Furnace Safety and Carbon Monoxide: What Every Chicago Homeowner Should Know

Carbon monoxide (CO) is a colorless, odorless gas produced when fuel does not burn completely. A cracked heat exchanger or a blocked flue on a gas furnace can leak it into your home—and because you cannot see or smell it, a working CO detector is your only reliable early warning. Every winter, faulty fuel-burning equipment sends thousands of people to emergency rooms, and most of those cases are preventable with detectors and an annual furnace inspection.

How does a furnace produce carbon monoxide?

Any furnace that burns natural gas, propane, or oil produces carbon monoxide as a byproduct of combustion. In a healthy system, that CO is contained inside the heat exchanger and vented safely outside through the flue. Problems start when combustion is incomplete or the exhaust cannot escape—usually from a cracked heat exchanger, a blocked or disconnected flue, poor combustion, or backdrafting that pulls exhaust back into the living space.

What are the warning signs of a carbon monoxide problem?

Signs from your furnace

• A yellow or flickering burner flame instead of a steady blue one.

• Soot or scorch marks around the furnace or vents.

• Excess moisture or condensation on windows near the unit.

• A stale, stuffy smell when the heat runs.

• Rust on the flue pipe or vent connector.

Symptoms of CO exposure in people or pets

• Headache, dizziness, or lightheadedness.

• Nausea, shortness of breath, or chest tightness.

• Unusual fatigue, confusion, or drowsiness.

• Symptoms that ease when you leave the house and return when you come back.

If your CO alarm sounds or several people feel these symptoms, get everyone outside to fresh air immediately and call 911. Do not go back inside until emergency responders say it is safe.

How to keep your furnace safe this winter

1. Install CO detectors on every level and near every sleeping area, test them monthly, replace the batteries yearly, and replace the units every 5–7 years.

2. Schedule an annual furnace inspection that includes a heat-exchanger and combustion check.

3. Keep the area around the furnace clear, and never store gasoline, paint, or chemicals near it.

4. Make sure the flue and vents are intact and unobstructed—and keep exterior exhaust vents clear of snow and ice.

5. Never use an oven, range, grill, or portable generator to heat your home.

6. Change your air filter on schedule, since restricted airflow can worsen combustion problems.

Carbon monoxide detector rules in Illinois

Illinois law requires a working carbon monoxide alarm within 15 feet of every room used for sleeping. For the best protection, place alarms on each level of the home, keep them away from bathrooms and cooking appliances that can cause false alarms, and choose interconnected alarms so that when one sounds, they all do. Test every alarm monthly and note the replacement date printed on the unit.

When should you call a professional immediately?

• Your carbon monoxide alarm goes off.

• You see a yellow burner flame, soot, or a rusted flue.

• You smell a rotten-egg odor (that signals a natural gas leak—leave and call your gas utility or 911).

• Anyone in the home has unexplained headaches, nausea, or drowsiness that improves outdoors.

• The furnace short-cycles with an odor or visible exhaust indoors.

Frequently asked questions

Can a furnace leak carbon monoxide with no smell?

Yes. Carbon monoxide is completely odorless, which is exactly why a working CO detector is essential—you cannot rely on your senses to catch it.

Where should carbon monoxide detectors be placed?

On every level of the home and within 15 feet of each sleeping area, as Illinois law requires. Keep them clear of bathrooms and cooking appliances to avoid false alarms.

How often should a furnace be inspected for CO safety?

Once a year, ideally in the fall, including a heat-exchanger inspection and a combustion analysis by a licensed technician.

What does a yellow furnace flame mean?

A healthy gas flame is crisp and blue. A yellow or flickering flame can signal incomplete combustion and possible carbon monoxide—shut the furnace down and call a professional.

CCC Mechanical performs heat-exchanger inspections and combustion safety checks for homes across Chicago and the suburbs. If something seems off with your furnace, call 630-827-9024. This article is general safety information—if you suspect a carbon monoxide leak right now, leave your home and call 911 first.