HomeSmarter with Andrew

Know your system · Safety

What to know about carbon monoxide

If a CO alarm sounds: Get everyone outside immediately. Leave the exit door open as you go only if doing so doesn’t slow evacuation. Call 911 from outside, account for everyone, and stay out until responders say it’s safe. Don’t stay inside or go back in to open doors or windows.
FIELD STUDY / 05Good: how carbon monoxide should vent
Good: how carbon monoxide should ventCutaway of the same house showing an example of working venting, not proof that a home is CO-safe. Each appliance is labeled. The furnace and water heater flue pipes are whole and run out through the roof. The gas range has a flame under a cooking pot, with the vent hood well above it, its fan on, and a duct that runs outdoors. The gas dryer duct is clear and runs out the wall, and its exhaust leaves at a 45 degree angle, blowing away from the house. The window on the right wall is labeled. The washer is gray, for context only. The generator sits outdoors, drawn not to scale, so the picture does not show a distance. Puffs are illustrative: they travel up the flue pipes and out the top, up into the hood and out its duct, and out the dryer vent away from the house. This sketch gives the furnace and water heater separate flue pipes. Real layouts vary. The CO alarm on the wall shows a green light in its bottom right corner.GoodFan onWindowCOFurnaceWater heaterRangeDryerWasherGenerator

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Read the field notes
  • This is an example of working venting, not proof that any home is CO-safe. The puffs are illustrative. CO is invisible, and they show where it should go: outdoors.
  • Furnace and water heater: in this sketch each flue pipe is whole and runs out through the roof. Venting layouts vary from home to home, so this is one example, not a rule. The gas dryer duct is clear and runs outdoors, and its exhaust blows away from the house. The washer is context only.
  • Range: the flame emblem shows it is gas, and a pot is cooking on it. The vent hood sits well above the cooktop, its fan is on, and its duct runs outdoors. A hood that vents outdoors helps, but it does not catch everything. In Berkeley Lab tests of seven home hoods, capture ranged from under 15 percent to over 98 percent, and the hoods did better on back burners than front burners. The height in the sketch is for clarity. Follow the hood maker's instructions for real mounting height.
  • Portable generator: outdoors only, and far from doors, windows and vents that could let CO come indoors, with the exhaust pointed away from the home. The drawing is not to scale and does not show a distance.
  • The alarm is quiet in this sketch. An alarm is a backup, not a substitute for working vents. How fast an alarm responds varies, so do not count on it to catch a leak right away.
Schematic field study. Not to scale or an installation plan.
FIELD STUDY / 05Bad: where carbon monoxide comes from
Bad: where carbon monoxide comes fromCutaway of a house with each appliance labeled. Gas appliances that can release carbon monoxide: furnace, water heater, range and oven with a vent hood above, and dryer. The furnace flue is cracked and separated, the water heater flue is blocked by debris at the top, and the dryer duct is clogged with lint. This sketch gives the furnace and water heater separate flue pipes. Real layouts vary. Red arrows and smoke puffs show exhaust leaking into the house. The washer is labeled in gray and is shown beside the dryer for context only. The range hood fan is off, so the range exhaust drifts out into the room instead of going up the hood. A large lint plug fills the dryer duct. The window on the right wall is labeled. The generator is the Bad example: it sits too close to the house, right against the wall under the window and next to the dryer vent, and its exhaust puffs go in through the window. A CO alarm on the wall warns you: its red light, in the bottom right corner, is on.BadFan offWindowCOFurnaceWater heaterRangeDryerWasherGenerator

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Read the field notes
  • Carbon monoxide has no color and no smell. Fuel-burning appliances and generators can make it.
  • Gas appliances in the sketch: furnace, water heater, range with burners and oven, and dryer. Many homes have electric versions of some of these. The puffs are illustrative. CO is invisible, and they show where it could leak from each one.
  • Vents, as they apply: in this sketch the furnace and water heater each have their own flue pipe. Venting layouts vary from home to home, so this is one example, not a rule. Here the furnace flue is cracked and separated, and the water heater flue is blocked by debris at the top. The gas dryer exhaust duct is clogged with lint. The U.S. Department of Housing and Urban Development's (HUD) inspection standard says a gas dryer's exhaust carries combustion gases, including carbon monoxide, outside, and rates a blocked or damaged gas dryer exhaust as life-threatening. The red arrows and smoke wisps show exhaust leaking into the house.
  • The range has no flue. Here the hood fan is off, so the range exhaust drifts out into the room instead of going up the hood.
  • The washer is beside the dryer for context only. It is not a source of carbon monoxide.
  • The generator placement is the unsafe example. Never run one inside the home or garage, or close to a window, door, or vent. Here it sits against the wall under a window and beside the dryer vent, so its exhaust drifts in. Run it outdoors, well away from the house.
  • The alarm on the wall is going off. How fast an alarm responds varies, so do not count on it to catch a leak right away. It does not stop the leak. It backs up working vents and does not replace them.
  • Alarms go on every level of the home and near bedrooms. Have fuel-burning appliances and their vents inspected. During renovations, make sure vents and chimneys are not blocked by tarps or debris.
Schematic field study. Not to scale or an installation plan.

Carbon monoxide (CO) is a gas you cannot see or smell. It comes from burning fuel. Furnaces, water heaters, ranges, fireplaces, generators and running vehicles can all make it.

What are the signs of CO poisoning?

Early signs feel like the flu without a fever: headache, tiredness, shortness of breath, nausea and dizziness. Higher levels bring confusion, vomiting, loss of coordination and loss of consciousness. Slow leaks are often mistaken for the flu.

If you suspect CO poisoning, follow the emergency steps above immediately. Once outside, seek medical attention and say you suspect CO. Do not wait to see if it passes.

Where to put alarms

The U.S. Consumer Product Safety Commission (CPSC) advises CO alarms on every level of the home and outside sleeping areas. CPSC says to skip attics and basements unless they include a sleeping area. If a basement or attic holds a gas furnace or gas water heater, put a CO alarm in that space too. It warns you early if the appliance is not venting properly, and it shows which area the problem is coming from. Interconnected alarms are best, because when one sounds they all sound. Do not let furniture or drapes cover an alarm.

Test alarms monthly and replace the batteries once a year. Replace the alarm itself by the date printed on it or in its manual. The date depends on the make and model. If you cannot find a date or do not know how old the alarm is, replace it. An alarm is added protection, not a substitute for keeping fuel-burning appliances in good shape.

How to stay safer around carbon monoxide

Leave appliance work to a pro. CPSC says never to service fuel-burning appliances without the knowledge, skill and tools. If a doctor confirms CO poisoning, have a qualified service person check the appliances before anyone uses them again.

Make a household exit plan

Work this out together before an emergency:

For a fire, follow NFPA's fire-specific escape guidance; the CO exit-door instruction above is not fire-door advice. Ready.gov helps you write and practice a plan that fits your household.

Sources

CPSC's fact sheet supports the signs, the fresh-air and fire-department steps, annual servicing, and the garage, generator, charcoal and gas-appliance rules. CPSC's carbon monoxide information page and 2023 storm-safety release support keeping a generator away from the house and the no porch or carport rule. CPSC's fact sheet supports checking that vents and chimneys are not blocked during renovations. CPSC's alarm page supports placement and monthly testing. The Centers for Disease Control and Prevention (CDC) supports the attached-garage rule, keeping a generator away from windows, doors and vents, annual servicing by a qualified technician and replacing a CO alarm on the maker's schedule. The U.S. Environmental Protection Agency (EPA) says carbon monoxide can build to deadly levels in a garage even with the door open. The kitchen hood and dryer exhaust notes in the drawings come from Lawrence Berkeley National Laboratory's range hood study and the U.S. Department of Housing and Urban Development's (HUD) clothes dryer exhaust inspection standard. CPSC's responder guide supports staying outside until responders authorize re-entry. It is written for trained responders and is not a home testing procedure.

CPSC's CO questions and answers support getting outside immediately, calling emergency services, accounting for everyone and staying out until responders permit re-entry. NFPA's escape-planning guidance and plan handout support clear routes, an outdoor meeting place, assigned assistance and practice. Ready.gov supports making and practicing a household plan.

CO response and household plan claims checked October 9, 2026. Other CPSC, CDC and EPA claims last read October 1, 2026; CPSC responder guide last read September 30, 2026.

An active alarm or symptoms are an emergency, not a reason to browse.