Furnace Combustion Analysis and Tuning
Prove gas input, read combustion numbers as a pattern, trace high carbon monoxide to its cause and document a furnace that burns clean at every stage.
Table of contents
Anyone can hold a probe in a flue. The senior skill is knowing what the numbers mean together, and what to do when they are wrong. A furnace with high carbon monoxide (CO) does not need a guess. It needs a method that checks gas input, air, burners and the vent in order, and a retest after each change. Gas and combustion work must be done by a qualified, licensed technician, or under one, as your state and employer require. These figures are common targets, and the rating plate and the manufacturer's instructions always control the job.
What You Will Be Able to Do
- Check gas input by clocking the meter.
- Test a furnace at every firing rate.
- Read oxygen, air-free CO, stack temperature and rise as one pattern.
- Trace high CO through a fixed order of causes.
- Decide when a test result means the furnace must stay off.
Set the Stage Before You Test
Combustion numbers mean nothing if the furnace is firing at the wrong rate or with the wrong airflow. Check these first:
- Check the air around you with a personal CO monitor before and during the job.
- Confirm filter, blower and static pressure are normal.
- Confirm the fuel type on the rating plate matches the fuel at the valve.
- Zero the analyzer in clean outdoor air, and confirm it is within its calibration date.
- Set the analyzer to the correct fuel.
- Close windows and doors, so the test reflects how the home normally runs.
Warning: Stop testing and ventilate if CO climbs in the air around you. Many programs use 9 ppm in living space as the level that needs investigation, and stop testing, shut off the appliance and ventilate above 35 ppm. Follow your employer's limit if it is stricter.
Clocking the Gas Meter
Clocking measures how much gas the furnace really burns. Use it on natural gas, with every other gas appliance off, including pilots if you can.
Formula: input in BTU per hour = (3,600 divided by seconds for one cubic foot) x heating value of the gas.
Worked example: a furnace is rated at 90,000 BTU per hour. The utility gives a heating value of 1,030 BTU per cubic foot.
- Time the one-cubic-foot dial with the furnace firing at steady state. It takes 40 seconds.
- 3,600 divided by 40 = 90 cubic feet per hour.
- 90 x 1,030 = 92,700 BTU per hour.
- That is about 3% over the plate.
Get the heating value from the utility, because it varies. Check the manufacturer's tolerance for input. Propane has no meter to clock, so you rely on the correct orifices and manifold pressure, set to the rating plate.
Test Every Firing Rate
A furnace can burn clean at high fire and poorly at low fire, or the reverse.
- Single-stage: test at steady state after at least 5 to 10 minutes.
- Two-stage: test at low fire and at high fire. Let each stabilize.
- Modulating: test at minimum and maximum, using the service mode if it has one.
Sample at the point the manual allows. On a condensing furnace that is usually a set test port. Empty the water trap often, because condensate can flood the analyzer's sensors.
Reading the Numbers as a Pattern
Typical targets are about 7 to 9% oxygen, with some systems 5 to 10%, and under 100 ppm air-free CO. Use the maker's figures where they exist.
| Pattern | What it suggests | Next check |
|---|---|---|
| High O2, low CO, low stack temperature | Low input or too much air | Clock the meter, check pressure |
| Low O2, high CO | Too much gas or too little air | Input, orifices, combustion air |
| Normal O2, high CO | Flame hitting metal, or dirty burners | Burners, alignment, flame shape |
| CO that keeps climbing | Vent problem or flue gas returning | Vent, intake, recirculation |
| CO or O2 jumps when the blower starts | Air reaching the flame | Heat exchanger |
| High stack temperature | Overfiring or low airflow | Input, rise, static pressure |
Always use air-free CO, which corrects for the extra air in the sample. Most analyzers calculate it. The formula is raw CO x 20.9, divided by (20.9 minus O2).
Worked example: raw CO is 180 ppm with 9% oxygen. Air-free CO is 180 x 20.9 divided by 11.9, or about 316 ppm. It is well over the usual target and heading toward the limit where the furnace must stay off.
Prove the Heat Going Into the House
The analyzer shows what leaves the flue. Rise and airflow show what stays in the home.
Formula: BTU per hour delivered = 1.08 x CFM x temperature rise.
Temperature rise is supply air minus return air, and the rating plate gives its allowed range. A rise above the range means the heat exchanger is running too hot. A rise below the range on an 80% furnace can cool flue gas enough to cause condensation in the vent.
Worked example: a furnace with a rated output of 72,000 BTU per hour shows a rise of 45°F at 1,400 CFM. Output is 1.08 x 1,400 x 45, or about 68,000 BTU per hour. That is close to rated, so airflow and input agree.
Tracing High CO
Do not change several things at once. Work in this order, and retest after each step.
- Burners. Dust, rust flakes and spider webs block air ports. Clean them.
- Gas input. Check manifold pressure and clock the meter. Confirm orifices match the fuel and the altitude.
- Burner alignment. Flame should go into the heat exchanger cell cleanly, not hit metal. A flame that touches metal cools and makes CO.
- Combustion air. Look for blocked intakes, closed-off rooms and exhaust fans that pull down the space.
- Vent. Look for a blockage, poor draft or flue gas returning to the intake.
- Heat exchanger. Check for damage that disturbs the flame.
Most modern inshot burners have no air shutter to adjust. High CO on these furnaces means something else is wrong. Do not modify burners or orifices to change the numbers, and never drill an orifice.
When the Reading Decides for You
| Result | Action |
|---|---|
| Under about 100 ppm air-free, steady | Record it and finish the job |
| Between about 100 and 400 ppm air-free | Find the cause, correct it and retest |
| Above about 400 ppm air-free | Shut the furnace down until it is corrected |
| CO keeps climbing and will not settle | Investigate before you leave it running |
| CO in supply air or living space | Investigate, and treat it as serious |
About 400 ppm air-free is a widely used limit in furnace standards and in ANSI/BPI-1200 work. State rules, gas utilities or your employer may set a lower one. To shut down, close the gas valve to the unit, turn off the power, tag it and tell the customer in writing.
Worked Diagnosis
A 12-year-old 80% furnace shows these numbers at steady state.
| Reading | First test | After burner cleaning | After correcting pressure |
|---|---|---|---|
| Oxygen | 6.2% | 6.5% | 8.1% |
| Air-free CO | 340 ppm | 210 ppm | 42 ppm |
| Stack temperature | 460°F | 450°F | 400°F |
| Manifold pressure | 4.2 in. w.c. | 4.2 in. w.c. | 3.5 in. w.c. |
Low oxygen with high CO points to too much gas or too little air. Cleaning the burners helped, but not enough. A manometer showed manifold pressure above the rating plate value, and clocking confirmed overfiring. Setting pressure to the plate brought CO down, and oxygen and stack temperature back in range. Each step was tested before the next. If you had changed both at once, you would not know which one mattered.
Documenting the Burn
Record these for each firing rate:
- Oxygen, raw CO, air-free CO and stack temperature.
- Manifold and inlet pressure.
- Temperature rise and static pressure.
- Ambient CO on arrival and at the end.
- Anything you changed, and the before and after readings.
The next technician gets a baseline, and you have proof of what you found.
Common Mistakes
| Mistake | What it costs |
|---|---|
| Testing only at high fire | A dirty low-fire burn goes unseen |
| Changing several things at once | You cannot tell what worked |
| Testing with a dirty filter | Rise and stack numbers mislead you |
| Clocking with a water heater running | A false input reading |
| Assuming a low CO reading proves a safe vent | A spillage problem goes unseen |
| Drilling or altering an orifice | Overfiring, and a code violation |
Check Your Understanding
You clock the meter while the water heater is firing. What happens to your result? It reads high, because the meter is measuring both appliances. Turn the others off and time it again.
An analyzer shows raw CO of 60 ppm with 15% oxygen. Why not record 60 ppm? The sample holds extra air. Air-free CO is 60 x 20.9 divided by 5.9, or about 213 ppm.
You watch flame hitting the side of a heat exchanger cell. What would you expect the analyzer to show? High CO, even if oxygen looks normal, because a flame that touches metal cools and makes CO.
A two-stage furnace reads 45 ppm air-free at high fire. Are you finished? No. Test low fire as well.
CO keeps climbing during a test and will not settle. Where do you look? The vent and the intake, because flue gas may be returning to the burner. Investigate before you leave it running.
Key Takeaways
- Confirm airflow and gas input before you trust combustion numbers.
- Clock the meter on natural gas, with other appliances off.
- Test every firing rate.
- Read oxygen, air-free CO, stack temperature and rise as a pattern.
- Trace high CO in order, and retest after each change.
- Above about 400 ppm air-free, the furnace stays off until it is corrected.