Carbon Monoxide and Chimneys: How Venting Failures Poison Homes and How to Stop Them

A working chimney relies on draft: hot flue gases are lighter than the cooler air around them, so they rise and pull fresh combustion air into the appliance behind them. The U.S. Department of Energy calls this the "stack effect." Draft depends on four things:

1. Heat in the flue. A cold flue drafts weakly. The DOE notes that when the chimney air is colder than the house air, the house can become "a better chimney than the chimney," pulling cold, smelly chimney air into the room.
2. An open, correctly sized path. Blockages, collapsed tiles, nests and clogged caps reduce or stop flow. A flue much larger than the appliance needs cools gases too quickly.
3. A sealed path. Cracks and gaps in liners let gases leak into walls, attics, adjacent flues and rooms.
4. Pressure balance in the house. Exhaust fans, dryers, range hoods and tight construction can pull the house into negative pressure, strong enough to reverse the flow in a chimney.

Break any of these and CO can enter the house. With wood appliances, you usually also see and smell smoke, which acts as a warning. With gas appliances, combustion products are often invisible and have little odor, which makes gas furnaces, boilers and water heaters on failed chimneys the most dangerous combination.

Each type below includes a definition, how to identify it, the typical 2026 cost of the fix (estimated from national price surveys, HomeGuide and Angi, unless noted) and when it applies.

1. Blocked flue

Definition. Something inside the flue restricts or stops flow: bird or squirrel nests, leaves, fallen tile pieces, a collapsed liner, heavy creosote, or debris from a deteriorating crown.

How to identify it. Smoke rolling into the room when a fire is lit; a gas appliance's draft hood spilling (a hand near it feels warm, moist air); a CO alarm; bird or animal sounds in spring and summer; pieces of tile in the firebox or at the cleanout.

Typical cost. Sweep and debris removal $130–$380; animal removal $200–$1,500; if tiles have collapsed, relining with a stainless liner $1,500–$5,000.

When it applies. Any chimney, especially uncapped chimneys and older clay-tile flues. Chimney swift nests are federally protected; see chimney codes and regulations.

2. Cracked, gapped or deteriorated liner

Definition. The flue liner (clay tile, metal or cast-in-place) has cracks, open joints or missing sections, so flue gases escape into the masonry and nearby spaces.

How to identify it. Only a camera (Level II inspection) confirms it. Clues include smoke smell in rooms away from the fireplace, staining on walls near the chimney, and CO alarms that sound when a furnace runs.

Typical cost. Level II inspection $250–$600; stainless liner $1,500–$5,000 (up to about $7,000 for large or complex jobs).

When it applies. Older clay-tile chimneys, chimneys that have had a chimney fire, chimneys exposed to acidic gas condensate, and chimneys that moved in earthquakes or settling.

3. Oversized or orphaned flue

Definition. A gas water heater or furnace connects to a flue that is too large for it, often because a second appliance was removed. Flue gases cool before reaching the top, draft weakly, and condense acidic moisture that eats mortar and tile.

How to identify it. A furnace was replaced with a high-efficiency model that vents through the wall, leaving the water heater alone in the chimney. White or rust staining at the base of the chimney, deteriorated mortar at the cleanout, water at the thimble, weak draft at the draft hood.

Typical cost. Aluminum or stainless liner sized for the water heater $1,500–$3,000 (estimate); some owners switch to a direct-vent or heat pump water heater instead.

When it applies. Any home where the number or type of appliances on a chimney changed. NFPA 211 requires a Level II inspection when appliances are added, removed, or replaced with one of different type, input or efficiency. See chimney inspection levels.

4. Backdrafting from negative house pressure

Definition. The house is at lower pressure than outdoors, so air flows down the chimney instead of up, carrying combustion gases into the house.

How to identify it. Smoke or odors come down the chimney when exhaust fans, a range hood, a clothes dryer or a central vacuum run; a gas appliance spills at the draft hood when the bathroom fans are on; the fireplace smokes when you light it but improves when you open a window.

Typical cost. Draft and pressure diagnosis $150–$400 (estimate); makeup air for large range hoods, outdoor combustion air, or switching to sealed-combustion appliances can range from a few hundred to several thousand dollars.

When it applies. Tight or newly weatherized houses, homes with powerful range hoods, basements with atmospherically vented appliances, fireplaces on exterior walls.

5. Cold-chimney startup spillage

Definition. When a fire is first lit or a gas appliance starts, the flue is cold and draft is weak, so gases spill until the flue warms.

How to identify it. Smoke puffs into the room during lighting; brief spillage at a gas draft hood for the first minute or two of a cycle.

Typical cost. Often behavioral (preheat the flue, crack a window). An insulated liner ($1,500–$5,000) or a taller chimney can help chronic cases.

When it applies. Exterior masonry chimneys, oversized flues, cold climates, short chimneys.

6. Damaged, disconnected or corroded connector

Definition. The pipe between the appliance and the chimney (connector or stovepipe) is loose, rusted through, sagging, sloped the wrong way, or pulled out of the thimble.

How to identify it. Visible gaps at joints, rust holes, soot around joints, missing screws. The IRC requires connectors to slope upward at least 1/4 inch per foot toward the chimney.

Typical cost. Connector replacement $100–$500 (estimate).

When it applies. Furnaces, boilers, water heaters and wood stoves, especially older installations and damp basements.

7. Gas logs and closed dampers

Definition. Vented gas logs installed in a wood fireplace require the damper to be open or permanently blocked open. A closed or partly closed damper sends combustion products into the room.

How to identify it. Gas log set installed but no damper clamp; soot on the fireplace face; headaches when the logs run.

Typical cost. Damper clamp or stop $50–$150 (estimate); damper repair $100–$250; replacement $160–$520.

When it applies. Fireplaces converted to gas logs. See gas fireplace service.

8. Clogged cap, spark arrestor or termination

Definition. The cap screen or vent termination is plugged with creosote, leaves, ice or snow, or a high-efficiency appliance's PVC vent outside the wall is buried by snow.

How to identify it. Visible buildup on the screen; draft problems that start in winter; a high-efficiency furnace that shuts down or short cycles after snowfall.

Typical cost. Cleaning included in a sweep; new cap $75–$850.

When it applies. Wood stoves burning wet wood (screens clog fast), snowy regions, homes with sidewall vents near grade.

9. Shared flue and cross-contamination

Definition. Two appliances share one flue, or two flues in the same chimney leak into each other through a missing divider (wythe), so exhaust from one appliance comes out of the other's opening.

How to identify it. Furnace odors from the fireplace; soot at a basement cleanout; smoke test shows smoke from one flue emerging in another.

Typical cost. Smoke test with Level II $250–$600; relining one or both flues $1,500–$5,000 each.

When it applies. Older multi-flue masonry chimneys. The IRC requires a masonry wythe at least 4 inches thick between adjacent flues in new work.

10. Wrong appliance for the chimney

Definition. An appliance vented into a chimney or vent not listed or sized for it: a wood stove connected to a gas vent, an insert pushed into a fireplace without a liner, an unvented gas log set installed in a factory-built fireplace not listed for it.

How to identify it. Data plates that do not match; missing permits; creosote in a pipe marked for gas.

Typical cost. Correct venting or appliance replacement, often $1,500–$6,000 (estimate).

When it applies. DIY installs, used appliances, flipped houses.

Inhaled CO forms carboxyhemoglobin in the blood, which, as the EPA explains, inhibits the blood's ability to carry oxygen. Symptoms are easy to mistake for flu. The CDC lists headache, dizziness, weakness, upset stomach, vomiting, chest pain and confusion. The CDC warns that severe exposure can cause loss of consciousness and death, and that some people die before they ever notice symptoms, which is why a sleeping household depends on alarms.

Clues that point to CO rather than flu:

• Symptoms improve when you leave the house and return when you come home.
• Several people (and pets) in the house feel sick at the same time.
• Symptoms start or worsen when the furnace, water heater, fireplace or stove is running.
• No fever.

Higher-risk groups. The CDC notes that infants, older adults, and people with chronic heart disease, anemia or breathing problems are more likely to get sick from CO. The CDC's 1999–2004 analysis found higher death rates among adults over 65 and men.

• Annual toll. More than 400 deaths, more than 100,000 emergency department visits and more than 14,000 hospitalizations per year from unintentional, non-fire CO poisoning (CDC).
• Seasonality. From 1999 to 2004, an average of 439 people died each year from unintentional non-fire CO poisoning. The average daily death count was 2.07 in January and 1.97 in December, versus 0.67 in July and August (CDC MMWR).
• Consumer products. CPSC estimates about 170 deaths each year from CO produced by consumer products, not counting vehicles.
• Heating season overlap. The months with the most CO deaths are the same months chimneys, furnaces and fireplaces run hardest.

Backdrafting is the most misunderstood CO pathway because the chimney itself can be in perfect condition. The problem is the house.

How it happens. Every cubic foot of air an exhaust device pushes out of the house must be replaced. If the house is tight and has no dedicated makeup air, the easiest replacement path can be down the chimney. The EPA names "back-drafting from furnaces, gas water heaters, wood stoves and fireplaces" as a leading indoor CO source. Common culprits:

Why tight houses raise the risk. Air sealing and new windows save energy but reduce the leaks that used to supply combustion air. Atmospherically vented appliances (the kind with an open draft hood on top) were designed for leaky houses.

How professionals test for it. Energy auditors and heating contractors perform a worst-case depressurization test, often called a combustion appliance zone (CAZ) test: close exterior doors and windows, turn on every exhaust fan and the dryer, close or open interior doors to create the worst case, then fire each appliance and measure draft with a manometer and check for spillage and CO. A chimney sweep can do a simpler version during a Level II.

Simple homeowner check (not a substitute for testing). With the water heater running, hold a lit incense stick or smoke pencil near the draft hood's edge. The smoke should be pulled into the hood. Repeat with the range hood, dryer and bath fans on. If smoke is pushed away from the hood, stop using the exhaust devices together and call a heating contractor.

Fixes, from simplest to most thorough:

1. Run large exhaust fans only with a window cracked open nearby.
2. Add makeup air for large range hoods (many jurisdictions require it above a certain fan capacity; ask your building department).
3. Supply outdoor combustion air to the appliance room.
4. Reline an oversized flue so it drafts more strongly.
5. Replace atmospheric appliances with sealed-combustion (direct-vent) or power-vented models, or electric heat pumps, which do not depend on chimney draft.

Types of CO alarms

Buy alarms listed to UL 2034, the US standard CPSC helped develop. Combination units should also be listed to the smoke alarm standard.

Where to put them

• Every level of the home and outside each sleeping area (CDC and CPSC).
• Inside bedrooms that contain a fuel-burning appliance, such as a gas fireplace, where state law or your code requires it. Minnesota, for example, requires alarms within 10 feet of each bedroom, or inside a bedroom with a fuel-burning appliance.
• In Illinois, state law requires alarms within 15 feet of every sleeping room.
• On the level with the furnace and water heater, but not right next to them.
• Avoid kitchens, and do not place alarms directly above fuel-burning appliances (CPSC); avoid bathrooms, dead-air corners and spots near open windows or fans.
• Follow the height recommendation in the alarm's instructions; manufacturers vary.

Maintenance

• Test monthly with the test button.
• Replace batteries at least twice a year if not sealed; the CDC suggests doing it when clocks change for daylight saving time.
• Replace the whole alarm at the end of its life. The CDC says every 5 years or per the manufacturer; many units chirp or display an end-of-life signal.
• Vacuum dust from vents.
• Keep the instruction sheet: it explains the difference between alarm, low-battery and end-of-life signals.

For state-by-state alarm laws, see the table in chimney codes and regulations.

1. Do not ignore it and do not try to find the source. CPSC advises immediately moving to fresh air and calling 911.
2. Get everyone out, including pets. If anyone is drowsy, confused or unconscious, call 911 immediately and tell the dispatcher you suspect CO poisoning.
3. Call 911 or the fire department from outside. Fire departments respond to CO alarm calls and typically carry meters to check the house.
4. Do not go back in until emergency responders say it is safe.
5. Seek medical care for anyone with symptoms. Tell the doctor you suspect CO; a blood test can measure carboxyhemoglobin, and it drops once the person is in fresh air, so prompt testing matters.
6. Have appliances shut off. Responders or the gas utility may shut off or tag appliances as unsafe. Do not restart them.
7. Book the right professionals. A heating contractor for furnaces, boilers and water heaters (combustion analysis, heat exchanger check), and a certified chimney sweep for a Level II inspection of the flue, especially if the alarm sounded when an appliance was running.
8. Fix the cause before resuming use. Get written findings: what was wrong, what was repaired, and a post-repair CO and draft test.
9. Replace the alarm if it is near end of life, and add alarms where coverage was missing.

If the alarm chirps (not a full alarm): that is usually low battery or end of life. Read the unit's label to distinguish chirp patterns.

If you feel symptoms and there is no alarm: treat it the same way. Get out, call 911.

• UL 2034 CO alarms on every level and outside every sleeping area; interconnected if possible.
• Annual chimney inspection and sweeping as needed (CDC, NFPA 211, CSIA).
• Annual service for gas and oil furnaces, boilers and water heaters.
• Level II inspection after any appliance change on a chimney.
• Chimney cap with screen to keep animals out; check it is not clogged.
• Gas logs: damper clamped open or removed per the log set's instructions.
• Wood burned dry and hot; no smoldering overnight fires with the damper nearly closed.
• Exhaust fans tested for backdrafting against atmospheric appliances.
• Sidewall vents of high-efficiency appliances kept clear of snow, leaves and shrubs.
• No generators, grills or vehicles running in the garage or near windows and doors; the CDC advises keeping generators at least 20 feet from windows and doors.
• Never use a gas oven to heat the home.

Ranges are estimates based on 2026 national surveys (HomeGuide, Angi) and vary by region.

The following are illustrative scenarios, not real customers. Costs are estimates based on 2026 national price ranges.