Carbon Monoxide and Your Furnace: A Safety Guide for Chicago Homes

Carbon monoxide is called the invisible killer for a reason. It has no color, no smell and no taste, and at high concentrations it can overcome people before they realize anything is wrong. Every gas furnace produces carbon monoxide as part of normal combustion. When the furnace is working and venting properly, that gas goes outside. When something goes wrong, it can end up in your living space.

Chicago homes rely on gas heat for months every year, often with older furnaces, boilers and chimneys. This guide explains how furnaces can create a carbon monoxide hazard, the symptoms to watch for, what Illinois law requires for CO alarms, and exactly what to do if an alarm sounds.

If a carbon monoxide alarm is sounding right now: get everyone outside to fresh air immediately and call 911 from outside. Do not stay to investigate.

What Is Carbon Monoxide?

Carbon monoxide, or CO, is a gas produced when fuels such as natural gas, oil, wood or gasoline burn. When you breathe it in, it replaces oxygen in your blood, starving the heart, brain and other organs of oxygen. Low-level exposure over time can cause ongoing symptoms. High concentrations can cause loss of consciousness and death.

Because you cannot see or smell it, a working carbon monoxide alarm is the only reliable way to know it is present.

How a Furnace Can Leak Carbon Monoxide

A gas furnace burns fuel in a sealed combustion area and sends the exhaust out through a vent or chimney. The air that heats your home passes over the outside of the heat exchanger, a metal chamber that keeps the combustion gases separate from the household air. CO problems usually come from one of these situations:

• A cracked or corroded heat exchanger. Thousands of heating and cooling cycles stress the metal. Overheating from poor airflow or an oversized furnace makes cracking more likely. A crack can let combustion gases mix with the air the blower circulates.
• Blocked venting. Bird nests, debris, collapsed chimney liners, or snow and ice over the PVC vent pipes of a high-efficiency furnace can prevent exhaust from leaving.
• Poor chimney draft. In many Chicago homes, the furnace and water heater share a masonry chimney. If the furnace is replaced with a high-efficiency model, the water heater may be left alone in a chimney that is too large, which can cause poor draft. Deteriorated chimney liners cause similar problems.
• Incomplete combustion. Dirty burners, incorrect gas pressure or not enough combustion air can make a furnace produce more CO than it should.
• Disconnected or damaged vent pipes. Vent connectors can separate or corrode, especially in older installations.
• Back-drafting. Powerful exhaust fans, a tight house or a closed-off furnace room can pull exhaust back into the home instead of sending it up the flue.

Warning Signs of a Furnace CO Problem

An alarm is the best warning, but there can be other clues that a furnace has a combustion or venting problem:

• Soot or black streaks around the furnace, vent pipe or registers.
• Excess moisture or condensation on windows near the furnace.
• A burner flame that is mostly yellow, lazy or flickering instead of mostly blue.
• A flame that moves or changes when the blower starts, which can suggest a cracked heat exchanger.
• A pilot light that keeps going out on older furnaces.
• Rust or corrosion on the vent pipe or inside the furnace.
• A banging sound when the furnace lights. Our guide to furnace noises explains why delayed ignition matters.

None of these signs proves there is carbon monoxide in your home, but each one is a reason to have the furnace inspected.

Symptoms of Carbon Monoxide Poisoning

According to the CDC, common symptoms of CO poisoning include:

• Headache
• Dizziness
• Weakness
• Upset stomach
• Vomiting
• Chest pain
• Confusion

These symptoms are often described as flu-like, which is why CO poisoning is easy to miss in winter. Watch for these patterns:

• Several people in the household feel sick at the same time.
• Pets seem sick or lethargic too.
• Symptoms improve when you leave the house and return when you come back.
• Symptoms appear or worsen when the furnace runs.

People who are asleep or have been drinking can be overcome before they notice symptoms. Infants, older adults and people with heart or lung conditions are especially vulnerable. If you suspect CO poisoning, get to fresh air and call 911.

What Illinois Law Requires for CO Alarms

The Illinois Carbon Monoxide Alarm Detector Act, in effect since January 1, 2007, requires at least one approved carbon monoxide alarm in working order within 15 feet of every room used for sleeping in covered dwelling units. That includes single-family homes and each unit in a multi-unit building.

In rental housing, the law divides responsibilities:

• The owner supplies and installs the required alarms, makes sure batteries work when a tenant moves in, and gives the tenant written information on testing and maintenance.
• The tenant tests and maintains the alarms, replaces batteries, reports problems in writing and allows access for repairs.

There are limited exemptions for buildings with no fossil-fuel-burning equipment and no attached garage, but most Chicago homes with gas heat are covered. Tampering with or disabling a required alarm is a criminal offense under the act.

Where to Place Carbon Monoxide Alarms

Beyond the legal minimum, good practice includes:

• An alarm within 15 feet of each sleeping area, and ideally in the hallway outside bedrooms.
• An alarm on every level of the home, including the basement.
• An alarm near, but not right next to, the furnace and water heater. Follow the manufacturer's spacing guidance.
• An alarm near any door to an attached garage.

Follow the manufacturer's instructions on mounting height and distance from appliances, windows, bathrooms and vents. Combination smoke and CO alarms are convenient, but a standard smoke detector does not detect carbon monoxide.

Testing and Replacing Alarms

• Test monthly using the test button.
• Replace batteries as the manufacturer recommends, or when the low-battery chirp starts.
• Replace the alarm itself at the end of its rated life. CO sensors wear out, and many alarms have an end-of-life signal and a replace-by date printed on the back.
• Do not disable an alarm because it chirps. Fix the cause.

What to Do If Your CO Alarm Goes Off

1. Get out. Move everyone, including pets, outside to fresh air immediately.
2. Call 911 from outside.
3. Do not go back in until emergency responders tell you it is safe.
4. Get medical attention for anyone with symptoms.
5. Have the source found and fixed. Do not use the furnace, water heater or other gas appliances until they have been inspected.

Do not try to find the source yourself while the alarm is sounding, and do not simply open windows and stay inside. If you also smell gas, which has an added rotten-egg odor, leave and call Peoples Gas at 866-556-6002 from outside.

Other Carbon Monoxide Sources in Chicago Homes

The furnace is a common CO source, but it is not the only one. When an alarm sounds, any fuel-burning appliance or engine could be responsible:

• Gas water heaters. They often share a chimney with the furnace in Chicago homes, and poor draft or a blocked flue can affect both.
• Boilers. Many greystones, two-flats and older apartment buildings use gas boilers for steam or hot water heat. They need the same venting and combustion checks as furnaces. See our boiler repair page.
• Gas stoves and ovens. Using an oven to heat a home, or running burners for long periods with poor ventilation, can raise CO levels.
• Gas fireplaces and wood-burning fireplaces. Blocked or poorly drafting chimneys are a risk.
• Attached garages. Running a car, snowblower or generator in an attached garage, even with the door open, can push CO into the house.
• Portable generators and grills. These should only ever be used outdoors, well away from windows, doors and vents. Never use them in a basement, garage or porch.

Power outages during winter storms are a classic time for CO incidents, when people improvise with generators or gas appliances for heat. If your heat goes out, see our guide on what to do when your heat goes out in a polar vortex for safer options.

Common Carbon Monoxide Myths

• "I would smell it." You would not. CO has no odor. The rotten-egg smell is an additive in natural gas, which is a different hazard.
• "A new furnace cannot leak CO." A new furnace with a blocked vent, poor draft or an installation error can still create a hazard. That is why combustion testing at installation matters.
• "Opening a window fixes it." Ventilation does not remove the source. Leave the building and call 911 if an alarm sounds.
• "The alarm is just being sensitive." Treat every alarm as real until professionals say otherwise. A chirp is different from an alarm. A chirp usually means a low battery or an alarm at end of life.

What a Furnace Safety Inspection Includes

After a CO alarm, or as part of a regular tune-up, a technician checking a furnace for carbon monoxide safety will typically:

• Inspect the heat exchanger for cracks, holes, rust and signs of overheating.
• Check the vent pipe, chimney connection and draft.
• Check outdoor vent terminations on high-efficiency furnaces.
• Inspect and clean burners and check the flame.
• Check gas pressure and combustion air supply.
• Measure carbon monoxide in the flue gases and in the living space.
• Test safety controls such as limit switches, rollout switches and pressure switches.

If a heat exchanger is cracked, the furnace should not be used. On older furnaces, a cracked heat exchanger usually means replacement. Our guides on how long furnaces last and furnace replacement costs can help you plan.

How to Reduce Your CO Risk Every Winter

• Schedule a furnace inspection and tune-up every fall. See our tune-up and maintenance page.
• Change furnace filters regularly so the heat exchanger does not overheat.
• Keep the area around the furnace and water heater clear.
• After heavy snow, clear snow and ice from high-efficiency furnace vent pipes outside.
• Never run a generator, grill or car engine in a garage, basement or near windows.
• Never use a gas oven or stovetop to heat your home.
• Have chimneys and liners inspected, especially after a furnace or water heater change.

Get Your Furnace Checked

If your CO alarm has gone off, if you see soot or a yellow flame, or if your furnace has not been inspected in a while, schedule a safety inspection. Call (312) 638-0812 to book furnace service in neighborhoods such as Uptown, Albany Park and Humboldt Park. If an alarm is sounding now, leave the building and call 911 first.