Incomplete Combustion in Homes: Warning Signs and Testing

Safety First

If a carbon-monoxide alarm activates or anyone experiences headache, dizziness, nausea, confusion, weakness, chest pain or difficulty breathing, move everyone into fresh air and call 911 or the fire department. Do not remain inside to troubleshoot the equipment.

If you smell gas — a sulfur or rotten-egg odor — leave the building, avoid light switches, appliances, phones and anything that could create a spark or flame, and call your gas utility's emergency line or the fire department from outside.

Complete combustion of natural gas produces carbon dioxide and water vapor. When combustion is incomplete, the same appliance can also produce carbon monoxide, nitrogen dioxide, aldehydes, soot and unburned hydrocarbons. Whether any of that reaches the people in the house depends on the appliance, its venting and the pressures in the building.

This guide covers what a homeowner can reasonably observe, what only instruments and manufacturer specifications can determine, and where the line is between looking and touching.

What Incomplete Combustion Is

Burning fuel requires the right amount of air, mixed correctly, with a clean burner and a working vent to carry products of combustion outdoors. Interference with any of those — restricted primary air, a fouled burner, a blocked or undersized flue, a depressurized appliance closet, incorrect gas pressure, a damaged component — can shift combustion away from complete, and the products change accordingly.

Carbon monoxide is the byproduct that makes this a safety issue rather than an efficiency issue. It is colorless and odorless, and CDC's carbon monoxide information describes sources, symptoms and the actions to take. The U.S. Consumer Product Safety Commission's carbon monoxide information center covers alarms and appliance safety for homeowners.

Warning Signs Worth Investigating

None of the following confirms a diagnosis on its own. Each is a reason to have a qualified technician test the appliance.

What you notice	Why it warrants investigation
A change in flame appearance, such as lazy, yellow or unstable flames on an appliance that previously burned steadily	Can accompany a burner, air-supply or gas-pressure problem — but flame appearance alone does not confirm safe or unsafe combustion
Soot or dark deposits on or near an appliance, vent connector or register	Deposits indicate something about combustion or venting has changed and should be examined
A persistent burning or chemical odor during operation	Requires investigation; a brief dust smell at the first heating cycle of the season is common, a persistent odor is not
A pilot or burner that repeatedly fails to stay lit	Can accompany draft, air-supply or control problems
Heavy condensation or moisture near the appliance or on nearby surfaces when it runs	Can accompany flue-gas spillage; also has non-combustion causes
A carbon-monoxide alarm that has activated, at any time	Follow the safety box above first; the alarm is doing its job
Symptoms consistent with CO exposure in one or more occupants	Get into fresh air and call 911 or the fire department; then have appliances tested before they are used again

Flame Appearance: Useful, Not Conclusive

Flame color is the one indicator homeowners can see, and it is routinely over-interpreted.

A change in a burner's flame from its normal appearance is meaningful information. But flame appearance cannot confirm that combustion is safe, and it cannot confirm that it is unsafe. An appliance can produce elevated carbon monoxide while its flames look unremarkable, and appliances can look different from one another by design. Dust, humidity, a nearby air current and viewing angle also change what you see.

Design differences matter especially with decorative gas appliances — gas logs, decorative fireplaces and similar products. Some are designed to produce a yellow, luminous flame deliberately, and each has its own installation, operating and inspection requirements. Follow the manufacturer's instructions for the specific appliance, and have it inspected and serviced according to those instructions rather than judged against a furnace burner.

What not to do: do not adjust burners, orifices, gas pressure, air shutters, venting or combustion-air openings. Those are licensed work, they require instruments to verify, and an incorrect adjustment can create the exact hazard you are trying to avoid.

Common Causes

Burner and appliance condition

Debris in burner ports, blocked primary-air openings, corrosion, physical damage, misalignment causing flame impingement, and dirt carried in from a poorly filtered airstream. Corrected by cleaning, repair or component replacement by a qualified technician, then verified by measurement.

Combustion air

Fuel-burning appliances need air. A closet or mechanical room sealed off from the rest of the house, combustion-air openings that are blocked or absent, and competing exhaust from range hoods, bath fans, dryers or a leaky return duct can all reduce available air or depressurize the space around the appliance. Corrections follow the applicable code and the appliance manufacturer's requirements for that installation.

Venting and draft

A blocked, disconnected, corroded, incorrectly sized or incorrectly pitched vent prevents combustion products from leaving. Negative pressure can pull them back into the house through an atmospheric appliance's draft hood. Draft has to be measured, not assumed.

Equipment wear

Heat-exchanger corrosion and cracking, control faults affecting the ignition or fuel sequence, inducer problems on power-vented equipment, and gas-valve or pressure-regulation issues. Diagnosis and repair belong with professional furnace service.

How Combustion Is Actually Tested

Professional combustion testing uses calibrated instruments and reads several parameters together — flue-gas carbon monoxide, oxygen, carbon dioxide, stack temperature, draft, gas input and pressures, and temperature rise across the heat exchanger — while observing the appliance through a full operating cycle.

The critical point about interpretation: there is no universal pass/fail table. Acceptable oxygen, carbon monoxide, draft, temperature-rise and efficiency values depend on the appliance type and design, the manufacturer's published specifications for that model, the fuel and altitude, the vent configuration, and the applicable safety standards and codes. A number that is normal for one design can be out of range for another. That is why results are compared against the manufacturer's specifications for the specific equipment rather than against a generic chart — and why we do not publish one here.

A competent combustion evaluation also includes tests that no chart captures:

• Draft and spillage checks, including with the house put into a worst-case depressurization condition using exhaust equipment.
• Ambient CO measurement in the appliance space and in occupied areas.
• CO measurement in the supply airstream with the blower running, where applicable.
• Verification that venting, clearances and combustion air match the installation requirements.

Heat exchangers: what a finding does and does not mean

A cracked or failed heat exchanger is a legitimate reason to take a furnace out of service, and it is also the single most over-claimed diagnosis in the trade. Two things are true at once:

• A crack does not automatically prove that carbon monoxide is entering the supply air. Whether, when and how much depends on the location and size of the defect, pressures on either side of it, burner operation and the blower's effect during the cycle. Establishing what is happening requires measurement.
• Visual inspection alone cannot rule a heat-exchanger problem out. Much of the surface is not visible, defects can be small or open only when hot, and a clean-looking inspection is not proof of integrity. Camera inspection, pressure or tracer methods, ambient and supply-air CO measurement across the cycle, and the manufacturer's own inspection procedure are how the question is approached.

If a technician reports a failed heat exchanger, ask what was measured, how, and against which manufacturer specification — and it is entirely reasonable to get a second opinion from another licensed contractor before replacing equipment.

Appliance-Specific Notes

Furnaces. Atmospheric and induced-draft furnaces depend on the vent and on the pressure conditions around them, so they are more sensitive to depressurization and vent faults. Condensing furnaces are typically sealed-combustion, drawing air from outdoors, and add condensate drainage and secondary heat-exchanger considerations. Both are tested against their own manufacturer's specifications.

Water heaters. Atmospheric tank water heaters with a draft hood are the appliance most likely to spill combustion products when the house is depressurized by exhaust equipment, and they run year-round. Power-vented and sealed-combustion models change that behavior. A draft and spillage test under worst-case conditions is the meaningful check.

Ranges and ovens. Use a range hood that exhausts outdoors while cooking; a recirculating hood does not remove combustion products. Never use a cooking appliance to heat a room.

Dryers. Restricted or damaged exhaust ducting and lint accumulation affect both safety and performance, and the vent needs periodic cleaning.

Fireplaces and decorative appliances. Vented units need the damper and chimney condition verified. Vent-free units release their combustion products into the room and are restricted or prohibited in some jurisdictions and some room types. Follow the manufacturer's instructions and local code for the specific product, and keep a listed CO alarm nearby.

What Correction Looks Like

Depending on what testing shows: burner and appliance cleaning and repair, restoring or adding combustion air per code, vent repair, replacement or resizing, addressing house pressure imbalances created by exhaust equipment or duct leakage, control or gas-train repair, or replacing an appliance whose heat exchanger or vent system cannot be made safe. After any of it, the appliance is retested and the readings documented against the manufacturer's specifications.

Gas appliance work is licensed work. Homeowner tasks are the ones that do not involve the fuel or venting system: keeping the area around appliances clear, changing filters so airflow stays where the manufacturer expects it, keeping vent terminations clear of snow and debris, maintaining listed CO alarms, and scheduling service when something changes.

Our $89 Comfort Audit includes combustion safety checks on gas appliances alongside airflow and static-pressure measurement, and our heating service handles the repair work that follows.

Where to Read Next

• Home Comfort and Indoor Air Quality Guides — the full Home Comfort hub.
• Low-Level Carbon Monoxide in Homes — alarms, sources and limitations.
• Indoor Air Quality Guide for North Texas Homes — how combustion fits with particles, moisture and gases.
• Why Does My Heat Smell Like Burning? — odor at the first heating cycle.

Frequently Asked Questions

Can I tell whether my furnace is burning safely by looking at the flame?

No. A change from an appliance's normal flame appearance is worth reporting, but flame appearance cannot confirm safe combustion and cannot confirm unsafe combustion. Appliances can produce elevated carbon monoxide while looking normal, and some decorative gas appliances are designed to burn with a yellow flame. Determining combustion quality requires instruments and comparison against the manufacturer's specifications for that model.

Does a cracked heat exchanger mean carbon monoxide is in my air?

Not automatically. Whether combustion products enter the supply airstream, and how much, depends on the location and size of the defect, the pressures on each side of it, and how the burner and blower operate through the cycle. It has to be measured. At the same time, a visual inspection that finds nothing does not rule a heat-exchanger problem out, because much of the surface cannot be seen and some defects only open when hot.

Are there standard numbers my combustion test should hit?

There is no universal pass/fail chart. Acceptable carbon monoxide, oxygen, draft, temperature-rise and efficiency values depend on the appliance design, the manufacturer's published specifications for that model, the fuel, the vent configuration and the applicable standards. Results should be compared against the specifications for your specific equipment, and you can ask for the measured values and what they were compared to.

How often should combustion be checked?

Have fuel-burning appliances inspected and serviced by a qualified technician on the schedule in the manufacturer's instructions — commonly annual for heating equipment — and have them checked promptly any time something changes: a new odor, soot, a flame that looks different, an appliance that will not stay lit, or a CO alarm activation. Age alone is not the trigger; a change in behavior is.

Can I adjust my burners or gas pressure myself?

No. Burner, orifice, air-shutter, gas-pressure, venting and combustion-air work requires a licensed technician, instruments to verify the result, and compliance with the manufacturer's instructions and local code. An adjustment made without measurement can create a carbon-monoxide or fire hazard, and it can void equipment warranties.

What should I do if I smell gas rather than something burning?

Treat it as an emergency and leave. Do not operate switches, appliances, garage doors or phones inside, do not light anything, and do not try to find the leak. From outside the building, call your gas utility's emergency number or the fire department, and let them clear the building before anyone re-enters.