HVAC
Combustion analysis and flue-gas tuning field guide
Put a calibrated analyzer in the flue, read oxygen, carbon monoxide, temperature, and draft, and prove the appliance burns clean, safe, and efficient instead of guessing.
Direct answer
Combustion analysis puts a calibrated analyzer probe in the flue to measure oxygen, carbon monoxide, flue temperature, and draft, then reads excess air and efficiency to confirm a gas appliance burns clean, safe, and efficient. Carbon monoxide is a life-safety hazard, so the manufacturer's targets, the fuel-gas code, and the AHJ govern every limit.
Key takeaways
- Combustion analysis puts a calibrated analyzer probe in the flue to measure oxygen, carbon monoxide, stack temperature, and draft, then calculates excess air and efficiency.
- High or rising air-free carbon monoxide means shut the appliance down and investigate; the governing limit is in the manufacturer's instructions and adopted code.
- A well-burning gas appliance commonly produces air-free CO well under about 100 ppm at steady state; higher or climbing readings get investigated.
- Zero the analyzer in clean fresh air until O2 reads near 20.9 percent and CO near zero before the probe enters the flue; electrochemical cells last about 2 to 3 years.
- Never over-fire past the rated input: set manifold pressure to the rating plate with a manometer, clock the meter to confirm input, and check draft and spillage including a worst-case depressurization test.
What combustion analysis is and why it is not optional
Combustion analysis is the work of placing a calibrated analyzer probe in the flue, measuring the oxygen, carbon monoxide, and temperature of the products of combustion along with the draft pulling them out, and reading the excess air and efficiency that those numbers calculate to. It tells you whether the burner is making a clean, safe, efficient flame or quietly making something dangerous. Without it you are guessing, and a flame that looks blue and steady through the sight glass can still be pouring out carbon monoxide or wasting a quarter of the fuel up the stack.
A gas appliance can run for years in a state no one would accept if they could see it. The burner lights, the house gets warm, the customer is happy, and nothing on the front of the equipment says the combustion is wrong. The flue is the only place the truth lives, and you cannot read it with your eyes. You read it with an instrument.
This guide stays on the analysis and the tuning. For how a furnace fires in sequence and how to run down a no-heat call, see the gas furnace operation and troubleshooting guide. For bringing a commercial boiler online and proving its safety chain, see the boiler startup and commissioning guide. The combustion side is common to all of them, and it is the side that hurts people when it is ignored.
Why it matters: carbon monoxide can kill, and bad combustion wastes fuel
Carbon monoxide is the reason this is a life-safety job and not a tune-up. CO is colorless, odorless, and toxic, and incomplete combustion is how an appliance makes it. A heat exchanger crack, a blocked flue, a starved burner, or an over-fired one can put CO into the air people breathe, and the first symptom is often a person who is already in trouble. You do not get to smell your way to safety. The analyzer is how you find CO before it finds the occupants.
The second reason is money and equipment. Burn with too much air and you heat the outdoors, sending the fuel you paid for up the flue as warm excess air. Burn with too little air and you make CO, soot the heat exchanger, and shorten its life. A burner tuned to the manufacturer's target sits in the narrow band where it burns complete and clean without throwing heat away.
The third reason is that the work requires it. Manufacturers publish target O2, CO, and temperature or efficiency figures for setup, the fuel-gas code and the equipment listing expect the appliance to be set and verified, and many jurisdictions and utilities ask for combustion readings on commissioning. Confirm what the manufacturer's instructions, the adopted code, and the AHJ require for the specific appliance in front of you.
The combustion analyzer and keeping it honest
A combustion analyzer pulls a flue-gas sample through a probe with a small pump and reads it on electrochemical cells, one for oxygen and one for carbon monoxide, often a third for nitric oxide, plus a thermocouple in the probe tip for stack temperature and a port for draft. The instrument measures O2, CO, and temperature directly and calculates the rest from them. Treat the calculated numbers as only as good as the cells feeding them.
Those cells drift and they die. A CO or O2 electrochemical cell commonly lasts somewhere around 2 to 3 years and then needs replacement, and the whole instrument needs periodic factory or bench calibration, often on a 6-to-12-month cycle. Confirm the interval and the cell life against the analyzer manufacturer's instructions, because they vary by brand and by how hard the tool is run.
Before every job you zero the analyzer in clean, fresh air, away from the appliance, vehicle exhaust, and your own breath. Start the pump with the probe in ambient air and let it settle for a couple of minutes so the O2 cell reads near 20.9 percent and the CO cell reads near zero before the probe ever enters the flue. An analyzer zeroed in a flue-gas-contaminated space lies to you from the first reading, and an uncalibrated analyzer is worse than no analyzer because it gives a wrong number you might trust.
What does a combustion analyzer measure?
An analyzer measures three things directly and calculates the rest. Oxygen, carbon monoxide, and stack temperature are read by the sensors. Draft is read at the same probe or a separate port. Carbon dioxide, excess air, air-free CO, and efficiency are computed from the measured values and the fuel you tell the instrument it is burning. Set the wrong fuel and the calculated numbers come out wrong even when the sensors are fine.
Each number answers a different question. Oxygen tells you how much excess air you have. CO tells you whether the burn is complete and whether it is safe. Stack temperature tells you how much heat is going up the flue. Draft tells you whether the products are leaving the way they should. Read together, they tell you whether to leave the appliance alone, tune it, or shut it down.
| Reading | Measured or calculated | What it tells you |
|---|---|---|
| Oxygen (O2) | Measured | How much excess air; low O2 risks CO, high O2 wastes heat |
| Carbon monoxide (CO) | Measured | The safety number; rising CO means incomplete, unsafe combustion |
| Stack temperature | Measured | Heat leaving the flue; drives efficiency and condensing concerns |
| Draft | Measured | Whether the flue is pulling the products out as intended |
| Carbon dioxide (CO2) | Calculated | Tightness of combustion; rises as excess air falls |
| Excess air | Calculated | Air above what the fuel needs to burn complete |
| Air-free CO | Calculated | CO corrected for excess air, so readings compare |
| Efficiency | Calculated | Heat captured vs. lost up the flue, net or gross |
Oxygen in the flue shows excess air
Oxygen is the first number to read because it sets up everything else. The air feeding the burner is about 20.9 percent oxygen. The burn consumes oxygen, so the leftover O2 in the flue is a direct measure of how much air you put in above what the fuel actually needed. That leftover is excess air, and it has a sweet spot.
Too little oxygen and there is not enough air to finish the burn, so the appliance starts making carbon monoxide. Too much oxygen and the surplus air absorbs heat and carries it up the flue, which drops efficiency and can cool the flue below where it should run. You are looking for a band, not a single point: enough air to burn complete with a safety margin, and not so much that you are heating the sky.
Typical flue O2 for a gas appliance often lands somewhere in the low-to-mid single digits up into the higher single digits depending on the equipment, and atmospheric appliances usually run more excess air than power burners. Do not set to a generic number. Set O2 to the band in the manufacturer's instructions for the specific appliance, because the target moves with burner type, fuel, and design.
What is a safe CO level in flue gas?
Carbon monoxide is the safety number, and it gets read air-free so it compares from appliance to appliance. A well-burning gas appliance commonly produces a low air-free CO, often well under about 100 ppm at steady state, and many techs treat a stable reading under roughly 100 ppm air-free as acceptable while anything higher gets investigated. Confirm the acceptable figure against the manufacturer's instructions and the adopted code, because the number that governs is theirs, not a rule of thumb.
What matters as much as the number is the trend. CO that climbs and will not settle is more dangerous than a higher number that is stable, because rising CO can keep rising past anything safe while you stand there. A reading that drifts up after the appliance reaches steady state is telling you something is wrong with the burn, the air, or the heat exchanger.
Read CO as a hazard, not a performance metric. Efficiency is about money. CO is about whether the people in the building are safe. When the two ever conflict, CO wins, and you do not trade a safe burn for a tenth of a point of efficiency.
Air-free CO and why it is the number that compares
Air-free CO is the carbon monoxide reading corrected to remove the diluting effect of excess air. Raw, as-measured CO in the flue is mixed with whatever excess air the burner is pulling, so two appliances making the same actual CO can read very differently if one runs more excess air than the other. Correcting to air-free strips that dilution out, so the number reflects the combustion itself and can be compared against a target.
The analyzer does the correction for you from the measured CO and O2, which is another reason the oxygen reading has to be right. A wrong O2, from a tired cell or a probe not in the flue stream, throws the air-free CO off in the same motion. The math is only as honest as the sensors.
When you record CO, record whether it is air-free or as-measured, and compare like to like. Manufacturer and code limits are usually stated air-free. Mixing an as-measured reading against an air-free limit is how a passing appliance looks failing or, worse, a failing one looks fine. Verify which basis the target uses before you call it.
Carbon dioxide and its relationship to oxygen
Carbon dioxide is calculated, not measured, and it moves opposite to oxygen. As you cut excess air, oxygen in the flue falls and CO2 rises, because the combustion products are less diluted. Higher CO2 means tighter, more complete combustion, up to the point where you run out of air and start making CO. So CO2 climbing is good until it is not.
Every fuel has a theoretical maximum CO2 it can reach at perfect stoichiometric combustion with zero excess air, which for natural gas is about 11.7 to 12 percent and runs higher for propane and oil, while real field operating values land in the high single digits. You never reach the theoretical max in the field, and you should not chase it, because the last bit before it is the unsafe edge where CO takes off.
Treat CO2 as a cross-check on the oxygen reading rather than the primary target. Set the burn by O2 and CO to the manufacturer's instructions, and let CO2 confirm the story those two are telling. If CO2 reads unusually high with low O2, you are tight on air, so watch the CO closely. Verify any CO2 target against the equipment and fuel.
Stack temperature and what it does to efficiency
Stack temperature is the temperature of the flue gas, and it is the largest single loss in most appliances. Heat going up the flue is heat you did not put into the building. The analyzer reads stack temperature at the probe and uses it, along with the combustion air temperature, to calculate efficiency. The bigger the difference between flue temperature and room air, the more heat is leaving, and the lower the efficiency.
Hotter is not better and colder is not always better either. Too hot a stack means you are throwing heat away, often from over-firing or a fouled heat exchanger that is not transferring well. Too cool a stack on a non-condensing appliance is its own problem, because flue gas that drops below its dew point inside the vent condenses, and that condensate is acidic and corrodes a vent that was never built to handle it.
Analyzers report efficiency on a net or a gross basis, and the two are not the same number, so know which one the instrument and the target are using. Net, or lower-heating-value, efficiency reads higher than gross because it does not count the latent heat in the water vapor. Compare net to net and gross to gross, and verify the basis the manufacturer's target uses.
Draft pulls the products out, and spillage means they are not leaving
Draft is the pressure that moves the products of combustion out of the appliance and up the flue, and it is read with the analyzer or a manometer at the breech or over the fire. Proper draft pulls the flue gas out the way the design intends. Too little draft and the products can back up into the space. Too much draft and you can pull excess air through the appliance, cool the flue, and waste heat.
Spillage and backdraft are the dangerous failures here, and they put combustion products, including CO, into the room instead of up the flue. On an atmospheric appliance with a draft hood, you check that the hood is capturing the products and not spilling them at the relief opening. If the flue gas is rolling out of the draft hood instead of going up, the venting is not doing its job and the space is at risk.
Draft targets are small numbers in inches of water column and they vary by appliance and venting type, so read draft at the location and under the conditions the manufacturer's instructions specify. Atmospheric, induced-draft, and direct-vent equipment each behave differently, and the number that is correct for one is wrong for another. Verify the target and the test point against the equipment and the adopted code.
Excess air is the balance you are tuning
Excess air is the air you supply above the exact amount the fuel needs to burn completely, and tuning combustion is really tuning this balance. No real burner runs at perfect stoichiometric air, because a burn with zero margin tips into making CO the instant anything shifts. So you give it a margin. The skill is giving it enough margin to stay safe and complete without giving it so much that the surplus air cools the flue and carries heat away.
Oxygen in the flue is how you read excess air, and CO is how you know you have not cut it too far. Drop the air and O2 falls, CO2 rises, efficiency improves, until you reach the point where CO starts to climb. The safe target sits back from that edge with a margin, not on it. Tune to the manufacturer's excess-air or O2 target, which already builds that margin in for the specific appliance.
How do you perform a combustion analysis?
Run the appliance to steady state before you trust any number. A burner that just lit is still warming the heat exchanger and the flue, and the readings drift until it settles, often after several minutes of firing. Take readings on a cold appliance and you are recording a transient, not the operating condition. Let it stabilize first.
Put the probe in the right spot. For most appliances that means in the flue past the heat exchanger and ahead of any draft hood or dilution air, at the location the manufacturer specifies, so you are sampling true flue gas and not flue gas already mixed with room air. A probe in the wrong place reads diluted and tells you the burn is leaner and cooler than it is.
Record at every firing rate the appliance runs. A modulating or two-stage appliance has to be checked at high fire and at low fire, because the combustion can be tuned correctly at one rate and wrong at the other. Drive it to each stage, let it settle, and capture O2, CO, stack temperature, draft, and the calculated values at each. Then read them together, not one at a time, because the safe call comes from the whole picture.
Tuning the burn, and never past the rated input
Tuning is adjusting the fuel and, where the burner allows, the air, to land the readings on the manufacturer's targets. On many residential appliances the gas side is what you adjust, by setting the manifold pressure with a manometer to the value on the rating plate. On power burners you also have an air adjustment, and you trim the air and fuel together to hit the target O2 and CO across the firing range. Make one change at a time and let the appliance restabilize before you read again.
The hard limit is the input. Do not over-fire. The rating plate gives the input rating and the manifold pressure that produces it, and pushing more gas to chase heat or a faster recovery over-fires the appliance, overheats the heat exchanger, drives stack temperature up, and can make CO. More fire is not more performance. It is a shorter equipment life and a safety problem.
Tune to the targets, then verify the input is right by clocking the meter where you can. Hitting a clean O2 and CO while the appliance is over-fired is not a pass. The manufacturer's instructions give the target readings and the input, and both have to be right at the same time. Confirm the figures against the rating plate and the instructions for the specific unit.
Manifold and inlet pressure, and clocking the meter
Gas pressure is set and confirmed with a manometer, not by feel. You read inlet pressure ahead of the gas valve and manifold pressure downstream of it, and both have to fall in the ranges the manufacturer and the fuel-gas code give. Low inlet pressure starves the appliance and can drop manifold pressure under load. Manifold pressure set wrong throws the input off, which moves every combustion reading with it.
Manifold pressure is the field adjustment for input on many appliances. Natural gas runs at a much lower manifold pressure than propane, and the rating plate gives the value to set for the fuel in use. Setting a natural-gas appliance to a propane pressure, or the reverse, is a serious error that over-fires or starves the burner, so confirm the fuel and the plate value before you touch the regulator.
Pressure alone does not prove the input, so clock the gas meter to verify the firing rate where the meter and the situation allow it. Timing the meter dial against the gas heating value gives you the actual input in Btu per hour, which you compare to the rating plate. The manometer sets the pressure, clocking confirms the input, and the two together are how you know the appliance is firing where it should. Verify the procedure and the values against the manufacturer's instructions and the adopted code.
What CO reading means shut the appliance down?
High or rising carbon monoxide means stop, investigate, and do not leave the appliance running. There are published action figures, often citing the low hundreds of ppm air-free as a shutdown point, but the figure that governs is the one in the manufacturer's instructions and the adopted code, so confirm it for the appliance you are on. The principle does not change: when CO is high or climbing, the appliance comes off and stays off until the cause is found and fixed.
Rising CO at any level is its own alarm. CO that will not stabilize and keeps climbing is more dangerous than a steady higher reading, because you have no idea where it stops. A burner with rising CO gets shut down and investigated, not watched. Do not chase a tune on an appliance whose CO is unstable.
When you shut an appliance down for CO, follow the red-tag and notification process the AHJ and your company require, and tell the occupant plainly that it is a safety shutdown, not an inconvenience. The appliance does not go back in service on a hunch. It goes back when the cause is corrected and a fresh analysis confirms the burn is safe. This is the part where being blunt is the job: a CO problem you talked yourself out of is the one that hurts someone.
Ambient CO protects the people in the building, and you
Flue-gas CO tells you about the burn. Ambient CO tells you about the room, and you test both. A personal CO monitor and an ambient reading in the combustion appliance zone catch carbon monoxide that is getting into the space from spillage, a crack, a blocked flue, or another appliance entirely. The flue can read acceptable while the room is filling, if the products are not leaving the way they should.
Wear a personal CO monitor whenever you work on combustion equipment. It protects you in a space you may be in for an hour, and it warns you of a problem the appliance gauges will not show. Check the space, not just the stack, and treat an ambient CO reading as a reason to stop and find the source. The occupants are why the analysis exists, and ambient testing is how you confirm the air they breathe is clean.
Spillage, backdraft, and the worst-case test
Spillage is flue gas escaping into the room instead of going up the vent, and it is checked at the draft hood or diverter on atmospheric appliances. Soon after the appliance fires, you look and feel for products rolling out of the relief opening instead of being drawn up. Spillage that continues past the first minute or two of operation is a venting failure, and it puts CO into the space.
Backdraft is spillage driven by the building. Exhaust fans, a clothes dryer, a kitchen hood, and the air handler can depressurize the combustion appliance zone enough to overcome a natural-draft vent and pull the products back into the room. This is why a worst-case test matters: you turn on the exhaust appliances and close the house up to create the worst depressurization the zone will see, then check whether the appliance still vents or starts to spill.
The worst-case condition is the one that bites in real life, because the appliance that vented fine on a calm test day spills the night every fan in the house runs at once. Run the depressurization test the way the protocol and the AHJ specify, and if the appliance spills under worst case, the problem gets corrected before it is left in service. Verify the test method against the adopted code and recognized procedure.
High CO can mean a cracked heat exchanger
A cracked heat exchanger is the failure combustion analysis is built to help catch, and it is dangerous because it lets combustion products, including CO, into the air stream the building breathes. High or rising CO, CO that changes when the blower starts, or a flame that moves when the indoor fan kicks on are patterns that point at the exchanger. The analyzer gives you the CO number, and the behavior around the blower gives you the pattern.
Combustion readings support the diagnosis, they do not complete it by themselves. Confirming a cracked heat exchanger usually takes inspection with a camera, mirror, or other manufacturer-accepted method, because you are condemning an appliance or a major component and that call has to hold up. For how this ties into the furnace sequence and the visual and pressure checks that go with it, see the gas furnace operation and troubleshooting guide.
When the evidence says the heat exchanger is compromised, the appliance comes out of service. A cracked exchanger is not a tune-and-monitor situation. Follow the manufacturer's instructions, your company policy, and the AHJ for condemning and red-tagging, and do not let an appliance suspected of breaching combustion into the living space keep running.
Condensing vs non-condensing appliances
The two designs want different stack temperatures, so the targets differ. A non-condensing appliance is built to keep the flue gas above its dew point all the way out, so it runs a hotter stack and is vented in material rated for that heat. Pull its stack temperature too low and the flue gas condenses inside a vent never meant to handle acidic condensate, which corrodes it.
A condensing appliance does the opposite on purpose. It is designed to pull so much heat out of the flue gas that the water vapor condenses inside the heat exchanger, which is where the extra efficiency comes from, so it runs a cool stack and vents in corrosion-resistant material with a condensate drain. The condensate is acidic and has to drain and often be neutralized per the manufacturer and the code.
Know which one you are on before you read the stack temperature against any target, because a stack temperature that is correct for a condensing appliance would be a venting hazard on a non-condensing one. Verify the target stack temperature, the venting material, and the condensate handling against the manufacturer's instructions and the adopted code for that specific appliance.
Natural gas, propane, and oil each have their own targets
The fuel changes the targets, the orifices, and the pressures, so the analyzer has to be set to the right fuel and the appliance set to the fuel it is actually burning. Natural gas and propane are not interchangeable: propane runs a much higher manifold pressure and uses different orifices, and a conversion kit is required to switch an appliance between them. Running an appliance on the wrong fuel setup over-fires or starves it and is a real hazard.
Each fuel has its own combustion targets. The theoretical CO2 maximums, the typical O2 bands, and the air-free CO expectations differ between natural gas, propane, and oil, and the analyzer applies the right calculation only when you select the correct fuel. Select the wrong fuel on the instrument and the calculated CO2, excess air, and efficiency come out wrong even with good sensor readings.
Oil-fired equipment adds a smoke test and falls under its own fuel code rather than the gas code. If you are analyzing an oil burner, set the analyzer to oil, follow the oil-specific procedure including smoke, and verify the targets against the manufacturer's instructions and the oil-burning equipment code. Confirm the orifice, pressure, and target figures for the specific fuel and appliance every time.
When should you do combustion analysis?
Do it at install and setup, at annual service, after any combustion-related repair, and on any CO or comfort complaint. At commissioning it is how you prove the appliance was set correctly before you left. At annual service it catches the drift, the fouling, and the venting problem that built up over a year while the appliance kept running.
After a repair, an analysis confirms you actually fixed the combustion and did not leave it worse. Replace a gas valve, a burner, an orifice, an inducer, or anything that touches the air or fuel path, and the burn can change, so you verify it. On a complaint, especially anything that sounds like CO, the analysis plus ambient testing is how you find out whether the appliance is safe.
Treat analysis as part of the work, not an upsell. An appliance that is never analyzed is an appliance running on the assumption that nothing changed since the factory, which is an assumption the field disproves constantly. The manufacturer's instructions and the code expect the appliance to be set and verified, so confirm the required cadence for the equipment and the jurisdiction.
Records: the before, the after, and the trend
Record the readings before you touch the appliance and again after you tune it, because the pair is the proof of what you did and the baseline for the next visit. The before tells you what the appliance was doing. The after tells you what you left it doing. The gap is the value of the service, and it is the record that defends the work if anyone asks later.
The trend over visits is where a slow failure shows itself. CO that creeps up year over year, stack temperature climbing, efficiency falling, draft weakening across services point at a problem developing long before it becomes an emergency. A single reading is a snapshot. The history is the diagnosis, and it only exists if someone wrote the numbers down each time.
Capture the readings on the work order and keep them with the equipment record, and a field tool like FieldOS keeps the before-and-after and the trend attached to the appliance so the next tech sees them. A combustion reading that lives on a sticky note in a truck is a reading nobody can use. Tie it to the job and the equipment so it is there next year.
What to document
Write down the readings, what each one means, and the target you held to, before and after the tune. The record has to let the next person reproduce the call, so it names the appliance, the fuel, the firing rates checked, and the basis for CO and efficiency. Verify every target you record against the manufacturer's instructions, because the note is only as good as the figures behind it.
| Reading to record | What it means | Note |
|---|---|---|
| Oxygen (O2), each fire rate | Excess air present | Verify the manufacturer's target band |
| CO, air-free, each fire rate | Safety of the burn | Record basis; verify the limit per manufacturer and code |
| Stack temperature | Heat lost up the flue | Verify target; note condensing vs non-condensing |
| Draft | Products venting as intended | Verify target and test point per the appliance |
| CO2 and excess air | Tightness of combustion | Cross-check against O2 |
| Efficiency, net or gross | Heat captured | Record the basis used |
| Manifold and inlet pressure | Input set point | Set to the rating plate for the fuel |
| Clocked input (Btu/h) | Actual firing rate | Confirm against rating plate; do not over-fire |
| Ambient CO | Safety of the space | Test with a personal monitor |
| Before and after readings | Proof and baseline | Keep with the equipment record for the trend |
Common mistakes
- Never doing a combustion analysis and tuning by eye, so the burn is set on a guess.
- Running an uncalibrated analyzer, or zeroing it in contaminated air instead of clean ambient.
- Ignoring high or rising CO, or watching an unstable CO climb instead of shutting the appliance down.
- Over-firing past the rated input to chase heat, which overheats the heat exchanger and can make CO.
- Skipping the draft and spillage check, including the worst-case depressurization test on atmospheric appliances.
- Missing a cracked heat exchanger pattern, the CO and flame change when the blower starts.
- Comparing an as-measured CO against an air-free limit, or selecting the wrong fuel on the analyzer.
Field checklist
Want this checklist to run itself on every job — with photo proof and a signed record crews can hand the customer? That's FieldOS.
Standards and references
The appliance manufacturer's installation and service instructions are the first authority, because they give the target O2, CO, stack temperature or efficiency, draft, manifold pressure, and input for the specific equipment. No general figure overrides the plate and the instructions for the unit in front of you. Set and verify to those, and confirm them every time, because they vary by model.
For gas appliances the fuel-gas code governs the installation, venting, and gas piping, commonly the National Fuel Gas Code, NFPA 54, also published as ANSI Z223.1, or the International Fuel Gas Code, IFGC, depending on the jurisdiction. Oil-burning equipment falls under the oil code, commonly NFPA 31. The exact provisions shift between editions, so confirm them against the edition the AHJ has actually adopted and any local amendments.
The analyzer manufacturer's instructions govern calibration interval, cell life, zeroing, and probe placement, and the worst-case depressurization and spillage procedure should follow a recognized protocol and the AHJ's requirements. The bottom line does not move: carbon monoxide is a life-safety hazard, so shut the appliance down on high or rising CO; tune to the manufacturer's target O2 and input without over-firing; and check draft and spillage so the products actually leave the building. Verify every target, ppm, and temperature against the manufacturer's instructions and the adopted code, and confirm the call with the AHJ.
Units and terms
Combustion readings show up in a few units, and the same idea reads differently across an analyzer screen, a rating plate, and a code. Oxygen, carbon dioxide, and excess air are percentages. Carbon monoxide is parts per million, ppm, and should be labeled air-free or as-measured. Stack temperature is in degrees Fahrenheit or Celsius. Draft is in inches of water column, in. w.c., sometimes written in. wg, or in pascals.
- Combustion analysis
- Measuring flue-gas O2, CO, and temperature plus draft with a calibrated analyzer to read excess air and efficiency and confirm safe, clean combustion
- Excess air
- The air supplied above what the fuel needs to burn completely; read from flue oxygen
- Air-free CO
- Carbon monoxide corrected for excess air so readings compare against a target; calculated from measured CO and O2
- O2 (oxygen)
- Leftover oxygen in the flue, a direct measure of excess air; about 20.9 percent in fresh air
- Draft
- The pressure that moves combustion products out through the flue, measured in inches of water column
- Stack temperature
- The temperature of the flue gas, the largest heat loss in most appliances, used to calculate efficiency
FAQ
What is combustion analysis?
Combustion analysis puts a calibrated analyzer probe in the flue to measure oxygen, carbon monoxide, and temperature, plus draft, then calculates excess air and efficiency. It confirms a gas or oil appliance burns clean, safe, and efficient. The manufacturer's targets and the adopted fuel-gas code govern the limits.
What is a safe CO level in flue gas?
A well-burning gas appliance commonly produces low air-free CO, often well under about 100 ppm at steady state, with higher or rising readings investigated. The figure that governs is in the manufacturer's instructions and the adopted code. CO that climbs and will not stabilize means shut the appliance down regardless of the number.
What does oxygen in the flue tell you?
Flue oxygen is a direct measure of excess air. Too little O2 means not enough air to finish the burn, so the appliance makes carbon monoxide. Too much O2 means surplus air carrying heat up the flue and wasting fuel. Set O2 to the manufacturer's target band for the specific appliance, not a generic number.
How often should you do combustion analysis?
Do combustion analysis at install and setup, at annual service, after any combustion-related repair, and on any CO or comfort complaint. Commissioning proves the appliance was set correctly, annual service catches drift and venting problems, and post-repair confirms the fix. Confirm the required cadence against the manufacturer's instructions and the jurisdiction.
What CO reading means shut the furnace down?
High or rising air-free CO means shut the appliance down and investigate. Published shutdown figures often cite the low hundreds of ppm air-free, but the limit that governs is in the manufacturer's instructions and the adopted code. Rising, unstable CO at any level is its own reason to stop, because it can keep climbing.
What is air-free CO versus as-measured CO?
Air-free CO is carbon monoxide corrected to remove the diluting effect of excess air, so readings compare appliance to appliance and against a target. As-measured CO is the raw flue reading mixed with whatever excess air is present. Manufacturer and code limits are usually air-free, so compare like to like and label which basis you recorded.
Why does over-firing a furnace make it dangerous?
Over-firing pushes more gas than the rating plate input, which overheats the heat exchanger, drives stack temperature up, wastes fuel, and can make carbon monoxide. More fire is not more performance. Set manifold pressure to the rating plate with a manometer, clock the meter to confirm the input, and never exceed the rated input to chase heat.
What is the difference between condensing and non-condensing stack temperature?
A non-condensing appliance runs a hotter stack to keep flue gas above its dew point, so a stack that is too cool condenses acidic moisture inside a vent not built for it. A condensing appliance runs a cool stack on purpose, condensing the water vapor for efficiency, vented in corrosion-resistant material with a drain. Verify the target per the manufacturer.
Can combustion analysis find a cracked heat exchanger?
Combustion analysis supports the diagnosis. High or rising CO, or CO and flame that change when the blower starts, point at a cracked heat exchanger letting products into the air stream. Confirming it usually takes camera or mirror inspection by a manufacturer-accepted method, and a confirmed crack means the appliance comes out of service and gets red-tagged.
Why do you test ambient CO and not just the flue?
Flue CO tells you about the burn. Ambient CO tells you about the space the occupants and you are in. Spillage, backdraft, a blocked flue, or a crack can put CO into the room while the flue reads acceptable. Wear a personal CO monitor on every combustion job and treat an ambient reading as a reason to stop.
People also ask
Codes cited in this guide
This guide is written and reviewed against the published standards below. Always confirm the current adopted edition with the authority having jurisdiction.