How Much Carbon Monoxide Does a Furnace Produce

Furnaces can produce carbon monoxide (CO) under normal and malfunctioning conditions, and understanding typical emission levels, health thresholds, and mitigation steps is critical for household safety. This article explains how CO is generated by furnaces, typical parts-per-million (ppm) outputs under various scenarios, measurement standards, causes of elevated CO, detection methods, and practical prevention and response measures.

Furnace Condition	Typical CO Range (ppm)	Notes
Properly Tuned Gas Furnace (Vented)	0–10 ppm	Close to ambient indoor levels when combustion and venting are correct.
Slight Combustion Imbalance / Incomplete Combustion	10–50 ppm	Often indicates burner adjustment or minor venting issues; long exposure increases risk.
Moderate Malfunction / Partial Blockage	50–200 ppm	Significant health risk over hours; immediate inspection recommended.
Severe Malfunction / Backdrafting	200–400+ ppm	Acute danger; can cause poisoning within hours or minutes at higher concentrations.

How Furnaces Produce Carbon Monoxide

Carbon monoxide forms whenever carbon-containing fuels burn with insufficient oxygen, producing CO instead of carbon dioxide (CO2). Natural gas, propane, oil, and wood furnaces all can produce CO if combustion is incomplete.

Common furnace-related causes include cracked heat exchangers, dirty burners, blocked flue or chimney, improper venting or draft, and faulty controls that cause incomplete combustion cycles.

Typical Carbon Monoxide Levels From Furnaces

Measured CO is reported in parts per million (ppm). A properly operating, vented gas furnace usually contributes 0–10 ppm above background indoor levels. Slight imbalances or minor venting issues can raise levels into the 10–50 ppm range.

Moderate malfunctions, partial blockages, or intermittent backdrafting commonly produce 50–200 ppm. Severe failures, a blocked flue, or prolonged backdrafting can generate >200 ppm quickly; levels above 400 ppm are life-threatening in short periods.

Health Thresholds And Exposure Risks

Public health guidance frames risk by ppm and exposure time. The EPA and CDC recognize that 9 ppm is a typical long-term indoor ambient reference, while OSHA permits occupational short-term exposure up to 50 ppm as an 8-hour time-weighted average for workers.

The National Institute for Occupational Safety and Health (NIOSH) and CDC identify 35 ppm as the 1-hour recommended exposure limit for the general public in some guidelines; higher concentrations lead to symptoms: 100 ppm causes mild headache in hours, 200 ppm causes lightheadedness and nausea in 2–3 hours, 400 ppm causes severe headache and life-threatening effects within a few hours, and >800 ppm can be fatal within an hour.

Factors That Increase Furnace CO Production

Fuel Type And Combustion Efficiency: Older oil or wood furnaces and poorly tuned gas burners burn less efficiently and risk higher CO outputs.

Ventilation And Draft: Blocked chimneys, bird nests, snow-clogged vents, or negative house pressure (caused by exhaust fans or tight building envelopes) can cause backdrafting and CO buildup.

Heat Exchanger Condition: Cracked or corroded heat exchangers can allow combustion gases to mix with conditioned air, increasing indoor CO even with otherwise normal venting.

Maintenance And Age: Lack of annual servicing, soot accumulation, loose burner components, or failed controls correlate strongly with increased CO emissions.

How To Measure Furnace CO Output

Accurate assessment uses calibrated instruments. Combustion analyzers measure flue gas CO and CO2, oxygen level, and combustion efficiency directly at the vent or flue. Portable electronic CO detectors measure indoor ambient CO concentrations in ppm.

For HVAC professionals, flue gas readings during steady-state operation provide the best indication of furnace CO production. Residential CO detectors are designed to warn occupants at defined ppm/time thresholds, not to quantify precise flue emissions.

Interpreting Detector Readings And What To Do

Residential CO alarms often follow ACCA, UL, or local code response curves. Common alarm triggers: 70 ppm triggers within 1–4 hours, 150 ppm triggers within 10–50 minutes, and 400 ppm triggers within 4–15 minutes depending on device certification.

If a CO alarm sounds, the recommended steps include: move everyone outdoors to fresh air, call 911 if symptoms occur, contact a qualified HVAC technician, and do not re-enter until the source is identified and corrected.

Testing, Inspection, And Professional Evaluation

Annual furnace inspection and tune-up by a licensed HVAC technician reduces CO risk. Inspections should include burner cleaning, flame pattern check, heat exchanger inspection, vent/chimney evaluation, and combustion analyzer testing when indicated.

When intermittent CO readings appear or occupants report symptoms, professionals should perform tracer/gas flow checks, pressure diagnostics for backdrafting, and a full combustion analysis at the flue to quantify CO production.

Mitigation Strategies To Reduce CO Production

Schedule Annual Maintenance: Regular combustion tuning, burner cleaning, and heat exchanger assessment keep CO production minimal.

Keep Vents Clear: Routinely inspect exterior vents, chimneys, and flue terminations to remove obstructions such as debris, nests, snow, or ice.

Address House Pressure Issues: Reduce negative pressure by balancing exhaust systems, adding make-up air, or adjusting ventilation strategies in tightly sealed homes.

Replace Aging Equipment: Consider upgrading old furnaces with high-efficiency models that include safer combustion control features; ensure proper venting and sealed combustion where possible.

Install And Maintain CO Detectors: Place detectors near sleeping areas and on each level; test monthly and replace according to manufacturer instructions.

Codes, Standards, And Regulatory Guidance

Building codes and standards affect furnace installation, venting, and CO detection requirements. The International Residential Code (IRC) and National Fuel Gas Code (NFPA 54) include venting and appliance installation rules. Local jurisdictions may require CO alarms as part of residential safety standards.

For workplaces, OSHA sets occupational exposure limits and requires controls and monitoring when CO is a hazard. Manufacturers’ installation instructions and local code inspectors provide additional compliance requirements.

Emergency Response And Medical Considerations

Symptoms of CO poisoning include headache, dizziness, nausea, confusion, and loss of consciousness. If multiple household members develop similar unexplained symptoms, CO exposure should be suspected and occupants evacuated.

Medical evaluation is important when symptoms occur. Health providers may use blood carboxyhemoglobin measurements to confirm exposure; hyperbaric oxygen is considered in severe cases depending on COHb level and clinical status.

Common Misconceptions

Myth: CO detectors are only needed near furnaces. Reality: Detectors should be placed near sleeping areas and on each level because occupants may be affected by CO that migrates away from the source.

Myth: If the furnace flame looks blue, there is no CO. Reality: A blue flame generally indicates good combustion, but hidden issues like intermittent backdrafting or a cracked heat exchanger can still cause hazardous CO in living spaces.

Practical Checklist For Homeowners

• Install UL-listed CO detectors near sleeping areas and test monthly.
• Schedule annual professional furnace inspection and tune-up.
• Keep vents and chimneys clear of obstructions year-round.
• Watch for signs: soot, yellow/tipped flames, pilot outages, or unusual odors.
• Evacuate and call emergency services if a detector alarms or occupants have symptoms.

Frequently Asked Questions

Can a properly functioning furnace produce dangerous CO levels?

Properly functioning, vented furnaces produce minimal CO. Dangerous CO arises mainly from malfunctions, blocked vents, backdrafting, or damaged heat exchangers.

How long does it take for CO to reach dangerous levels?

Time to dangerous concentration depends on ppm. At 400 ppm, life-threatening effects can occur in a few hours; at higher concentrations the danger accelerates to minutes. Continuous monitoring and prompt mitigation are essential.

Are battery CO detectors enough or are wired ones better?

Both types work if installed properly. Hardwired interconnected detectors with battery backup offer enhanced reliability and city-code compliance in many areas. Replace batteries and detectors per manufacturer guidance.

Should a homeowner perform CO testing with a handheld meter?

Handheld CO meters can give useful ambient readings but are not a substitute for professional combustion analysis when diagnosing furnace problems. If suspicious readings are found, contact a qualified technician.

Resources And Further Reading

Authoritative sources for more information include the U.S. Consumer Product Safety Commission (CPSC), Centers for Disease Control and Prevention (CDC), National Fire Protection Association (NFPA), and local building or health departments. HVAC professionals and utility providers also offer guidance and safety inspections.

Key Takeaway: Most furnaces produce little to no CO when well-maintained and vented properly; however, malfunctions, blockages, or damaged components can raise CO to hazardous levels. Regular inspection, proper venting, and functional CO detectors are the most effective defenses against furnace-related CO risk.