What it is

A combustion analyzer is a probe-and-meter that samples flue gas and tells you how well a gas appliance is actually burning. You stick the probe in the flue (in the right spot), and it reports oxygen, carbon monoxide, flue temperature, and a calculated combustion efficiency. It turns "the flame looks blue, probably fine" into hard numbers — which matters because you cannot see carbon monoxide, and a furnace can look perfect while producing dangerous CO.

For any tech doing gas work, this is the tool that proves an appliance is safe and tuned, not just lit. Eyeballing a flame is not combustion analysis.

How it works

Complete combustion of natural gas or propane ideally turns fuel and oxygen into carbon dioxide and water vapor. In the real world it's never perfect, so you measure how close it is:

  • Oxygen (O2): how much unburned air came along for the ride. The analyzer measures O2 directly with an electrochemical cell. More O2 in the flue = more excess air (the burner is pulling in more air than it strictly needs). Too little O2 means not enough air — incomplete combustion and CO.
  • Carbon dioxide (CO2): usually calculated from O2 and the fuel type, not measured directly. CO2 peaks when combustion is most complete; it falls as excess air rises (dilution).
  • Carbon monoxide (CO): measured directly with a CO cell, in parts per million. CO is the danger signal — it's the product of incomplete combustion. The analyzer often also reports air-free CO, which mathematically removes the dilution effect of excess air so you can compare appliances apples-to-apples.
  • Flue/stack temperature and a net efficiency the analyzer computes from the stack temp, O2, and ambient.

The relationship that runs it all: as you reduce excess air (less O2), combustion gets more complete and efficient — up to a point. Push too far and you starve the flame, CO shoots up. The sweet spot is enough excess air for clean, complete, safe combustion with CO well in check.

In the field

A typical combustion check:

  1. Let the analyzer warm up and zero in clean air before you start — the cells calibrate to ambient. Don't zero it in a fume-filled mechanical room.
  2. Run the appliance to steady state. Combustion numbers drift during warm-up; take readings after the heat exchanger is hot and the burner has stabilized (usually a few minutes).
  3. Sample in the right spot — in the flue, before any dilution air (for atmospheric appliances, before the draft hood/dilution; for induced-draft, per the manufacturer's test location). Sampling in the wrong place gives meaningless dilution-skewed numbers.
  4. Read O2, CO, CO2, and stack temp. Compare CO and air-free CO to safe limits, check O2/excess air is in range, and verify stack temp is sane (not so low it condenses in a non-condensing flue, not so high you're throwing heat away).
  5. Cross-check manifold gas pressure and input against the rating plate while you analyze — combustion, gas pressure, and firing rate are one system.
  6. Test under worst case where relevant — other appliances running, exhaust fans on — to catch backdraft/spillage that only shows up under depressurization.
  7. Document the numbers before and after any adjustment so you have proof of a safe, tuned appliance.

Normal values & targets

These are general field references — always defer to the appliance's rating plate and instructions:

  • Air-free CO: aim for under ~100 ppm air-free on a properly operating residential appliance; many clean furnaces run well under that (single/double digits). Above ~400 ppm air-free is a clear problem demanding correction; high readings warrant red-tagging per your AHJ's threshold.
  • O2 in the flue: commonly in the roughly 5–9% range for many residential gas appliances (excess air present but not wild). Very low O2 points to insufficient air; very high points to too much excess air or dilution.
  • CO2: typically lands around roughly 8–10% for natural gas when combustion is good (it's the inverse of excess air). Exact target depends on fuel and appliance.
  • Stack temperature: has to stay above the flue gas dew point in a conventional (non-condensing) vent to avoid condensation, yet not be so high you're wasting heat — net stack temps vary widely by appliance; follow the manufacturer.
  • Ambient CO in the space: should be near zero. Any meaningful CO in the living space or mechanical room is a spillage/venting alarm.

Common faults & what they mean

  • High CO / high air-free CO: incomplete combustion — insufficient combustion air, overfiring (gas pressure too high), a dirty or misaligned burner, flame impingement on a cool/cracked heat exchanger, or a blocked/restricted flue. Find the cause; don't just walk away.
  • Low O2 with rising CO: the burner is starved for air — combustion-air opening blocked, filter/airflow problem, or overfiring. More air needed.
  • High O2, low CO2, cool stack: too much excess air or dilution leaking in — loose connections, draft hood pulling extra air, or a low firing rate. Efficiency suffers.
  • CO present in the living space: spillage or a cracked heat exchanger letting flue products into the airstream — a life-safety stop. Shut it down.
  • Numbers swing and won't settle: appliance not at steady state yet, cycling on a limit, or a draft/venting instability.

Tech tips & gotchas

  • You can't see CO — measure it. A textbook-blue flame can still produce dangerous CO. The analyzer is the difference between "looks fine" and "is safe."
  • Air-free CO is the honest comparison. Raw CO ppm can be masked by excess-air dilution; air-free strips that out, so use it to judge the burner and compare appliances.
  • Zero in clean air, read at steady state. Zeroing in a contaminated room or reading during warm-up gives false numbers — a common rookie mistake. Sample location matters as much as the meter: before dilution for atmospheric units, the manufacturer's port for induced/condensing.
  • Combustion, gas pressure, and firing rate are one system. Don't tune CO without checking manifold pressure and input rate against the plate.
  • Replace/calibrate sensor cells on schedule (an out-of-cal analyzer can pass a furnace it shouldn't), and protect the probe from condensate on condensing appliances — a slug of water in the cell wrecks readings.

Safety / code notes

  • A reading that exceeds your AHJ's CO action threshold (commonly cited around 400 ppm air-free, but follow local policy) means take the appliance out of service and red-tag.
  • Combustion-air provisions are sized per the fuel gas code (IFGC) / IMC §701 (e.g., the air-opening sizing methods) — inadequate combustion air is a leading cause of high CO.
  • Venting category, clearances, and termination follow the IFGC/IMC and the appliance listing; a venting fault is a CO hazard.
  • Any CO in occupied space is a life-safety event — evacuate/ventilate and find the source before restoring operation.