Combustion Analysis for Gas Furnaces

Why Combustion Analysis Matters

A furnace with poor combustion can be unsafe, inefficient, or both. The visual flame check tells you very little. A furnace can produce dangerous CO levels with a flame that looks completely normal. The only way to know is to measure.

What You're Measuring

O2 (Oxygen): Residual oxygen in the flue gas. For residential gas furnaces, target is typically 6-9% O2. Low O2 (rich combustion) = elevated CO risk. High O2 (lean combustion) = excess air and efficiency loss.

CO (Carbon Monoxide): Measured in ppm in the flue gas. Target is under 400 ppm. Values above 400 ppm warrant investigation. Values above 1,000 ppm indicate a serious problem. CO in ambient air is the safety emergency — flue gas CO is the diagnostic indicator.

CO2 (Carbon Dioxide): For natural gas, target is typically 9-12% CO2. Low CO2 indicates excess air.

Stack temperature: High stack temp = poor heat transfer (dirty heat exchanger or scale buildup). Low stack temp = potentially condensing, or low firing rate.

Combustion efficiency: Calculated from O2 and stack temperature. Most modern furnaces should achieve 78-82% combustion efficiency on a well-tuned system.

Performing the Analysis

Location of sample: For induced draft furnaces, sample at the flue outlet just downstream of the heat exchanger. For atmospheric furnaces, sample in the vent connector.

Timing: Let the furnace run for 5 minutes minimum before sampling. You need steady-state conditions.

Zero the CO sensor in fresh air before sampling. Enter the fuel type — natural gas and propane have different stoichiometric requirements.

What High CO Indicates

CO production comes from incomplete combustion. The causes:

Insufficient combustion air: Blocked combustion air opening, closed damper, tight house that's depressurized by other exhaust fans.

Delayed ignition: Gas builds up before ignition, causing a rich ignition. You'll often hear a rumbling or puff on startup.

Dirty burners: Debris on the burner ports disrupts the flame pattern and creates rich zones. Clean the burners and retest.

Heat exchanger crack: Combustion products mixing with supply air. Any furnace with elevated CO and no obvious combustion cause gets a thorough heat exchanger inspection.

Heat Exchanger Inspection

Visual inspection through the sight glass with a mirror or camera. Smoke test under negative pressure. CO test — elevated CO in the supply air plenum during furnace operation is a strong indicator of a compromised heat exchanger.

When you find a cracked heat exchanger, red-tag the unit. Explain to the homeowner that this is a safety issue, not a performance issue. Document everything.