Home/Guides/Carbon monoxide and your furnace

Guide

Carbon monoxide and your furnace: alarms, warning signs and what to do

Carbon monoxide is a poisonous gas you cannot see or smell. Fuel-burning furnaces, boilers and water heaters can produce it when something goes wrong. Alarms and yearly inspections are the main defenses.

The short answer

If a carbon monoxide alarm sounds, or people feel sick and better outside, get everyone out and call 911 from outside. Do not go back in until responders say it is safe.

The U.S. Consumer Product Safety Commission (CPSC) advises CO alarms on every level of the home and outside sleeping areas, and a professional inspection of the heating system every year.

What carbon monoxide is and where it comes from

Carbon monoxide (CO) is a gas produced when fuel burns: natural gas, propane, oil, wood, charcoal or gasoline. It has no color, smell or taste. The CPSC says more than 200 people in the United States die every year from accidental non-fire CO poisoning associated with consumer products.

A furnace that is working properly sends its combustion gases out through a flue or vent pipe. CO becomes a danger when that path is blocked, leaking or disconnected, when burning is poor, or when the heat exchanger – the metal wall that keeps combustion gases separate from the air blown through your home – cracks. Electric heat pumps and electric furnaces do not burn fuel and do not produce CO, but other appliances in the home may.

CO alarms: where and what kind

  • Where: the CPSC says to install CO alarms on every level of the home and outside sleeping areas.
  • Power: battery-operated alarms, or plug-in or hard-wired alarms with battery backup, so they work during a power outage.
  • Standard: choose an alarm that meets the current UL 2034 safety standard, as the CPSC advises. Combination smoke and CO alarms are available.
  • Upkeep: test alarms regularly with the test button and replace batteries and the alarms themselves as the maker directs. CO alarms wear out.

An alarm is not a substitute for maintenance, and maintenance is not a substitute for an alarm. You need both.

Warning signs of CO poisoning

Early CO poisoning feels like the flu without a fever: headache, dizziness, weakness, nausea or vomiting, shortness of breath and confusion. People who are asleep or have been drinking can be badly poisoned before they notice anything.

Clues that point to CO rather than a bug:

  • Several people in the home, or pets, feel sick at the same time.
  • Symptoms ease when you leave the house and return when you come back.
  • Symptoms start when the heating comes on or a fuel-burning appliance is used.

Around the furnace, watch for soot or scorch marks near the burners or vent, a yellow, lazy flame where it is normally blue, unusual moisture on windows near the appliance, or a vent pipe that has come loose. Any of these means turn the appliance off if it is safe to do so and call a technician.

What to do if the CO alarm goes off

  1. Get everyone out into fresh air right away, including pets. Do not stop to open windows or look for the source.
  2. Call 911 or the fire department from outside.
  3. Stay out until responders tell you it is safe to go back in.
  4. If anyone has symptoms, tell the 911 dispatcher and get medical care.
  5. Before using the furnace or any fuel-burning appliance again, have it inspected and repaired by a qualified technician.

If you also smell gas, leave first and call your gas utility or 911 from outside. Our emergency guide covers what to do when the heat is out.

Never do these

  • Never run a generator inside the home, a basement or a garage, even with doors and windows open. Keep it outdoors, well away from doors, windows and vents.
  • Never use a grill, charcoal or camp stove indoors or in a garage or tent.
  • Never heat the home with a gas oven or range.
  • Never run a car or other engine in an attached garage, even with the door open.
  • Never ignore a CO alarm, or disconnect one because it keeps going off. Repeated alarms mean find the cause.

Keep vents and flues clear

Fuel-burning equipment needs a clear path for exhaust out and for combustion air in.

  • After snow, check outside vents. High-efficiency furnaces and many water heaters vent through plastic pipes in a side wall, often near the ground, and drifts or ice can block them.
  • Keep vents free of leaves, nests and stored items. Do not let siding, decks or landscaping close them in.
  • Do not block the furnace’s air openings or crowd the furnace closet with storage.
  • Look at the vent pipe where it leaves the furnace: it should be connected, sloped and free of rust holes. If not, call.
  • Have chimneys used by fuel-burning appliances checked so they are not blocked.

Yearly inspection and the heat exchanger

The CPSC says to have the heating system professionally inspected and serviced every year to ensure proper operation. ENERGY STAR’s maintenance checklist says a contractor should check gas (or oil) connections, gas pressure, burner combustion and the heat exchanger.

A cracked heat exchanger can let combustion gases, including CO, into the air your furnace blows through the house. Finding one takes a technician; it is not something to diagnose yourself. If you are told yours is cracked, ask to see the crack or the test result. On an older furnace, a cracked heat exchanger often leads to a replacement decision – see repair or replace. For what a fall tune-up should include, see furnace maintenance.

Sources

Find an HVAC company near you

Local heating and cooling companies by metro area, listed A to Z with phone numbers, services and emergency options.

Show HVAC companies

Questions

Can a furnace leak carbon monoxide?

Yes. A gas, propane or oil furnace can release CO if its vent is blocked or disconnected, if burning is poor, or if the heat exchanger is cracked. The CPSC advises CO alarms and a professional inspection of the heating system every year.

Where should carbon monoxide detectors be placed?

The CPSC says to install battery-operated CO alarms, or alarms with battery backup, on every level of the home and outside sleeping areas, and to choose alarms that meet the current UL 2034 standard.

What are the signs of carbon monoxide poisoning?

Headache, dizziness, weakness, nausea, shortness of breath and confusion – like the flu without a fever – especially if several people feel ill at once and feel better away from home.

What should I do if my carbon monoxide alarm goes off?

Get everyone outside into fresh air, call 911 from outside, and stay out until responders say it is safe. Have the furnace and other fuel-burning appliances inspected before using them again.

Related guides

Furnace not working? What to check firstA furnace that will not start is often stopped by a safety switch doing its job. Here is how to find which one – and when to leave the house instead.Furnace maintenance: a fall checklist for safe heatA gas furnace burns fuel inside your home, so fall maintenance is about safety as much as comfort. Here is what to do yourself and what the yearly professional inspection should cover.HVAC maintenance checklist: what to do each seasonA few minutes a month and one professional visit a year prevent most breakdowns. Here is the checklist, season by season.

Updated October 2026 by the Heating and Cooling editorial team. General information for homeowners, not professional advice – rules and equipment differ. When gas, carbon monoxide, electricity or anyone’s safety is involved, call 911 or your utility. How we write our guides · HVAC glossary

This field is for validation purposes and should be left unchanged.
So an HVAC company can reach you. We do not use it for anything else.
What is happening and where. Optional.
Find HVACRequest