Carbon monoxide is invisible and odorless, and fuel-burning heating equipment can produce it. This guide covers how it happens, how to protect your household and what to do the moment an alarm sounds.
Carbon monoxide, or CO, is a gas produced when fuel burns incompletely. It has no color, no smell and no taste, so you cannot detect it with your senses. Breathing it prevents the blood from carrying oxygen, and at higher concentrations it can cause serious harm or death.
Many things in a home can produce CO, including gas furnaces, boilers, water heaters, ranges, fireplaces, generators and vehicles running in an attached garage. Furnaces are on the list because they burn fuel, even though a furnace in good condition vents its combustion gases safely to the outdoors.
A gas, oil or propane furnace is designed so that combustion gases pass through a sealed heat exchanger and out through a flue or vent pipe, never mixing with the air that circulates through the house. Problems arise when something interrupts that design.
Early symptoms resemble the flu without the fever: headache, dizziness, weakness, nausea and shortness of breath. A key clue is the pattern. Symptoms that affect everyone in the house at the same time, that ease when you leave and return when you come back, or that affect pets as well, may point to CO. People who are asleep or intoxicated can be overcome before they notice anything.
Do not wait for a diagnosis if you suspect CO. Get into fresh air, and call 911.
Pets and plants can be early hints, but they are not a reliable detector. Never use how you feel as a substitute for an alarm, and never assume the absence of symptoms means the air is safe.
Treat every CO alarm as real until emergency responders say otherwise. Do not assume a low battery or a false alarm. Follow these steps in order:
Make a plan before you ever need one. Agree on a meeting point outside, make sure everyone knows the alarm sound, and keep the gas utility's emergency number where it can be found quickly. Children, older adults and guests should know what the alarm means and that the answer is always to go outside first.
CO alarms are the only reliable warning. Install them according to the manufacturer's instructions and local requirements, which vary by state and city. General guidance from safety organizations is to have alarms on every level of the home, and outside every separate sleeping area. Many people also put one near the furnace or other fuel-burning equipment, following the alarm maker's placement advice.
Alarms need upkeep. Press the test button on a regular schedule, replace batteries as directed, and replace the alarm itself when it reaches the end of its life. The manufacturer's label and instructions state the lifespan, and most units also sound a distinct end-of-life signal. Combination smoke and CO alarms are available, and they follow the same rules.
Alarms that chirp once at intervals usually signal a low battery or an end-of-life condition, which is different from the loud repeating alarm pattern that signals CO. Read the instructions for your model so you know the difference, and if you are not sure which signal you are hearing, treat it as a CO alarm and get outside.
Prevention is mostly maintenance. Have fuel-burning heating equipment inspected by a qualified professional each year before the heating season. A proper inspection looks at the burners, the heat exchanger, the venting and combustion, and the safety controls, as described in what is included in an HVAC tune-up. Keep the area around the furnace clear, and keep exterior intake and exhaust vents free of snow, leaves and obstructions.
Replace the filter on schedule, because airflow problems stress heating equipment. See how often to change your furnace filter for the details. Never use a gas oven or range to heat the house, and never run a generator, grill or vehicle inside a garage or other enclosed space, even with the door open.
If your furnace shuts down repeatedly, makes new noises or shows soot or discoloration near the burner or vent, request service through our furnace and heating repair page. If it will not start, our furnace checklist covers the safe checks, with one rule that always comes first: if you smell gas or an alarm is sounding, leave and call for help.
Electric heating, including electric furnaces and heat pumps, does not burn fuel in the unit and does not produce carbon monoxide itself. A home with only electric appliances has fewer sources, but other equipment, a fireplace or an attached garage can still be one. Many safety authorities still recommend CO alarms in every home.
Get everyone outside into fresh air, then call 911 or your gas utility from outside. Do not go back in until responders say it is safe.
Yes, if the heat exchanger is cracked or the venting is blocked or damaged, or if combustion is incomplete. Annual inspection helps catch these problems.
Follow the manufacturer and local requirements. Common guidance is one on every level and outside each sleeping area.
No, a heat pump does not burn fuel. Other fuel-burning sources in the home can still produce CO, so alarms remain a good idea.
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