are the usual suspects.

Excess-air signature: high O₂ (10%+), low CO, low stack temperature. The appliance is pulling in too much dilution air through the draft hood — common on atmospheric equipment in cold weather when stack effect over-drafts the vent. Efficiency suffers because you’re heating and then exhausting room air.

Fouled heat-exchanger signature: normal O₂, low-to-moderate CO, high stack temperature. Combustion is fine but heat isn’t transferring into the load — so flue gas leaves the appliance hotter than it should. On a boiler this is often waterside scale; on a furnace it’s often dust or soot on the exchanger fins, or an over-fired condition.

Dangerous-CO signature: normal O₂, high CO (hundreds of ppm), normal stack. The appliance looks fine to every other test — inducer runs, pressure switch proves, flame lights, nothing locks out — but the CO number betrays that something about combustion is wrong. Cracked heat exchanger, wrong orifice for altitude, or contaminated combustion air (laundry fumes, cleaning products in a closet install, nearby salt spray).

When a combustion analyzer is warranted

Not every call needs an analyzer. The analyzer earns its setup time on specific call types.

The pattern: use it when the complaint involves combustion products (CO, soot, smell, spillage), when you’re commissioning new equipment, or when the controls diagnosis doesn’t explain the symptom. Don’t bother when the complaint is clearly electrical or airflow.

Taking the reading

Combustion analysis — residential gas appliance

procedure

1. Power on the analyzer outside the appliance, in clean room air. Let it self-calibrate through its warmup cycle. Most units take 30–90 seconds. During this time the O₂ sensor references itself to 20.9% atmospheric O₂.
2. Select the correct fuel type on the analyzer — natural gas or propane. Getting this wrong throws off CO₂, excess-air, and efficiency calculations because fuel stoichiometry is baked into the math.
3. Locate or create a test port in the flue. Many appliances have one built in — look for a small capped opening in the flue collar or vent elbow. Otherwise drill a 1/4-inch hole in the flue pipe about 6 inches downstream of the appliance collar, on the hot side of any barometric damper or draft hood. Plug the hole with a plug or metal tape when you’re done.
4. Call for heat. Wait for the burner to stabilize — three to five minutes of steady run on most residential equipment. Cold-start readings are not representative of operating performance.
5. Insert the probe through the test port and position the tip in the center of the flue stream. Secure with the probe cone or tape so it doesn’t move during sampling.
6. Read and record the four primary values after they stabilize: O₂, CO, stack temperature, combustion efficiency. Note ambient combustion-air temperature separately.
7. Remove the probe. Cap the test port. Compare readings to the appliance’s commissioning spec (if known) or to general residential norms.

Common diagnoses from combustion readings

”High CO but nothing looks wrong electrically”

A classic combustion-analyzer save. The furnace runs, the flame sensor proves, the burner looks normal to the eye. But the analyzer shows 380 ppm air-free. Options to investigate:

• Primary orifice size wrong for altitude. Manufacturer supplies NG orifices calibrated for sea level; above ~2000 feet you’re supposed to derate with smaller orifices. Wrong orifice for altitude is rarely caught at install time and produces elevated CO for years.
• Wrong manifold pressure. Too high = over-fired = incomplete combustion = CO. Manometer will confirm.
• Partial heat-exchanger crack. Combustion gases getting diverted or mixed with excess air in abnormal ways. Check with soap solution at the joints and with a careful visual inspection. CO pattern can be irregular as the crack opens and closes thermally.
• Contaminated combustion air. Halogens (from cleaning products, laundry detergent, bleach fumes in a closet install), salt air in coastal installs, or silicone compounds pulled from nearby activity can poison burners over time. Sign: CO climbs slowly over months or a season.

”Stack temperature very high on an 80% furnace”

The appliance is operating but wasting heat up the flue. Causes in order of likelihood:

• Dirty secondary heat exchanger. Dust and lint accumulated on the exchanger fins insulate them from the flue gas. The exchanger stops absorbing heat as efficiently. Efficiency drops, stack temperature climbs, homeowner calls because “the furnace runs forever but the house isn’t warm.”
• Over-fired condition. Wrong orifice, excessive manifold pressure, or a field modification that increased input beyond the exchanger’s transfer capacity. Confirm with manometer.
• Restricted airflow. Dirty filter, closed registers — the blower can’t move enough heat away from the exchanger, so the exchanger rides hot, and stack gas rides hot with it. Check filter and register coverage before doing more.

”Low stack temperature on atmospheric equipment”

Rare but dangerous. Indicates flue gas cooling below the dew point inside the vent, which means condensation in the flue, which means an atmospheric vent turning into a corrosion-accelerated leak path. Causes:

• Way too much excess air (dilution cooling) — check for over-drafting, missing draft diverter, oversized vent.
• Over-sized appliance on a short-cycling call profile — short cycles don’t let the vent warm up before the burner shuts off.
• Derated appliance running below minimum rated input — field modification beyond spec.

Quick reference

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Check your understanding

0 / 5

01A combustion analysis shows O₂ of 2.0%, CO (air-free) of 420 ppm, and stack temperature 440°F on an 80% NG furnace. What do these readings together tell you?

ANormal operation for this equipment typeBStarved combustion — insufficient air producing incomplete combustion and CO; unsafe and must be correctedCExcess air condition — lower efficiency but safeDHeat exchanger fouling

02Why does the combustion analyzer display 'CO air-free' in addition to raw CO?

AAir-free is the CO measurement before the sensor decaysBIt's a calibration check for the CO sensorCAir-free corrects for dilution by excess air and reveals the actual CO produced at the burnerDBoth numbers are always identical — it's redundant

03Why must you power on a combustion analyzer in fresh air before inserting the probe?

ATo warm up the LCDBTo self-calibrate the O₂ sensor to 20.9% atmospheric oxygenCTo preheat the thermocoupleDFresh air startup is a recommendation, not a requirement

04A 90%+ condensing modcon boiler reads stack temperature of 95°F. Is this concerning?

AYes — dangerously low stack temp on any equipmentBNo — condensing equipment is designed to cool flue gas below the dew point; low stack is the expected normalCYes — below 100°F on any appliance indicates heat-exchanger failureDInconclusive without more context

05A homeowner reports that their CO alarm has triggered twice in the past month. You arrive, appliance is running normally. Controls diagnostic shows nothing wrong. Should you run combustion analysis?

ANo — the alarm is probably failedBYes — every CO-alarm call should include combustion analysis, regardless of what controls diagnosis showsCOnly if the alarm triggers again while you're thereDOnly on the highest firing stage

Before you close the chapter

You should now understand what each of the four primary combustion readings measures, why they’re interpreted together rather than one at a time, and which call types warrant pulling out the analyzer versus which ones don’t. You should also recognize the three or four combustion fault signatures — sooting, excess air, fouled exchanger, dangerous CO — by the pattern they leave on the analyzer display.

A combustion analyzer is not an entry-level tool and it’s not a universal one. But on calls where it’s warranted, it’s the difference between “I think this is fixed” and “I can prove this is fixed.” That’s the diagnostic payoff, and it’s substantial.

The next chapter moves on to meter safety and CAT ratings — the final piece of the test-equipment foundation before we move into control systems.