Carbon Monoxide and Your Chimney

If a CO alarm is sounding right now

The U.S. Consumer Product Safety Commission (CPSC) gives this same instruction: get outside to fresh air, then call 911. Chimney service is for after the immediate danger has been handled. Do not use the fireplace, stove or connected appliance again until the cause has been found and corrected.

What carbon monoxide is and where it comes from

CO forms when fuel burns without enough oxygen to burn completely. Wood, oil, natural gas, propane and charcoal can all produce it. You cannot see it, smell it or taste it.

The U.S. Environmental Protection Agency lists leaking chimneys and furnaces, and back-drafting from furnaces, gas water heaters, wood stoves and fireplaces, among the sources of CO indoors. CPSC lists early symptoms of CO poisoning as flu-like: headache, fatigue or weakness, shortness of breath, nausea and dizziness, with confusion and sleepiness at higher exposures. EPA notes that symptoms that clear up when you leave the house are a warning sign.

Smoke from a fireplace is easy to notice. CO can be present without visible smoke, which is why alarms matter.

How a chimney can let CO into the house

These are the common ways the venting path breaks down. Any one of them can apply to a fireplace flue or to the flue that serves a furnace, boiler or water heater.

• Blockage. Nests, leaves, fallen liner tiles, debris or heavy creosote narrow or close the flue. CPSC specifically mentions birds, animals and insects nesting in vents and blocking exhaust.
• Damaged liner. Cracks, open joints and missing tiles let exhaust escape into the masonry and, from there, into living spaces or an adjacent flue.
• Oversized or cold flue. A small, efficient appliance venting into a large old masonry flue may not create enough warm, rising exhaust to draft reliably. Exhaust can linger or spill back. See The orphaned water heater problem.
• Disconnected or loose vent connectors. The metal pipe between a furnace or water heater and the chimney can separate or rust through. CPSC advises checking that vents are not loose or disconnected.
• Backdrafting. When exhaust fans, dryers or a strong draft from another appliance lower the air pressure in the house, outside air can be pulled in through a chimney. CSIA notes this can draw exhaust down an unused chimney, and lists makeup air or a top-mounted damper as ways to address it.
• Closing the damper too early. CPSC advises keeping a fireplace damper open until the ashes are cool, because smoldering embers keep producing CO.

CO alarms: where and what kind

Alarms are the second line of defense after proper venting. CPSC recommends CO alarms on every level of the home and outside sleeping areas, and notes that interconnected alarms are better because when one sounds, they all do. CPSC also advises using alarms that meet the UL 2034 standard and testing them regularly with the test button.

• Put alarms near bedrooms so they wake people up.
• Test them on a schedule and replace batteries as the maker directs.
• Replace alarms at the end of the service life printed on the unit.
• Do not treat a smoke alarm as a CO alarm unless it is a combination unit labeled for both.

New Jersey has its own rules too. Under the state fire code, as summarized by local fire officials such as Wyckoff Township, one- and two-family homes need a certificate of smoke alarm, carbon monoxide alarm and portable fire extinguisher compliance before they are sold or rented, and the CO alarm must be near sleeping areas. Your local fire official can tell you exactly what applies to your house.

Why annual inspection matters for CO

CPSC has repeatedly urged a yearly professional inspection of all fuel-burning home heating systems, including furnaces, boilers, fireplaces, wood stoves, water heaters, chimneys, flues and vents, and specifically recommends having chimneys and flues checked for blockage and leakage. EPA likewise recommends annual professional inspection of central heating systems.

A chimney inspection looks at the venting side: is the flue open from bottom to top, is the liner intact, is the connector secure, is the termination clear, and is the flue appropriate for what is connected to it. On our visits, the assessment includes a camera look up the flue when the flue allows, and photos of what we find.

The appliance itself, such as burner adjustment on a furnace, is serviced by a heating technician. Venting and appliance service work best when both are done.

Which appliances share the risk

Many homes have more than one of these, often sharing one chimney structure with separate flues. Each flue is checked separately.

Clues that the venting may not be working

None of these proves a CO problem, and a CO problem can exist with none of them. Treat them as reasons to stop and get the system checked.

• Smoke or exhaust smell indoors when an appliance runs.
• Soot or scorch marks around a furnace or water heater vent connection.
• Moisture, rust or white streaks on the vent pipe or chimney base.
• Debris, tile pieces or a nest at the chimney cleanout.
• A fireplace that smokes back into the room.
• Symptoms that improve when people leave the house.

If you suspect a problem with a gas appliance and smell gas, leave the house and call your gas utility or 911 from outside.

Practical steps to reduce the risk

1. Install CO alarms on every level and near sleeping areas, and test them.
2. Have chimneys, flues and fuel-burning appliances inspected every year.
3. Keep chimney tops capped and screened to keep animals and debris out. See chimney caps.
4. Open the fireplace damper fully before lighting and leave it open until ashes are cold.
5. Never run a generator, grill or vehicle in the house or an attached garage.
6. Have the venting checked whenever you replace a furnace, boiler or water heater. A new appliance may need a different liner size.

If an inspection shows the liner is damaged or the flue is the wrong size for the appliance, relining is a common fix. See chimney liners.

Questions homeowners ask

Sources

1. U.S. CPSC: Carbon Monoxide Information Center
2. U.S. CPSC: Seasonal heating system inspections to prevent CO deaths
3. U.S. CPSC: Caution with heating as winter weather arrives
4. U.S. EPA: Carbon monoxide's impact on indoor air quality
5. CSIA: Homeowner FAQs
6. Township of Wyckoff, NJ: Smoke detector, carbon monoxide detector and fire extinguisher compliance

This guide is general information. It can't diagnose a specific chimney; an inspection can.