Carbon Monoxide and Your Chimney

Too Long To Read

• If a CO alarm sounds or anyone has headache, dizziness, nausea, confusion, weakness, or unusual fatigue, leave the home and call 911 from outside.
• A blocked flue, cracked liner, backdrafting appliance, or failed vent connector can move combustion gases into living space.
• CO detectors are essential, but they do not replace inspection of the appliance, vent, flue, liner, and chimney path.
• Source check: emergency and detector guidance is cross-checked against the CDC carbon monoxide page and CPSC home heating CO safety guidance.

If you suspect carbon monoxide in your home right now, do not read further. Leave the home immediately with everyone in it, call 911 from outside, and do not re-enter until emergency responders have cleared the building. Carbon monoxide is odorless and colorless. Symptoms include headache, dizziness, nausea, confusion, and fatigue. If anyone in the home is experiencing symptoms, call 911 and report a medical emergency. Never wait for a chimney contractor when CO is suspected.

Carbon monoxide risk from a chimney is real and underappreciated. Most homeowners understand that a fireplace or furnace produces combustion gases. The assumption is that the chimney carries those gases outside. When the chimney fails, that assumption fails with it.

How Combustion Produces Carbon Monoxide

Complete combustion of any carbon-based fuel, wood, natural gas, oil, or propane, produces carbon dioxide and water vapor. Incomplete combustion produces carbon monoxide. Incomplete combustion occurs when there is insufficient oxygen for the fuel, when the appliance is malfunctioning, or when combustion gas temperatures drop too low in the flue and the combustion reaction is not fully completed.

A healthy chimney draws adequate combustion air to the fire, maintains appropriate flue temperatures, and carries combustion products including any carbon monoxide outside the building. When any of those functions fails, combustion products can find pathways into the living space rather than exiting at the top.

Chimney Failure Modes That Create CO Risk

Cracked or deteriorated liner. The liner is the continuous channel that separates combustion gases from the surrounding masonry structure. Clay tile liners crack through freeze-thaw cycles and acid attack from flue gas condensate. When a liner has cracks that penetrate the full tile thickness, combustion gases can pass through into the surrounding masonry and migrate into wall cavities or, in worst cases, directly into interior spaces. See new liner warning signs for how liner deterioration progresses.

Blocked flue. A partial or full blockage in the flue, from fallen liner debris, animal nesting, or accumulated debris, reduces the flue’s ability to exhaust gases. Combustion products build up pressure inside the firebox and find the next available path, which can be the room air rather than up the flue. An animal nest near the flue top can cause CO risk even when the lower chimney appears intact.

Backdrafting. Negative pressure in the home, caused by tight construction combined with exhaust appliances such as range hoods, bathroom fans, or dryers, can reverse draft in the chimney. Instead of flowing upward and out, flue gases are drawn downward into the home. See fireplace smoke troubleshooting for a detailed breakdown of how negative pressure causes backdrafting.

Closed or damaged damper. A damper that is stuck closed or is missing entirely will prevent flue gases from rising when the fireplace or appliance is in use. This is a less common failure mode than the others, but a damper that has been closed and forgotten, or that has corroded and stuck, is a serious hazard on any active appliance.

Damaged smoke chamber. Gaps or cracks in the smoke chamber above the damper can allow combustion gases to escape the flue path before they reach the liner. Read fireplace smoke troubleshooting for what smoke chamber deterioration looks like.

Which Appliances Create the Highest Risk

Any combustion appliance vented through a chimney or flue can create CO risk if the venting system fails. Gas appliances deserve particular attention for two reasons. First, they run much more frequently than wood-burning fireplaces, so a small liner gap is exposed to combustion gases far more often. Second, high-efficiency gas appliances run at lower flue temperatures than older appliances or wood fires, which means the flue gases cool before they exit and can condense inside the liner, producing acidic condensate that attacks clay tile from the inside.

NFPA 54 governs gas appliance venting and requires that venting systems be maintained to prevent the escape of combustion products into the building. Annual inspection of a chimney venting a gas appliance should be treated as a baseline safety practice, especially when liner condition is unknown.

Carbon Monoxide Detectors Are Not a Substitute for Inspection

Carbon monoxide detectors are a critical safety device. Every home with a combustion appliance should have functioning CO detectors on every level and outside every sleeping area. However, detectors respond to CO that has already entered the living space. They do not detect a developing flue problem. A liner that has small gaps may allow combustion gases to slowly migrate into wall cavities for months without triggering a detector in the main living space.

Annual chimney inspection is the preventive step that identifies liner problems before they create a CO pathway. A NFPA 211 Level I inspection checks accessible portions of the flue. A Level II inspection with video scanning of the flue interior is the only way to assess the full liner condition. See the annual chimney inspection: what it covers for what each inspection level includes.

What Happens After a CO Alarm or Suspected Exposure

After emergency services have cleared the building and confirmed it is safe to return, the next step is a complete inspection of every combustion appliance and its venting system. Do not assume the source was identified just because you were cleared to re-enter. The emergency responders address the immediate hazard; identifying the source of the CO pathway is a professional inspection task.

That inspection should cover: the chimney liner condition from top to bottom, the smoke chamber, the damper condition and sealing, the appliance connection point, and any other penetrations in the venting path. If the source is not identified, it can recur.

Reducing CO Risk Through Maintenance

The preventive steps for CO risk from a chimney are straightforward:

Annual inspection. NFPA 211 is the industry standard commonly used for annual inspection planning on chimneys in service. For chimneys venting gas appliances, this is the minimum cadence. An inspection that identifies a deteriorated liner or a developing blockage before it creates a CO pathway is the most effective preventive measure.

CO detectors on every level. Test them regularly. Replace per manufacturer instructions. Do not disable or remove them because of nuisance alarms. If a detector alarmed and you do not know why, that is an unresolved situation that requires investigation.

Heating season start check. Before using a fireplace or activating a heating appliance at the start of each season, check that the damper opens fully, that there is no visible debris at the firebox opening, and that there is nothing blocking the top of the chimney. This does not replace annual inspection but catches acute problems.

Address draft problems promptly. If you notice smoke entering the room during fireplace use, or unusual odors near a gas appliance, do not continue using the appliance until the cause is diagnosed. Smoke rollback and combustion odors are symptoms of the same draft failure modes that create CO risk.

Schedule Your CO Safety Inspection

Delta - Chimney Repair and Services handles chimney inspection across the North Shore and northwest suburbs since 1987. We serve Arlington Heights, Schaumburg, Highland Park, and Buffalo Grove, along with the full Chicagoland service area.

A video inspection of the flue interior is the only way to know whether your liner is intact. We provide a written inspection report that documents findings at every component. A written estimate for any repair follows, so you know what the scope covers before any work begins. Call (847) 685-1043 or use our contact form to schedule your inspection.

Carbon monoxide has no smell and no color. The chimney that looked fine from the outside can still be allowing combustion gases into the house if the liner has failed or the flue is blocked.