What to Do When Your Carbon Monoxide Alarm Goes Off

A carbon monoxide (CO) alarm that sounds four loud beeps, over and over, is reporting CO in the air: get everyone outside to fresh air, call 911 from outside, and stay out until responders say it is safe to go back in. A single chirp every 30 or 60 seconds is a different signal. It usually means a low battery, and chirping that continues after a new battery means the alarm has reached the end of its life.

Alarm or chirp: what the sound means

The pattern tells you which problem you have. The National Fire Protection Association (NFPA) describes a continuous set of four loud beeps as the CO warning, and a single chirp every 30 or 60 seconds as a low battery.

End-of-life signals vary by manufacturer, so NFPA points to the user guide, or a search for the brand and model, as the reference for your alarm. A combination smoke and CO alarm uses a different pattern for smoke, which NFPA describes as three loud beeps.

Treat every four-beep alarm as real. The U.S. Consumer Product Safety Commission (CPSC) says not to try to find the source of the CO while the alarm is sounding.

What to do, step by step

The CPSC steps are short, and the order matters:

Move everyone outside to fresh air right away.
Call 911 or the fire department from outside.
Count heads to make sure everyone is out. Do not go back in until the responders give permission.
If the responders identify an appliance as the source, do not use it again until trained personnel have serviced it.

If the alarm sounds again within 24 hours of going back inside, CPSC advises repeating the first three steps and having a qualified appliance technician inspect every fuel-burning appliance in the home.

If anyone has symptoms of CO poisoning, CPSC says to get outside to fresh air immediately, call the fire department from outside to report the symptoms, and contact a doctor immediately. A physician, not a checklist, is the right judge of what exposure means for a particular person.

Why the furnace is a common suspect in winter

Fuel-burning equipment is where household CO comes from. The Centers for Disease Control and Prevention (CDC) names gas- and oil-burning furnaces, portable generators, charcoal grills, fireplaces and gas ranges among the sources. CPSC names malfunctioning furnaces, ranges, water heaters and room heaters in particular.

Deaths peak in the heating season. A CDC analysis published on March 3, 2017 counted 2,244 deaths from unintentional CO poisoning in 2010 through 2015, with the highest numbers each year in winter months. In 2015, 36% of the 393 deaths occurred in December, January or February.

CDC estimates that more than 400 Americans die each year from unintentional CO poisoning not linked to fires, and more than 14,000 are hospitalized.

What exposure looks like

CO has no color and no odor, which is why people rely on an alarm to detect it. CDC lists the common symptoms as headache, dizziness, weakness, upset stomach, vomiting, chest pain and confusion, and notes that they are often described as flu-like.

CDC adds that people who are asleep, or who have been drinking or using other substances, can die from CO before they notice symptoms, which is why alarms belong near bedrooms. It also names the people more likely to get sick from CO: infants, older adults, and people with chronic heart disease, anemia or breathing problems.

Those symptoms matter even when no alarm is sounding. CPSC’s instruction is the same either way: get outside to fresh air immediately, call the fire department from outside, and contact a doctor immediately.

Where CO alarms belong

Put an alarm outside each sleeping area and on every level of the home. That is the placement the U.S. Fire Administration (USFA) and CPSC recommend.

The 2024 International Residential Code (IRC), Section R311.3, requires alarms outside each separate sleeping area in the immediate vicinity of the bedrooms. It also requires an alarm inside a bedroom when a fuel-burning appliance is located in that bedroom or its attached bathroom.

CPSC adds practical rules:

Choose alarms that meet the UL 2034 safety standard, battery-operated or with battery backup.
Interconnected alarms are preferable, because when one sounds, they all sound.
Avoid spots near heating vents or where furniture or drapes can cover the alarm. CPSC does not recommend installing one in a kitchen or above a fuel-burning appliance.
Test the alarms monthly and replace the batteries once a year.

What the building code requires

In new homes, the model code requires CO alarms when the home has a fuel-fired appliance or an attached garage with an opening into the house. The rule is Section R311.2.1 in the 2024 IRC and Section R315.2.1 in the 2021 edition.

The code also reaches existing homes. Under Section R311.2.2 of the 2024 IRC (Section R315.2.2 in the 2021 edition), when alterations, repairs or additions that require a permit take place, the home must be equipped with CO alarms placed as for a new home. Work on plumbing, and on mechanical systems that are not fuel fired, is exempt, so permitted work on a gas furnace or boiler brings the alarm requirement with it. That trigger is not new in 2024: Philadelphia’s building department lists it among the changes from the 2018 edition to the 2021 edition, so a jurisdiction still enforcing the 2018 edition or an earlier one may not apply it.

Which edition applies depends on what your state or city has adopted, and many adopt the code with local amendments. The building department can tell you which rule it enforces.

Where CO gets into the house

An alarm tells you CO is present; it cannot tell you where it came from. CPSC’s advice for the annual heating inspection names the parts to check: chimneys and flues, for blockages, corrosion, partial or complete disconnections and loose connections.

Some causes are visible from outside. USFA advises making sure the vents for the dryer, furnace, stove and fireplace are clear of snow and other debris.

When a professional is needed, and what to ask

After the responders have cleared the house, the appliance that caused the alarm needs a qualified technician before it runs again. CPSC says not to operate it until trained personnel have serviced it.

Beyond the alarm itself, CDC recommends having the heating system, water heater and any other gas, oil or coal burning appliances serviced by a qualified technician every year, and CPSC recommends an annual professional inspection of the heating system.

Questions worth asking the technician:

Will you check burner combustion, gas pressure, gas connections and the heat exchanger? ENERGY STAR lists all four for a heating checkup.
Will you inspect the venting from the appliance to the outdoors, including the chimney or flue, for blockages, corrosion and disconnections?
If you find an unsafe condition, will you write down what you found and which part is involved?
Does the repair need a permit here, and does that trigger the local CO alarm requirement?
What license do you hold for this work? Licensing rules vary by state and locality, and the agency that issues the license can confirm it.