Reading a Combustion Analyzer

A flame's color can look fine while a furnace is producing dangerous carbon monoxide. A combustion analyzer measures what you cannot see. It samples the flue gas and tells you how much oxygen is left, how much carbon monoxide the burner is making and how hot the gases are. With those numbers, you can tell a clean, safe burn from a dangerous one.

What You Will Be Able to Do

• Explain what happens during complete and incomplete combustion.
• Take a flue sample at the correct point.
• Read oxygen, CO, CO2, stack temperature and efficiency.
• Explain what air-free CO is and why it matters.
• Decide when a reading means stop.

What Burns and What Comes Out

Natural gas is mostly methane. With enough oxygen, it burns to carbon dioxide, water vapor and heat:

CH4 + 2 O2 gives CO2 + 2 H2O + heat

Propane burns the same way with more carbon. When there is not enough oxygen or the flame is cooled or disturbed, combustion is incomplete, and some carbon turns into carbon monoxide (CO) rather than carbon dioxide.

Furnaces are built to run with excess air, more air than the chemistry strictly needs. This makes sure all the fuel burns. Too little air makes CO. Too much air carries useful heat up the vent.

Before You Test

1. Make sure the analyzer is calibrated and has been serviced on schedule.
2. Start the analyzer in fresh outdoor air, so it zeroes with no flue gas in it.
3. Check the water trap and filter.
4. Select the correct fuel, natural gas or propane.
5. Check the air around you with a CO monitor before you start. Wear a personal CO alarm.

Taking a Flue Sample

Testing a furnace is a job for trained technicians. A trainee should work under direct supervision.

1. Pick a test point the manufacturer approves. On an 80% furnace this is usually in the flue pipe before any draft hood or dilution air. On a condensing furnace, follow the manual, because many have a set test port.
2. Place the probe in the center of the flue gas stream.
3. Fire the furnace and let it reach steady state, usually at least 5 to 10 minutes.
4. Record the readings.
5. Watch for changes when the blower starts. A sudden jump in CO or oxygen can point to a heat exchanger problem.
6. Remove the probe, and seal the hole with an approved plug if you made one.

Warning: Do not leave a test hole open in a vent. A hole in a vent can leak flue gases into the home. Use only the plug or method the manufacturer allows.

What the Readings Mean

Stack temperature on a non-condensing furnace commonly falls around 250 to 450°F. On a condensing furnace it is much lower, often under about 150°F, because most of the heat has been taken out. Use the manufacturer's range. Propane has a somewhat higher CO2 target than natural gas.

The numbers work together. High oxygen with low CO2 means too much air. Low oxygen with rising CO means too little air. A dirty burner or poor flame can make CO even with enough oxygen.

Air-Free CO

A raw CO reading goes down as extra air dilutes the sample. That makes a high-CO burn look better than it is. Air-free CO corrects the reading to what it would be without the extra air. It is the number to trust, and most analyzers calculate it.

The formula is:

Air-free CO = raw CO x 20.9 / (20.9 minus O2 reading)

Worked example: raw CO is 60 ppm and oxygen is 9%. Air-free CO = 60 x 20.9 / 11.9 = about 105 ppm. A raw 60 ppm looks fine, but the true value is over the target.

When to Stop

• Under about 100 ppm air-free at steady state is the usual target.
• Above about 400 ppm air-free, shut the furnace down and correct the problem. That is a widely used limit in furnace standards.
• Any CO that is climbing and does not settle needs investigation.
• Any CO found in the supply air or living space needs investigation. 9 ppm in the living space is a common action level.

Common causes of high CO include:

• Dirty or misaligned burners.
• Blocked combustion air or vent.
• Incorrect gas pressure.
• Flame touching the heat exchanger.
• A cracked or damaged heat exchanger.

After You Test

1. Record the readings on the invoice or service log. Those numbers give the next tech a baseline.
2. Make any correction the manual calls for, such as cleaning burners or correcting gas pressure.
3. Retest after any change.
4. If the furnace is unsafe, shut it down, close the gas valve to the unit, tag it and explain why to the customer in writing.

Common Mistakes

Check Your Understanding

You read 150 ppm raw CO with 12% oxygen. Is that good? No. Air-free CO is 150 x 20.9 / 8.9, or about 352 ppm. It needs attention.

Air-free CO is 40 ppm at steady state, then jumps sharply when the blower starts. What might that mean? Air may be getting into the flame path through a crack in the heat exchanger. Investigate before you leave.

A blue flame looks great to the customer. Is a combustion test still needed? Yes. Flame color is not proof that CO is safe.

A condensing furnace shows a stack temperature of 120°F. Is that a problem? No. Condensing furnaces have cool flue gas. Compare with the manufacturer's range.

Key Takeaways

• Incomplete combustion makes carbon monoxide, which you cannot see or smell.
• Use air-free CO, not raw CO, to judge safety.
• Aim for under 100 ppm air-free, and shut down above about 400 ppm.
• Oxygen shows how much excess air is present.
• Stack temperature depends on the furnace type, and condensing furnaces run much cooler.
• Record your readings, because they are the proof of a safe burn.