Carbon Monoxide Detector Going Off: What to Do

Alarm, chirp or end-of-life signal?

Every CO alarm speaks in more than one sound, and only one of them means carbon monoxide, a dangerous gas you cannot see or smell, is in the house. Manufacturers give each signal its own pattern, and the label on the back or the manual explains each one. Learn yours on a quiet afternoon, not at three in the morning.

When you are not sure which sound you are hearing, act as though it is the full alarm. Walking outside costs nothing if it turns out to be a battery.

The first few minutes

New York City's housing agency sums it up in one line: when the CO alarm sounds, get out immediately and call 911. Everything else waits until responders arrive.

• Wake everyone and get them out, pets included, closing the door behind you
• Call 911 from the sidewalk or a neighbor's house; if you heat or cook with gas, the gas utility also takes these calls
• Count heads outside so nobody goes back in looking for someone already out
• Do not return for belongings, to switch off the boiler or to open windows
• Tell responders if anyone feels unwell, and let them decide when it is safe to go back in

What responders do when they arrive

Firefighters and utility crews carry meters that read carbon monoxide in the air. They walk the house, often starting at the boiler room or basement, and they may shut off an appliance or the gas supply to it and leave it off until it has been repaired. Ventilating the house clears the gas but does not tell anyone where it came from.

Do not restart a shut-off appliance yourself. The alarm going off means exhaust got into living space, and the path it took has to be found and closed.

Expect questions about what runs on fuel in the house: boiler, furnace, water heater, range, fireplace, a car in an attached garage. Answer as fully as you can, and mention any recent work such as a new boiler, a kitchen hood upgrade or new windows, because each of those can change how the chimney drafts.

Where the exhaust usually came from

In older brick houses across New York, New Jersey, Philadelphia and northern Delaware, the oil or gas boiler and the water heater commonly vent into a masonry chimney, and that heating flue is where a chimney technician looks first. The causes below run roughly from the ones found most often to the ones found less often.

• Fallen flue tile, mortar and soot piled at the base of an old clay-lined flue, choking the opening where the boiler pipe enters
• A liner that has cracked, separated or crumbled from years of acidic exhaust, so gases leak into the masonry and back toward the rooms
• A connector pipe from the boiler or water heater that has rusted through, sagged or slipped out of the thimble in the chimney wall
• A flue far too large for a newer, more efficient appliance, so the exhaust cools, drafts weakly and spills at the draft hood
• A nest, leaves or a broken cap blocking the top of the flue
• Strong exhaust fans, a clothes dryer or a very tight house drawing outside air in through the flue, reversing its draft

Alarms that sound and then stop

An alarm that goes off for a minute and falls silent is easy to write off. It deserves the same respect as one that keeps going. Exhaust can spill in short bursts when a boiler fires into a cold flue on a winter morning, when gusts from a nor'easter push down the chimney, or when a kitchen hood and a dryer run at the same time and the house pulls air backward through the heating flue. Those conditions come and go, and so does the alarm.

Power outages bring their own risk. A portable generator run in a garage, basement or near an open window can fill a house with carbon monoxide. Federal safety guidance says to run generators outdoors only, at least 20 feet from the home, with the exhaust pointed away.

When the alarm itself is the problem

False alarms do happen. A unit past its rated life, a failing battery, an alarm mounted right beside a boiler or in a steamy bathroom, and strong fumes from some solvents or cleaners can set off certain models. Replacing an old unit and moving a badly placed one fixes those.

Calling an alarm false is a conclusion to reach after the heating system and chimney have been checked, not before. A technician's meter reading at the appliance and at the flue settles the question far better than a guess.

Apartments, two-family houses and shared buildings

In a rental, the alarm going off is still a get-out-first event, whoever owns the building. New York City's housing agency says owners must provide and install at least one approved, working smoke and carbon monoxide detector in each apartment, and tenants are responsible for maintaining them, including testing and replacing batteries.

In a brownstone split into units or a two-family house with one boiler, tell the other households right away, because exhaust from a shared heating flue does not stay on one floor. Let the owner or the building's super know as well, since the boiler room and the chimney are usually outside any one tenant's reach.

Before the heat goes back on

A heating technician checks the burner and the appliance. The chimney is a separate inspection, and it matters just as much, because a perfectly tuned boiler still spills exhaust into a blocked or broken flue. Under NFPA 211, anything that may have damaged a chimney is a reason to inspect it, and a carbon monoxide event counts.

• The cleanout below the boiler connection, emptied and checked for fallen tile
• The connector pipe and the thimble where it enters the masonry
• The full length of the liner for cracks, gaps and missing sections
• The flue size compared with the appliance it serves
• The top of the chimney for blockage, cap condition and crown damage
• Draft with the appliance running, once repairs allow it