Carbon monoxide and your chimney: blocked flues, alarms and the law

Updated November 10, 2026 · KeystoneProGuide editorial team

What does a chimney have to do with carbon monoxide?

The chimney is the exhaust for everything in the house that burns fuel and is connected to it: a fireplace, a wood stove, and very often a furnace, boiler or water heater. Carbon monoxide is made whenever fuel burns incompletely, and the flue is what carries it outside. When the flue is blocked, cracked, disconnected or the wrong size, the gas stays in the house.

The federal agencies say this directly. The Environmental Protection Agency's list of indoor sources includes leaking chimneys and furnaces, back-drafting from furnaces, gas water heaters, wood stoves and fireplaces, and a flue that is improperly sized, blocked, disconnected or leaking. Philadelphia's Office of Emergency Management names plugged or cracked chimneys among the causes of dangerous levels.

How serious is the risk?

The Centers for Disease Control and Prevention says that each year more than 400 Americans die from unintentional carbon monoxide poisoning not linked to fires, more than 100,000 visit an emergency department and more than 14,000 are hospitalized. The Consumer Product Safety Commission counts a narrower group, deaths from carbon monoxide produced by consumer products other than cars, at about 170 a year on average, and includes malfunctioning furnaces, water heaters and fireplaces among those products.

It is a winter problem. Philadelphia's emergency management office says fifty percent of the carbon monoxide incidents reported to local fire departments occur between November and February, peaking in December.

What goes wrong inside a flue?

It gets blocked. The Commission has listed the causes over the years: birds, other animals and insects that build nests in vents over spring and summer, and mortar and other material dislodged from the chimney walls. CSIA, the trade's certification body, adds soot from oil burners and flakes of clay liner in flues that serve gas appliances. The Commission also warns that during renovations vents and chimneys can be covered by tarps or debris, and the U.S. Fire Administration reminds homeowners to keep vents for the furnace, stove and fireplace clear of snow.

It leaks. The Commission says an inspector should check chimneys and flues for corrosion, partial and complete disconnections and loose connections as well as blockages. In a 1997 release it gave homeowners something to look for: black stains on the outside of the chimney or flue, which it said can mean pollutants are leaking into the house.

The house pulls air down it. The EPA calls this back-drafting. CSIA explains how it happens in a tightly sealed house: a fireplace in use exhausts household air until the pressure indoors is negative, and replacement air comes down whichever flue is not in use. It says the remedy, a supply of make-up air, also removes the potential for carbon monoxide to be drawn back down the furnace chimney.

It no longer fits the appliance. CSIA says high-efficiency gas and oil furnaces often do not perform well when vented into existing chimney flues, which raises the chance of gases entering living spaces.

What are the symptoms?

The CDC lists headache, dizziness, weakness, upset stomach, vomiting, chest pain and confusion. The Commission describes the early stage as like the flu but without the fever, and warns that with a slowly developing problem both residents and doctors can mistake it for flu. The EPA's version is the most useful at home: flu-like symptoms that clear up after you leave the house.

People who are asleep can die before they notice anything, the CDC says. That is the argument for alarms.

What should you do if an alarm sounds?

The Commission's instructions are short.

1. Do not look for the source. Move outside to fresh air at once.
2. Call the fire department or 911.
3. Check that everyone is accounted for, and do not go back in until responders say you may.
4. If the cause turns out to be an appliance, do not run it again until it has been properly serviced by trained personnel.

Philadelphia's emergency management office adds one step for city residents: call your utility company as well.

Where should alarms go?

• The Commission says to install alarms that meet the current UL 2034 standard, battery-operated or with battery backup, on every level of the home and outside sleeping areas. It does not recommend putting them in kitchens or above fuel-burning appliances.
• The CDC says to place one near every sleeping area, check the batteries each spring and fall and replace the unit every 5 years or as the maker instructs.
• Pennsylvania's Ready PA program says one on each floor, with additional detectors near major fuel-burning appliances such as the furnace, water heater or wood stove.
• The University of Missouri Extension says not to mount one directly above or beside a wood stove, where start-up can trigger a false alarm.

The Commission's caution applies to all of this: an alarm is no substitute for proper use and upkeep of the appliances.

What does Pennsylvania law require?

Less than many people assume. The Carbon Monoxide Alarm Standards Act of 2013 does two things.

• In a multifamily dwelling, meaning a building with three or more households, each apartment that has a fossil fuel-burning heater or appliance, a fireplace or an attached garage must have an approved alarm near the bedrooms and near the heater or fireplace. The act counts wood as a fossil fuel for this purpose. In a rental the owner provides the alarm and the tenant maintains it and replaces batteries.
• When a house is sold, the seller must disclose on the property disclosure statement what carbon monoxide detectors are installed.

The act does not itself order an alarm into every one- or two-family house. It does let municipalities go further. West Conshohocken's code, for one, requires proof of hard-wired detectors within 20 feet of a heating unit before it issues a certificate of use and occupancy for a home. Willful failure to install a required alarm is a summary offense under the act, with a fine of up to $50.

How does a chimney inspection fit in?

It is on every agency's list. The CDC says to have your chimney checked or cleaned every year. The EPA says to have a trained professional inspect, clean and tune up the central heating system, naming furnaces, flues and chimneys, annually. The Commission says the yearly heating inspection should take in the chimney and flue.

One practical warning comes from the Commission's 1990 release: heating contractors do not normally check chimneys and flues, so ask for the venting to be included or have a sweep look at it. Gas and oil furnace chimneys covers that flue in detail, signs your chimney needs attention lists what to watch for, and how often a chimney should be swept gives the schedule. Local businesses are on the chimney sweeps directory page.

Sources

1. Carbon Monoxide's Impact on Indoor Air Quality, U.S. Environmental Protection Agency
2. Carbon Monoxide (CO) Poisoning Basics, Centers for Disease Control and Prevention
3. Carbon Monoxide Questions and Answers, U.S. Consumer Product Safety Commission
4. Inspect Home Furnace System For Hazards, Carbon Monoxide, U.S. Consumer Product Safety Commission
5. CPSC Urges Annual Fuel-Burning Appliance Inspection to Prevent Deaths, Fires, U.S. Consumer Product Safety Commission
6. Install a carbon monoxide alarm in your home: CPSC Urges Annual Furnace Inspection to Prevent CO Deaths, U.S. Consumer Product Safety Commission
7. Carbon Monoxide Poisoning Prevention, U.S. Fire Administration
8. Homeowner Resources, Chimney Safety Institute of America (CSIA)
9. Avoiding Carbon Monoxide Hazards, Chimney Safety Institute of America (CSIA)
10. Carbon Monoxide Alarm Standards Act (Act 121 of 2013), Pennsylvania General Assembly
11. Your Home, Only Safer, Ready PA (Commonwealth of Pennsylvania)
12. Carbon monoxide poisoning, City of Philadelphia
13. Winter weather outlook 2025: What to know to be prepared, City of Philadelphia
14. Code of the Borough of West Conshohocken, Chapter 65: Housing Standards, Borough of West Conshohocken, Montgomery County
15. Wood Stove Maintenance and Operation, University of Missouri Extension