Carbon Monoxide in Your Home

Furnaces, water heaters, gas stoves, fireplaces, attached garages, most rooms in a typical home are near something that can produce carbon monoxide if it malfunctions or vents improperly. Knowing the signs of carbon monoxide in your home is critical because CO has no smell and early symptoms mimic the flu. This guide walks through the most common household sources, from your furnace and water heater to your garage, explains the warning patterns that suggest a problem, and provides a practical carbon monoxide (CO) safety checklist you can act on today.

Quick Safety Summary

• Top home sources: furnaces/boilers, water heaters, gas stoves/ovens, fireplaces, and attached garages.
• Portable generators during outages are a leading cause of fatal CO poisoning, outdoors only, far from buildings.
• Warning pattern: "flu-like" symptoms that improve outdoors and affect multiple people or pets at once.
• If a CO alarm sounds: leave immediately, call for guidance/air testing, and don't re-enter until cleared.
• Annual appliance inspection + working CO alarms on every level are your best defense.

What Changed in the Building

Most CO advice treats a house as a fixed object with a broken appliance somewhere inside it. The question people arrive with is different: why did a furnace that ran through nine winters start making the household ill in the tenth. The usual answer is that the building changed around it.

Home-performance contractors measure this rather than argue about it, and the way they measure it has changed in a way most consumer guides have not caught up with. The test is still the same setup: the combustion appliance zone, the space housing the furnace or water heater, is driven as negative as the house can make it, with the clothes dryer running, every exhaust fan on, the air handler on, and interior doors positioned to worsen it. What changed is the pass mark. Older BPI practice compared the pressure against a table of pascal limits, which is where the "-5 pascals" figure in circulation comes from. The current standard, ANSI/BPI-1200-S, does not set a pascal limit at all: it tells the auditor to reach the greatest depressurization achievable and then judge the appliance on what it does under that condition, testing for flue-gas spillage at five minutes and measuring carbon monoxide in undiluted flue gas against Table 1, which allows 400 ppm air-free for a central furnace or boiler and 200 ppm for a water heater or a vented room heater. The distinction matters when you are buying the work: a house can sit at a comfortable pressure and still spill, and the spillage is the thing that reaches you. If you are booking an energy audit or a weatherization job, ask whether the worst-case CAZ test includes spillage and undiluted-CO measurement, not just a manometer reading. Tightening a house without one is how a back-drafting problem gets built in.

Two signals point the same way without a manometer. Condensation that keeps coming back to the windows through the heating season says the house is holding moisture that used to leave, and that is the same air movement that used to carry combustion gases out. The second is structural: an attached garage and the rooms beside it often share attic space, so exhaust has a route over the ceiling even when the connecting door everyone remembers to close is shut.

Where CO Comes From at Home

CO is produced when carbon-based fuels burn incompletely. In a well-installed system, combustion gases are vented outdoors. Problems occur when venting is blocked, equipment is damaged or misadjusted, or a device is used in a confined space. Because CO mixes with air and has no smell, alarms and safe habits are critical layers of protection.

Understanding how to prevent carbon monoxide in home environments starts with understanding airflow. In a properly functioning home, combustion gases travel up through vents, flues, and chimneys and exit the building. But several factors can disrupt this process. Tight weatherization (sealed windows, insulation upgrades) can reduce the air supply that combustion appliances need, leading to incomplete burning. Running multiple exhaust fans, kitchen range hoods, bathroom fans, clothes dryers, at the same time can create negative air pressure inside the house, which can pull combustion gases backward through vents and into living spaces (a problem called back-drafting). Even a strong wind from the wrong direction can overpower a chimney's draft for as long as it blows. Air supply, combustion and exhaust operate as one system, and a house can fail at any of the three.

Common Sources of Carbon Monoxide in the Home

The most common household CO sources are fuel-burning appliances and equipment. Risk is higher when maintenance is neglected, venting is blocked, or appliances are used in ways they were not designed for.

Furnaces, Boilers, and Water Heaters

Heating systems and water heaters can produce dangerous CO levels if burners are not operating correctly or if exhaust cannot vent outdoors. If you suspect a problem, stop using the appliance and arrange a professional inspection.

Stoves, Ovens, and Gas Ranges

Gas stoves and ovens can produce CO, especially with poor ventilation or abnormal combustion. Never use a gas oven to heat a room.

Fireplaces and Wood Stoves

Fireplaces can become CO sources if the chimney/flue is blocked, the damper is closed, or exhaust back-drafts into the room. Soot, staining, or persistent draft problems justify professional evaluation.

Other Fuel-Burning Appliances

Gas dryers and other fuel-burning equipment can also contribute if venting is incorrect.

The Garage Factor

An attached garage is a common "hidden" pathway for CO into a home. Vehicle exhaust can build up and seep through doors, walls, and ventilation paths, even with the garage door open. Never idle a vehicle in an attached garage, and avoid running gasoline-powered tools in enclosed garage spaces.

More on vehicle scenarios: Carbon monoxide in your car.

The wall between a garage and a house is rarely airtight, and exhaust finds the gaps around the door frame, the outlets and the plumbing penetrations. It also outlasts the car: move the vehicle out and the garage keeps feeding the house for a while. A snow blower, a pressure washer or a mower running on the garage floor does what the car does. Put a CO alarm on the living side of the shared wall, and start every engine outside.

Generators, Grills, and Outage Scenarios

Portable generators produce large amounts of CO and are a leading cause of fatal poisonings during storms and power outages. Generators must be used outdoors only, far from buildings and openings. Never use a generator in a garage, basement, crawlspace, or near windows and doors.

Charcoal grills and camping stoves can also produce deadly CO. Never use them indoors, in garages, or in enclosed/semi-enclosed areas.

Apartments and Shared Buildings

In multi-unit buildings, CO can sometimes affect more than one apartment due to shared mechanical rooms, venting paths, or attached garages. If your alarm sounds or you suspect exposure, follow emergency steps first. Then notify building management so qualified inspection can include shared equipment and venting.

The pressure logic above applies to a whole building at once. A rooftop exhaust fan or a corridor pressurization system can hold a stack of apartments negative, and a single undersized or failing vent in a shared boiler room then feeds every unit above it. The tell is an alarm that sounds in your apartment with nothing in your apartment to explain it. When that happens, ask management for testing of the shared mechanical plant rather than of your own kitchen, and say why.

Warning Patterns and Clues

CO exposure often announces itself through context rather than a smell. Warning patterns include:

• Multiple people feel unwell at the same time indoors (headache, dizziness, nausea, unusual fatigue).
• Symptoms improve when you leave the building and return when you come back.
• Symptoms appear when heat, hot water, or a fireplace is running.
• Pets appear unusually lethargic or unsteady at the same time.
• Soot/staining around appliances or vents, recurring pilot issues, or poor draft signs.

These clues are not proof. The safe response is to leave to fresh air and get professional guidance/testing when exposure is possible.

The Flame Check You Can Do in Ten Seconds

Look at the burner flame on a gas stove, furnace or water heater. Complete combustion burns blue with a crisp inner cone. Yellow or orange flames, and lazy flames that lift away from the burner ports, indicate incomplete combustion, which is the condition that makes CO. A gas fireplace burner that used to run blue and now runs yellow belongs in the same category. Decorative log sets are the exception, since they are built to burn yellow on purpose.

Two related signs are worth checking while you are down there: soot or brown-black staining on or above the appliance, and a pilot light that keeps going out. None of this confirms CO by itself, and a clean blue flame does not clear the appliance either, because a well-tuned burner still fills a house when its exhaust cannot get out. The flame tells you about combustion. It tells you nothing about venting.

What to Do If You Suspect Carbon Monoxide in Your Home

If your alarm sounds, or the pattern above fits what your household is feeling, work down this list in order. The first four steps are the ones that matter while you are still deciding whether it is real.

1. Get everyone outside, pets included. Do not stop to open windows, find the source, or collect anything.
2. Call the fire department from outside and ask for air testing. They arrive with a calibrated meter, which is the only thing on the scene that can settle it.
3. Say on the call that you suspect carbon monoxide. It changes what the crew brings and how fast they come.
4. If anyone is confused, faint, or struggling to breathe, treat it as an emergency and get them urgent care rather than waiting for the building to be cleared.
5. Stay out until responders say the building is clear. Levels can climb again once doors close and the appliance restarts.
6. Ask for the readings before the crew leaves, room by room if they took them that way, and write them down. That number is what a technician works from the next day.
7. Have every fuel-burning appliance and its venting inspected before anything is lit again, and tell the technician what the readings were.
8. Check the age of your alarms. An alarm that sounded correctly is still due for replacement on its own schedule, and the incident is when people find out theirs expired years ago.

Full emergency response steps, including what to do when the alarm sounds and nobody feels unwell: what to do if you suspect a carbon monoxide leak.

Protecting Your Home From CO

CO prevention is practical: use working alarms, maintain fuel-burning appliances, keep venting clear, never run generators or grills indoors, and never idle vehicles in attached garages.

There is a legal layer under this that most homeowners meet only once. Across the 51 US jurisdictions in our state law dataset, 39 apply their CO alarm requirement to owner-occupied homes at all. In a separate group of 11, selling or transferring the property is itself a trigger that brings the requirement into force whatever the building's appliances, and that group is where the deadline is real, because it surfaces at closing when there is no time left to deal with it.

The Eleven Where Selling Is the Trigger

Ten of these also cover owner-occupied homes. Pennsylvania is the exception: Act 121 of 2013 does not reach an owner-occupied house at all, and its transfer trigger runs through covered multifamily rental apartments instead.

• Colorado, where sale sits alongside permit-required alteration and tenant turnover as routes into C.R.S. 38-45-102 and 38-45-103.
• Connecticut, where transfer joins new construction and permit work in the trigger set.
• Louisiana, where R.S. 40:1581 ties the baseline duty to sale or lease status rather than to any appliance.
• Maine, where 25 M.R.S. 2468 drives coverage by transaction category and adds transfer certification steps of its own.
• Massachusetts, where the local fire department inspects at the sale checkpoint and the compliance document is part of the file.
• Nebraska, where section 76-606 fires on a change in tenant occupancy and 76-603 sets what a qualifying build owes per habitable floor.
• New Jersey, where the Uniform Fire Safety Act certificate has to be in hand before sale, lease or occupancy in covered properties.
• Oregon, where conveyance and the start of a new rental agreement are separate triggers under the ORS and OAR 837-047 pathways.
• Pennsylvania, through covered multifamily apartments with a fossil-fuel heater, a fireplace or an attached garage, not through owner-occupied houses.
• Rhode Island, where the local fire authority certifies in the window before title transfer.
• Vermont, where newly built single-family dwellings carry the duty and public-building provisions extend owner duties into multi-unit and lodging property.

Every other jurisdiction attaches the duty to the building or its appliances instead, so a sale changes nothing about what is owed. Our state-by-state CO detector law pages set out the trigger conditions and who carries the duty in each one.

What a Mandatory Inspection Looks Like

Every US source on this page, ours included, tells you to have the furnace checked once a year, and none of them can make you. Germany runs the same idea as law. Section 14 of the Schornsteinfeger-Handwerksgesetz gives the district's authorized chimney sweep, the bevollmaechtigter Bezirksschornsteinfeger, the job of carrying out a Feuerstaettenschau on the combustion appliances in the district, and the statute fixes the spacing rather than leaving it to the owner to arrange: no earlier than three years after the last one and no later than five.

The comparison is worth making for what it shows about the US position rather than the German one. A homeowner here who skips the annual service meets no inspector, so the alarm on the ceiling is the whole safety system. That is why the trigger conditions in the 51 jurisdictions above carry as much weight as they do, and why an alarm sitting years past its replacement date is a larger problem in a country with no scheduled inspection standing behind it.

Full checklist: Prevention of carbon monoxide poisoning.

Home CO Safety Checklist

• CO alarms installed on every level and near sleeping areas.
• CO alarms tested monthly; batteries replaced as needed; units replaced at end-of-life.
• Furnace/boiler inspected and serviced annually by a qualified technician.
• Water heater venting checked for proper connection and draft.
• Fireplace chimney/flue inspected and cleaned annually.
• Gas stove/oven: range hood used; never used for room heating.
• Attached garage: vehicles never idled inside; door between garage and home kept closed.
• Generator: used outdoors only, 20+ feet from building, exhaust directed away.
• Vents and exhaust paths checked for blockages (snow, debris, nests) seasonally.
• Space heaters: only models rated for indoor use; manufacturer instructions followed.

Sources & References

• CDC, Carbon Monoxide Poisoning
• US CPSC, Carbon Monoxide Information Center
• US EPA, Carbon Monoxide's Impact on Indoor Air Quality
• NFPA, Carbon Monoxide Safety
• NHS, Carbon Monoxide Poisoning
• Building Performance Institute and NREL, Combustion Appliance Zone (CAZ) Depressurization Testing quick guide, the six-step worst-case setup, ending at "record the greatest CAZ depressurization achievable" and then spillage and CO testing
• ANSI/BPI-1200-S Standard Practice for Basic Analysis of Buildings, section 7.9 for the CAZ procedure and 7.9.5 Table 1 for the CO thresholds; the standard sets no CAZ pascal limit
• DOE Building America Solution Center, Combustion Appliance Zone (CAZ) Testing
• Minnesota Department of Health, Carbon Monoxide Poisoning in Your Home. Included because it does the thing the federal pages do not: it publishes state-level counts, an average of 14 deaths and roughly 300 emergency department visits a year in Minnesota, alongside typical measured indoor and outdoor concentrations and the state's own alarm law. If you want to see what this subject looks like with real numbers attached to one population, this is the clearest example a US health department publishes.
• Schornsteinfeger-Handwerksgesetz section 14 (Feuerstaettenschau), in German, on the federal justice ministry's law portal, for the three-to-five-year statutory interval
• SilentShadow state CO detector law dataset, 51 jurisdictions, for the owner-occupied and sale-or-transfer trigger counts and the eleven named states

Last updated: August 12, 2026