Normal Carbon Monoxide Levels in Furnace Exhaust: Safe Ranges and Action Steps

Carbon monoxide (CO) in furnace exhaust is a critical safety and performance metric for homes. This article explains what constitutes normal CO levels in furnace exhaust, how levels are measured, typical ranges for healthy combustion, potential causes of elevated CO, detection and testing methods, and practical steps to fix problems and prevent CO exposure. The information focuses on U.S. standards and best practices for residential furnaces.

CO Range (ppm In Furnace Exhaust)	Interpretation	Suggested Action
<50 ppm	Good combustion; furnace operating normally	Routine maintenance and periodic checks
50–200 ppm	Marginal combustion; adjustment or service recommended	Schedule combustion tuning and inspect venting
200–400 ppm	High CO; indicates significant combustion inefficiency or venting issue	Stop use if symptoms appear; call HVAC professional immediately
>400 ppm	Very high CO; potential hazard to occupants if leakage occurs	Cease operation and evacuate if necessary; immediate professional intervention

What Is Carbon Monoxide And Why Furnace Exhaust Matters

Carbon monoxide is an odorless, colorless gas produced by incomplete combustion of fuels like natural gas, propane, oil, or wood. Furnaces vent CO as part of combustion products. Proper combustion and effective venting keep CO in the flue where it disperses safely outdoors. Elevated CO in furnace exhaust signals incomplete combustion, blocked flues, poor air supply, or equipment malfunction, and increases the risk of CO entering living spaces.

Measurement Units And Standards Relevant To Furnaces

CO concentrations are commonly reported in parts per million (ppm). Several regulatory and safety benchmarks guide interpretation:

• OSHA: workplace permissible exposure limit is 50 ppm as an 8-hour time-weighted average; not designed for residential furnace flue analysis but useful for context.
• NIOSH/CDC: recommended exposure limit is 35 ppm for a 10-hour workday; IDLH (immediately dangerous to life or health) is 1,200 ppm.
• EPA: ambient air standard is 9 ppm over 8 hours and 35 ppm over 1 hour.

For furnace combustion analysis, HVAC technicians use different practical ranges to assess appliance performance. These ranges help determine whether the furnace combustion is efficient or requires adjustment.

Typical Normal CO Levels In Furnace Exhaust

Furnace exhaust CO varies by fuel type, appliance age, burner condition, combustion air supply, and draft. Typical bench and field guidance for residential furnaces is:

• <50 ppm — Desirable and indicates good combustion when measured in the flue near the draft hood or vent connector on a steady-state run.
• 50–200 ppm — Marginal; suggests tuning may improve combustion efficiency and reduce CO.
• 200–400 ppm — Concerning; indicates poor combustion, burner problems, or venting/draft issues. Immediate service recommended.
• >400 ppm — High risk; could indicate substantial malfunction or blocked venting. Treat as a potential safety hazard.

These ranges are practical field guidelines used by many HVAC professionals. The acceptable flue CO level for a specific furnace can also be provided by the manufacturer and should be referenced during service.

How Technicians Measure CO In Furnace Exhaust

Accurate measurement requires the correct tools and technique. Common procedures include:

• Using a calibrated combustion analyzer or CO meter capable of measuring flue gas CO in ppm.
• Measuring at steady-state operation after the furnace reaches normal operating temperature.
• Placing the probe in the flue gas stream near the draft hood, vent collar, or vent connector while avoiding dilution from ambient air.
• Recording additional parameters like flue gas temperature, oxygen (O2) or carbon dioxide (CO2), and draft to diagnose combustion efficiency.

Combustion analyzers that display CO, CO2, O2, and temperature provide the best diagnostic picture and allow calculation of combustion efficiency and excess air.

Common Causes Of Elevated CO In Furnace Exhaust

Understanding causes helps prioritize fixes. Common contributors include:

• Incomplete combustion from clogged burners, incorrect gas pressure, or dirty heat exchangers.
• Poor air supply to the burner, especially in tightly sealed homes or when nearby registers/closets restrict airflow.
• Blocked or restricted venting from bird nests, debris, ice, snow, or collapsed vent pipes, causing backdrafting and higher flue CO.
• Cracked heat exchanger allowing combustion gases to leak into the furnace cabinet and potentially into living spaces.
• Improper installation or deterioration of vent connectors and chimney liners.

Often, more than one issue contributes to elevated CO; a comprehensive inspection is necessary for accurate diagnosis.

Interpreting Test Results: What To Watch For

When reviewing flue CO readings, technicians should consider context and complementary measurements:

• Low O2 or low CO2 with high CO suggests incomplete combustion due to burner problems.
• High flue temperatures with elevated CO may indicate poor heat transfer or blocked heat exchanger fins.
• Rapid fluctuations in CO may indicate intermittent backdrafting or variable draft caused by wind or house pressure changes.
• Even moderate flue CO levels become a higher concern if a cracked heat exchanger could allow gases into the home; visual and pressure diagnostics are critical.

Documenting multiple readings (start-up, steady-state, and post-run) helps ensure reliable evaluation.

When Elevated Flue CO Requires Immediate Action

Certain measurements or conditions demand urgent response:

• Any flue CO reading above 200 ppm should trigger immediate evaluation and corrective action.
• Signs of CO in the living space (detector alarms, occupants with headaches, nausea, dizziness) require stopping appliance use and ventilating the home.
• Visible damage to the heat exchanger, flames impinging on surfaces, or persistent backdrafting warrants shutting down the furnace until repaired.

Safety protocols dictate evacuation and professional intervention if people experience CO symptoms or if CO detectors alarm.

Testing At-Home: CO Detectors Versus Flue Gas Analysis

Residential CO detectors monitor indoor air and are essential for occupant safety. They do not measure flue gas inside the vent. Flue gas analysis requires professional equipment. Key distinctions:

• CO detectors: detect indoor CO concentrations and alert occupants; follow manufacturer replacement timelines and placement recommendations.
• Combustion analyzers: measure CO in the furnace exhaust and are used by trained technicians to diagnose appliance performance.

Both are important: detectors protect occupants, and combustion analysis identifies and fixes the source.

Steps To Reduce Furnace Exhaust CO And Improve Safety

Practical measures help prevent elevated CO in the flue and living spaces:

• Schedule annual professional furnace maintenance, including burner cleaning and combustion analysis.
• Ensure proper combustion air supply by avoiding blocked louvers or seals around appliance closets.
• Inspect and clear venting pathways, chimneys, and caps of obstructions seasonally.
• Install and maintain CO detectors on each level of the home, following placement guidance near sleeping areas.
• Address signs of a cracked heat exchanger promptly; replacement is often required.
• Avoid using unvented combustion appliances (portable heaters, gas stoves) as primary heat sources indoors.

Proactive maintenance reduces the chance of high CO and improves furnace efficiency and life span.

Codes, Standards, And Manufacturer Guidance

Local building codes, manufacturer installation instructions, and standards from organizations like the National Fire Protection Association (NFPA) and American Society of Heating, Refrigerating and Air-Conditioning Engineers (ASHRAE) govern safe installation and venting practices.

Manufacturers typically specify acceptable flue gas limits and required testing methods. Technicians should follow those specifications when assessing normal CO levels for a specific furnace model.

What Homeowners Should Do If Furnace Exhaust CO Is Suspected

If a homeowner suspects elevated CO or if a detector alarms, the recommended steps are:

1. Turn off the suspected fuel-burning appliance if safe to do so and ventilate the home by opening windows and doors.
2. Evacuate occupants, especially children, elderly, or those with health conditions, and seek fresh air.
3. Call 911 or the local emergency number if anyone has symptoms of CO poisoning or if the detector indicates hazardous levels.
4. Contact a licensed HVAC professional to perform combustion analysis and repairs before resuming appliance use.

Do not attempt repairs without proper training and equipment.

Choosing A Qualified HVAC Technician For Combustion Analysis

Look for technicians who are certified and experienced in combustion analysis and CO diagnostics. Relevant credentials include EPA certification (for some fuel types), NATE (North American Technician Excellence), or manufacturer-specific training.

Ask prospective technicians whether they use a calibrated combustion analyzer, can measure CO, O2/CO2, flue temperature, and draft, and whether they provide a written report including readings and recommended corrective actions.

Key Takeaways For Safe Furnace Operation

Normal CO levels in furnace exhaust are typically below 50 ppm for well-tuned residential furnaces; 50–200 ppm indicates a need for adjustment; readings above 200 ppm signal a problem requiring immediate attention. Regular professional maintenance, proper venting, functional CO detectors, and adherence to manufacturer and code requirements keep furnaces operating safely and efficiently.

For persistent concerns, documented flue gas readings and a comprehensive inspection by a licensed HVAC professional provide the most reliable path to diagnosis and remediation.