Carbon Monoxide Symptoms From a Furnace: Warning Signs, Risks, and What to Do

Carbon monoxide from a furnace is a silent danger: it is colorless, odorless, and can make people sick before they realize there is a problem. Knowing the symptoms of carbon monoxide poisoning, spotting furnace warning signs, and acting quickly can prevent serious harm. This guide explains how carbon monoxide forms, the symptoms to watch for, how detectors work, and the steps to take to protect a home during heating season.

What Carbon Monoxide Is And Why Furnaces Can Produce It

Carbon monoxide (CO) is a toxic gas produced by incomplete combustion of fuel. Gas, oil, wood, and propane furnaces all produce some CO during operation, which should be vented outdoors. When combustion or venting is faulty, CO can accumulate indoors to dangerous levels.

How Combustion Creates CO

A properly tuned burner mixes fuel with enough oxygen to produce carbon dioxide and water. If the mix is wrong, the flame is unstable, or the heat exchanger or flue is compromised, the burn becomes “fuel-rich” and produces CO. Yellow, lazy flames indicate incomplete combustion, a common precursor to CO generation.

Furnace Types And Risk Factors

• Conventional gas or oil furnaces: Relies on chimney draft; backdrafting or blocked flues can pull exhaust into living spaces.
• High-efficiency (condensing) furnaces: Uses PVC intake/exhaust; snow, ice, or debris blocking vents can lead to CO spillage.
• Wood and pellet systems: Lower combustion control and soot increase CO risk if not maintained.
• Risk factors: Cracked heat exchanger, inadequate makeup air, negative pressure from exhaust fans, unlined or damaged chimneys, and improper installation.

Symptoms Of Carbon Monoxide Exposure From A Furnace

CO symptoms often mimic viral illness but typically occur without fever and may improve after leaving the house. Multiple people or pets feeling ill at the same time indoors is a red flag for a CO problem.

Early Symptoms (Low To Moderate Exposure)

• Headache (often dull, frontal) and fatigue
• Dizziness or lightheadedness
• Nausea, queasiness, or upset stomach
• Mild shortness of breath, chest tightness, or palpitations
• Irritability and trouble concentrating

Moderate Symptoms

• Confusion, impaired judgment, or memory issues
• Worsening headache and vomiting
• Blurred vision or coordination problems
• Sleepiness and difficulty staying awake
• Chest pain, especially for those with heart disease

Severe Symptoms (High Exposure)

• Loss of consciousness or collapse
• Seizures or profound confusion
• Shortness of breath, rapid heartbeat, or arrhythmia
• Coma and death if exposure continues

Symptoms In Children, Pregnant People, Older Adults, And Pets

• Children: Irritability, vomiting, and lethargy can appear quickly.
• Pregnancy: CO crosses the placenta; fetal harm can occur even with mild maternal symptoms.
• Older adults: Higher risk of cardiac complications from CO exposure.
• Pets: Lethargy, unsteadiness, and vomiting often present before people notice symptoms.

Key clue: symptoms worsen at home (especially when the furnace is running) and improve outdoors or away from the building.

Physical Signs Your Furnace May Be Leaking Carbon Monoxide

While CO itself is undetectable by senses, furnaces that generate or spill CO often leave traces. Visual and performance cues can prompt a safety check and professional inspection.

• Yellow or orange burner flames on gas furnaces (healthy flames are steady blue).
• Soot, scorch marks, or smoke stains on or around the furnace, vent pipes, or registers.
• Frequent burner shutdowns, pilot outages, or tripping safety switches.
• Rust or water streaks on vent pipes (condensation can indicate venting issues).
• Heavy moisture on windows when the furnace runs, suggesting poor combustion or backdrafting.
• Exhaust odors or a “stale” air feeling (CO has no smell, but other combustion byproducts can be noticeable).
• Blocked or iced-over outdoor vents on high-efficiency systems.
• CO detector alarms or elevated digital readings at home.

Carbon Monoxide Levels, Timelines, And Alarm Behavior

CO health effects depend on concentration and time. Many standard CO alarms follow UL 2034, designed to alarm before life-threatening exposure while minimizing nuisance alarms. Digital displays and low-level monitors offer more insight than basic alarms.

UL 2034 standards aim to prevent fatalities but may not alarm at lower levels that still cause symptoms in sensitive individuals. Low-level CO monitors can provide earlier warnings for at-risk households.

Immediate Steps To Take If You Suspect Carbon Monoxide

If a CO alarm sounds or symptoms suggest exposure, treat it as an emergency. Do not wait for confirmation from a second device.

• Get everyone to fresh air immediately. Leave the building and move to an outdoor location or another building.
• Call 911 and report a suspected carbon monoxide leak and any symptoms.
• Do not reenter until first responders declare it safe. If safe to do so while exiting, open doors for ventilation.
• Do not attempt repairs or relight the furnace. Turn off the appliance only if it does not delay evacuation.
• Seek medical evaluation. Even if symptoms improve outdoors, CO can have delayed effects.
• Contact your gas or utility provider and a licensed HVAC contractor for inspection after clearance.

Diagnosing And Treating CO Poisoning

Medical teams assess exposure using history, symptoms, and a carboxyhemoglobin (COHb) level obtained via blood gas or a CO-oximeter. COHb reflects how much hemoglobin is bound to CO instead of oxygen.

Standard treatment is 100% oxygen via mask to hasten CO elimination. Severe cases, pregnancy with significant exposure, neurologic symptoms, cardiac ischemia, or high COHb may qualify for hyperbaric oxygen therapy. Follow clinical advice even if symptoms seem mild.

According to the CDC, thousands in the U.S. are treated for CO poisoning annually, and risk increases in winter and during power outages. Prompt care reduces complications such as heart injury and delayed neurologic effects.

Preventing Carbon Monoxide Leaks From A Furnace

Prevention focuses on safe combustion, reliable venting, and early detection. Annual professional maintenance significantly reduces risk and improves efficiency.

Professional Inspection And Combustion Safety

• Annual tune-up: Inspect burners, heat exchanger, ignition, flame sensor, and safety controls.
• Heat exchanger checks: Look for cracks, corrosion, or damage that could allow exhaust into supply air.
• Draft and spillage testing: Verify proper chimney draft and no backdrafting near the furnace or water heater.
• Flue gas analysis: Measure oxygen, CO, and combustion efficiency; adjust burner for clean, stable flame.
• Vent system inspection: Confirm intact vent connectors, proper slope, secure joints, and clear terminations.

Home Ventilation And Pressure Balance

• Ensure makeup air: Modern tight homes may require dedicated combustion air to prevent negative pressure.
• Manage exhaust devices: Large range hoods and bath fans can reverse draft; consider make-up air kits.
• Keep returns and supply registers clear and avoid enclosing furnaces without proper ventilation.

Safe Operation Habits

• Never use an oven or stove to heat the home.
• Keep high-efficiency intake/exhaust vents clear of snow, leaves, and nests.
• Store solvents and paints away from furnace rooms; vapors can disrupt combustion.
• After severe weather, verify rooftop or sidewall vents are open and chimneys are unobstructed.

CO Detectors And Monitors: Placement, Features, And Maintenance

Detectors save lives. Choose reliable devices, place them correctly, and maintain them. CO alarms are not the same as smoke alarms; both are needed.

Where To Install CO Detectors

• On every level of the home and near each sleeping area so alarms wake occupants.
• In or near the furnace room (at least one outside the mechanical room).
• Avoid installing right next to bathrooms, kitchens, or within supply register airflow to reduce false readings.
• In attached garages or along entry points from garages to catch vehicle-related CO.

Carbon monoxide mixes with air, so follow manufacturer height guidance; most detectors work effectively when mounted on walls or ceilings.

Features To Look For

• UL 2034 certification for residential alarms; consider models with a digital display.
• Interconnection (hardwired or wireless) so all alarms sound together.
• Battery backup for hardwired alarms; sealed 10-year batteries reduce maintenance.
• End-of-life chirp and easy test/silence button.
• Low-level monitors that display and alert to lower concentrations can benefit sensitive individuals.

Maintenance And Replacement

• Test monthly and replace batteries per instructions.
• Replace alarms every 5–7 years (check manufacture date).
• Vacuum dust from vents to maintain sensor performance.

The CPSC and EPA provide detailed guidance on detector use. Building codes such as the International Residential Code and NFPA 72 require CO alarms near sleeping areas in homes with fuel-burning appliances or attached garages.

Furnace Problems Most Likely To Produce Carbon Monoxide

Certain failures directly increase CO production or spread exhaust into living spaces. Proactive repairs are critical to safety.

• Cracked heat exchanger: Allows combustion gases into supply air; often linked with soot, odd smells, or frequent limit trips.
• Maladjusted burners: Incorrect fuel/air mix produces CO and soot; requires professional tuning.
• Blocked or undersized flue: Birds’ nests, corrosion, or improper slope cause spillage and backdrafting.
• Failed draft inducer or pressure switch: Prevents safe venting in modern furnaces, leading to shutdowns or unsafe operation.
• Improper retrofit: New high-efficiency furnace on old chimney without liner can cause condensation and vent failure.

Seasonal And Weather Considerations

Carbon monoxide incidents spike in winter with heavy furnace use and storms. Weather-driven hazards can quickly turn safe systems into risks.

• Snow and ice: Keep sidewall PVC vents and intakes clear; create a 12–18 inch clearance zone.
• Power outages: Operate portable generators outdoors, at least 20 feet from buildings, away from doors, windows, and vents.
• After storms: Inspect chimneys and caps for damage; check for animal nests before heating season.
• Cold snaps: Longer runtimes can magnify small combustion issues; schedule preventive maintenance early.

How To Tell Flu From CO Symptoms

CO exposure commonly gets mistaken for a virus. Recognize patterns that point to a furnace issue rather than illness.

• No fever: CO usually does not cause a high temperature.
• Timing and location: Symptoms appear or worsen at home, especially at night or when the furnace is on, and improve outdoors.
• Multiple people or pets affected: Several household members become ill simultaneously.
• CO alarm activity: Any alarm or elevated display reading supports suspicion.

What To Expect From A Professional CO And Furnace Safety Inspection

After an incident or as preventive care, an HVAC professional should conduct a comprehensive evaluation. Choose licensed, insured contractors with combustion-safety training.

• Interview and history: Symptoms, alarm data, appliance list, and building changes.
• Visual and mechanical checks: Heat exchanger, burners, ignition, vent connectors, and condensate systems.
• Combustion analysis: Measure CO, O2, CO2, and flue temperature; tune for efficient, clean burn.
• Spillage and draft testing: Verify proper draft with doors open/closed and exhaust fans operating.
• Air balance review: Check return/supply pressures and makeup air availability.

Documented readings before and after adjustments provide proof of safe operation and can satisfy warranty or insurance requirements.

Costs, Warranties, And Insurance Considerations

Safety upgrades are often affordable compared to the risks of CO exposure. Budget for detectors and maintenance as part of home ownership.

• CO detectors: $20–$50 per unit; interconnected or smart models $80–$150.
• Annual furnace tune-up: Typically $100–$250 depending on region and fuel.
• Chimney liner or vent repairs: A few hundred to several thousand dollars based on scope.
• Heat exchanger replacement: Often $1,500–$3,000; sometimes furnace replacement is more cost-effective.

Manufacturer heat exchanger warranties can be long, but labor is rarely covered. Homeowners insurance may cover sudden damage but not wear-and-tear or improper maintenance. Keep service records and alarm logs.

Frequently Asked Questions

Can An Electric Furnace Produce Carbon Monoxide?

Electric furnaces and heat pumps do not burn fuel and do not produce CO. However, they can circulate CO produced elsewhere, such as from a gas water heater, fireplace, or attached garage. CO alarms are still essential.

Is A Cracked Heat Exchanger Always A CO Leak?

A cracked exchanger allows combustion gases to mix with household air and can leak CO if the burner produces it. Any suspected crack requires immediate professional evaluation; operating the furnace is unsafe until cleared.

Will A Smoke Alarm Detect Carbon Monoxide?

No. Smoke alarms and CO alarms use different sensors. Install both, or choose combined units rated for smoke and CO with the correct certifications.

Can I Smell Or See Carbon Monoxide?

No. CO is colorless and odorless. Any perceived “furnace smell” is from other byproducts. Only detectors can reliably warn of CO.

Do Symptoms Go Away Quickly After Leaving?

Symptoms may improve with fresh air, but CO can linger in the body. Always seek medical care after significant exposure or when an alarm sounds, especially for children, pregnant people, and older adults.

How Often Should A Furnace Be Serviced?

At least once a year, ideally before the heating season. Schedule earlier if there are symptoms, alarms, or any performance changes.

What If The CO Alarm Chirps But Does Not Sound?

Intermittent chirping usually indicates low battery or end of life, not an active CO event. Replace batteries or the unit as instructed and verify function with a test.

Key Differences Between CO And Natural Gas Leaks

CO poisoning is not the same as a gas leak. Knowing the difference helps choose the right response.

• CO: No odor; causes illness. Use CO alarms; evacuate and call 911 if they sound.
• Natural gas or propane: Distinct “rotten egg” odor added. If the smell is strong, evacuate, avoid switches or flames, and call the gas utility and 911 from outside.

Household Checklist For Carbon Monoxide Safety

• Install CO alarms on every level and near bedrooms; test monthly.
• Schedule annual furnace maintenance including combustion testing and vent inspection.
• Keep vents and chimneys clear of debris, nests, snow, and ice.
• Never idle vehicles in an attached garage, even with the door open.
• Use generators outdoors, well away from openings.
• Teach household members symptoms and the evacuation plan.
• Record alarm events and share with HVAC technicians and medical providers if symptoms occur.

Resources And Standards For Further Guidance

Authoritative references provide detailed safety guidance and current recommendations. Bookmark and share with household members.

• Centers for Disease Control and Prevention (CDC): Carbon Monoxide Poisoning — symptoms, treatment, prevention.
• U.S. EPA: Carbon Monoxide and Indoor Air Quality — sources, health effects, mitigation.
• U.S. Consumer Product Safety Commission (CPSC): CO Poisoning — safety tips, recalls, detector guidance.
• NFPA — heating safety and alarm standards.
• OSHA: Carbon Monoxide — exposure limits and workplace guidance.

Why This Matters During Heating Season

Furnaces are essential for comfort, but they demand respect for combustion safety. Recognizing the symptoms of carbon monoxide from a furnace, acting immediately on alarms, and maintaining equipment can prevent tragedies. With detectors on each level, a trusted HVAC partner, and an informed household, families can heat their homes confidently and safely.