Carbon Monoxide and HVAC Safety

Carbon monoxide (CO) is an odorless, colorless, tasteless gas produced whenever fuel is burned. Your gas furnace, water heater, fireplace, and other combustion appliances all produce CO as a byproduct. When these appliances are properly maintained and vented, the CO is safely directed outside. But when something goes wrong, CO can enter your living space with deadly consequences.

According to the CDC, more than 400 Americans die from unintentional CO poisoning each year, and more than 100,000 visit emergency departments. Understanding the relationship between your HVAC system and carbon monoxide is essential for keeping your family safe.

How CO Enters Your Home from HVAC Systems

Cracked Heat Exchanger

The heat exchanger in your furnace is a metal chamber where combustion occurs. It keeps the burning gas separate from the air that circulates through your home. Over time, the metal expands and contracts with each heating cycle, and eventually cracks can form. These cracks — even tiny ones — allow combustion gases including CO to mix with your supply air.

A cracked heat exchanger is the most common way CO enters a home through the HVAC system. It can only be detected by a trained technician using specialized cameras or chemical tests.

Blocked or Damaged Flue Pipe

The flue pipe carries combustion gases from your furnace to the outside. If the flue becomes blocked by debris, a bird’s nest, or corrosion, combustion gases back up into your home. Similarly, if the flue pipe has rust holes or disconnected joints, CO can escape into living spaces.

Incomplete Combustion

If your furnace is not receiving enough oxygen, or if the gas pressure is incorrect, the burner produces incomplete combustion — generating more CO than normal. This can happen because of a dirty burner, incorrect gas pressure, or inadequate combustion air supply.

Backdrafting

In tightly sealed homes, exhaust fans, dryers, and other appliances can create negative pressure that pulls combustion gases back down the flue pipe instead of allowing them to vent outside. This is called backdrafting and is more common in homes with multiple combustion appliances.

Symptoms of CO Exposure

CO poisoning symptoms often mimic the flu, making them easy to dismiss:

Mild Exposure

Headache
Fatigue
Shortness of breath
Nausea
Dizziness

Moderate Exposure

Severe headache
Dizziness
Confusion
Nausea and vomiting
Blurred vision
Loss of coordination

Severe Exposure

Loss of consciousness
Seizures
Cardiac arrest
Death

Critical rule: If symptoms improve when you leave the house and return when you come back, suspect CO poisoning.

Essential Safety Measures

Install CO Detectors

This is the single most important step you can take:

Install detectors on every level of your home
Place one within 10 feet of each sleeping area
Follow manufacturer placement instructions — do not place near windows, doors, or ventilation that could affect readings
Replace batteries annually (unless using 10-year sealed models)
Replace the entire detector every 5 to 7 years — sensors degrade over time
Look for UL 2034 listed detectors

Two top-rated options: the First Alert CO400 Carbon Monoxide Detector offers a reliable battery-powered sensor with a 5-year life, while the Google Nest Protect Smoke and CO Alarm adds smartphone alerts, voice warnings, and a 10-year battery — making it ideal for connected homes.

Schedule Annual Furnace Inspections

A professional HVAC technician should inspect your furnace every year before heating season. This inspection should include:

Visual inspection of the heat exchanger for cracks
Combustion analysis to measure CO output
Gas pressure testing
Flue pipe inspection
Burner inspection and cleaning
Testing all safety controls

Maintain Proper Ventilation

Never block or cover return air grilles
Keep the furnace room clear of stored items
Ensure combustion air openings are unobstructed
Do not operate the furnace with the blower door removed

Never Use Gas Appliances for Supplemental Heat

Never use a gas oven to heat your home
Never use a portable gas generator indoors
Never use a charcoal or gas grill indoors
Never run a car in an attached garage, even with the door open

What to Do If Your CO Detector Alarms

Do not ignore it. Treat every alarm as real.
Move everyone outside immediately. Do not stop to gather belongings.
Call 911 from outside or a neighbor’s phone.
Do not re-enter until emergency services have tested the air and confirmed it is safe.
Contact an HVAC company to inspect your heating system before using it again.

Additional Safety Tips

Have your water heater and fireplace inspected annually along with your furnace
Ensure your garage has adequate ventilation if it shares a wall with living space
When purchasing a new furnace, choose a model with built-in CO safety features
Consider a low-level CO monitor in addition to standard detectors — these alert at lower concentrations that can still cause health problems over time

Carbon monoxide safety is not optional. A few preventive measures — detectors, annual inspections, and awareness — can protect your family from a threat you cannot see, smell, or taste.

The Complete HVAC Maintenance Checklist for Homeowners — includes annual furnace inspection steps that help prevent CO hazards
Signs It Is Time to Replace Your Furnace — a cracked heat exchanger is both a CO risk and a strong signal your furnace needs replacing
DIY vs Professional HVAC Repairs — know which furnace tasks require a licensed professional for safety