Can a Chimney Cause Carbon Monoxide in the House?

How a healthy flue keeps exhaust moving

Anything that burns fuel makes carbon monoxide: a wood fire, an oil burner, a gas boiler, furnace or water heater. Hot exhaust is lighter than the cooler air around it, so it rises up the flue and pulls fresh air in behind it. Chimney technicians call that pull draft, and the larger effect of warm air rising through a whole house is the stack effect.

Draft depends on a clear path, a sealed flue and enough heat in the gases. Take away any of the three and exhaust slows down, cools, and starts looking for another way out. The other ways out are the draft hood on the water heater, the barometric damper on an oil boiler, the fireplace opening, and any crack in the masonry that leads into a wall or a closet.

The heating flue that nobody watches

Homeowners think about the fireplace because they can see it. In brownstones, Philadelphia rowhomes, North Jersey two-families and pre-war brick houses on Long Island, the flue that matters more is usually the one serving the boiler or furnace in the basement. It runs every day from fall to spring, it often shares the chimney with a water heater, and its exhaust is wet and mildly acidic, which slowly breaks down old clay tile and mortar from the inside.

Replacing the boiler can create a new problem. A modern high-efficiency boiler often vents out the side wall through plastic pipe, leaving the old gas water heater as the sole appliance in a flue sized for something much bigger. That lone water heater cannot warm the flue enough, the exhaust condenses and drafts weakly, and spillage at the draft hood becomes likely. Technicians call it an orphaned water heater, and the usual fix is a properly sized liner.

Failure points, from most to least common

These are the chimney conditions found behind carbon monoxide complaints, roughly in the order they turn up on heating flues in older housing.

• Fallen tile, mortar and scale piled in the bottom of the flue, choking the opening where the boiler pipe enters
• A liner that is cracked, has open joints, or has crumbled away in sections
• A flue too large or too small for the appliance, so the draft is weak or unstable
• A connector pipe that has rusted through, pulled out of the thimble or sags back toward the appliance
• A blocked top: nests, leaves, a fallen cap or ice in a severe cold snap
• Exhaust fans, a dryer or a very tight house that reverses the flue's draft and sucks outdoor air down it

Fireplaces and wood stoves

A wood fire that is burning hot and bright makes less carbon monoxide than one that is smoldering. The risky moments come late in the evening, when the fire has died to glowing coals and someone closes the damper to keep the heat in. The coals keep producing carbon monoxide with nowhere to go but the room. The same happens with gas logs run with the damper shut or only partly open.

Stoves and inserts add a connection point. An insert set into an old masonry fireplace without a liner running to the top, or with a loose connection at the damper area, can spill exhaust into the smoke chamber and from there into the room.

Shared stacks and party walls

In attached houses, one chimney stack often holds flues for two or more homes, separated by a single layer of brick called a wythe. When that wythe cracks or its mortar washes out, exhaust from a neighbor's boiler can cross into your flue and come out of your fireplace, even if you have not lit a fire in years. The reverse can happen too.

That is why an unused fireplace in a rowhouse still deserves a look during a heating-season inspection, and why a smell of exhaust in a room with no working appliance is worth taking seriously. Repairs to a shared stack, such as relining one flue or rebuilding the top, go more smoothly when the neighbors know about the work in advance.

Warning signs at the appliance and the chimney

The alarm tells you carbon monoxide is present. The equipment can tell you the venting is struggling before that point. Any of the following is a reason to have the appliance and the chimney checked.

• Soot or scorch marks around the boiler, furnace or water heater draft hood
• A yellow or lazy flame on a gas appliance that normally burns blue
• Rust, white crust or water dripping at the connector pipe joints or the thimble
• Heavy condensation on windows near the heating equipment
• A burner that shuts itself off repeatedly, or a puff back on an oil system
• Smoke rolling into the room from the fireplace

What you can look at without tools

A homeowner can learn a fair amount from the basement and the living room without touching the equipment or going near the roof. Look, note what you see, and pass it on to the technician; do not take apart pipes or open a hot appliance.

• The connector pipe: it should rise steadily from the appliance to the chimney, without sagging, gaps or rust holes
• The thimble: the pipe should sit snugly in the masonry with no daylight, soot or crumbling mortar around it
• The small iron cleanout door in the basement: shards of tile or mortar on the floor in front of it are a clue
• The chimney top, seen from the street: a missing cap, a leaning flue tile or plants growing out of it
• The fireplace: a black smudge across the top of the opening shows smoke has been spilling into the room

If a CO alarm sounds

Leave first and let responders investigate. Get everyone out, pets too, and call 911 from outside; homes on natural gas can also call the gas utility. Do not go back in to switch off equipment or open windows, and do not return until responders say it is safe. Afterward, both the appliance and the chimney need checking before the heat runs again.

During winter power outages, keep portable generators outdoors and far from doors, windows and vents. A generator in a basement or garage is a carbon monoxide source no chimney can help with.

What an inspection checks on a heating flue

NFPA 211 puts chimneys and vents on a yearly inspection cycle. On a heating flue, that means opening the cleanout and removing whatever has fallen, checking the connector and thimble, examining the liner from top to bottom, comparing the flue size with the appliances connected to it, and looking at the cap and crown. A cap keeps nests and leaves out. A liner sized to the appliance keeps the draft steady and the masonry dry.

Test your CO alarms the way their maker says and replace them when the maker says to. A sound chimney and a working alarm do different jobs, and a house needs both.