Carbon monoxide in a house comes from anything that burns fuel. Gas furnaces, water heaters, stoves, fireplaces, space heaters, clothes dryers, and portable generators all produce carbon monoxide as a normal byproduct of combustion. The gas becomes dangerous when equipment malfunctions, ventilation fails, or fuel-burning devices are used in ways they weren’t designed for. Each year, more than 400 Americans die from unintentional carbon monoxide poisoning not linked to fires, and over 100,000 visit an emergency department.
Why Burning Fuel Creates Carbon Monoxide
Any time a fuel like natural gas, propane, wood, charcoal, or gasoline burns, it combines with oxygen. When there’s plenty of oxygen available, the fuel burns completely and produces carbon dioxide and water vapor, both relatively harmless. When oxygen is limited, the fuel burns incompletely and produces carbon monoxide instead. This incomplete combustion releases only about 52% of the energy in the fuel, which is why it also tends to produce a weaker, cooler flame.
Three conditions make incomplete combustion worse: low temperature, limited oxygen supply, and not enough time for the chemical reaction to finish. Inside a home, any of these can occur when a burner is dirty, a flame is starved of air, or exhaust gases can’t escape fast enough. Carbon monoxide is actually a normal intermediate step in the combustion process. In a well-functioning appliance, it quickly converts to carbon dioxide before leaving the flue. In a poorly functioning one, that conversion never finishes.
The Most Common Household Sources
Gas furnaces and boilers are the biggest concern because they run for long stretches during cold months and move large volumes of combustion gases. Water heaters are a close second, since they cycle on and off throughout the day year-round. Gas stoves and ovens produce carbon monoxide every time a burner lights. According to the EPA, levels near a properly adjusted gas stove typically run 5 to 15 parts per million, while a poorly adjusted stove can push 30 ppm or higher.
Fireplaces and wood stoves generate significant carbon monoxide, especially when burning unseasoned wood or when the damper is partially closed. Gas clothes dryers vent combustion exhaust outdoors through a duct, but a kinked, disconnected, or lint-clogged duct can send that exhaust back into the laundry room. Space heaters that run on kerosene or propane are particularly risky because many older models are unvented, meaning all combustion byproducts stay in the room.
The Consumer Product Safety Commission specifically warns against using gas ranges, ovens, or clothes dryers to heat your home. These appliances aren’t designed for continuous operation as heaters, and doing so can quickly raise indoor carbon monoxide to dangerous levels.
Cracked Heat Exchangers
Inside a gas furnace, the heat exchanger is the metal barrier that keeps combustion gases separated from the air circulating through your home. The burner heats one side of the exchanger, and your household air passes over the other side, picking up warmth without ever touching the exhaust. When this metal wall cracks, carbon monoxide and other combustion byproducts leak directly into the air your family breathes.
Cracks develop over time from repeated heating and cooling cycles that expand and contract the metal. Older furnaces are more prone to this, but even newer units can develop problems if they were improperly installed or transported. Warning signs include a chemical or burning smell (sometimes compared to formaldehyde), visible soot around the furnace, or a yellow, flickering burner flame instead of a steady blue one. A cracked heat exchanger isn’t something you can see easily on your own. It requires an HVAC technician with a combustion analyzer to confirm.
Backdrafting and Blocked Flues
Even when your furnace or water heater is working perfectly, carbon monoxide can enter your living space if the exhaust can’t get out. Backdrafting happens when the normal upward flow of air in a chimney or flue reverses direction, pushing combustion gases back into the house instead of outside. This reversal is caused by pressure imbalances inside the home.
Several things create these imbalances. Bathroom exhaust fans, kitchen range hoods, and clothes dryers all push air out of the house, which lowers indoor air pressure. If enough air is being exhausted, the easiest path for replacement air to enter may be down through the furnace flue, dragging carbon monoxide with it. Leaky ductwork in an HVAC system can amplify the problem. Even closing interior doors in a tightly sealed home can shift pressure enough to cause backdrafting in the room where the furnace sits.
Blocked flues cause a similar result through a simpler mechanism. Bird nests, fallen debris, ice buildup, or a collapsed liner can physically obstruct the exhaust path. The combustion gases have nowhere to go, so they spill out of joints in the vent pipe, gaps around the draft hood, or back through the burner opening. A horizontal vent pipe that sags or isn’t angled slightly upward toward the exterior wall can also trap exhaust and cause spillage.
Generators and Vehicles
Portable generators are one of the deadliest sources of residential carbon monoxide. A single generator can produce as much CO as hundreds of cars, and poisoning incidents spike during power outages when people run generators in garages, basements, or too close to open windows. The safe placement is outdoors, at least 20 feet from any door, window, or vent.
Cars, trucks, and lawn equipment running in an attached garage are another common source. Carbon monoxide migrates easily through the shared wall between a garage and living space, seeping through gaps around doors, wiring penetrations, and ductwork. Even briefly warming up a car in a closed garage can raise CO levels inside the home for hours.
How Detectors Respond to These Sources
Residential carbon monoxide alarms are designed to catch sustained or high-level exposure, not brief spikes. Under the UL 2034 standard used in the United States, an alarm exposed to 70 ppm must sound between 60 and 240 minutes. At 150 ppm, it must alarm within 10 minutes. At 400 ppm, which is immediately dangerous, it must trigger within 4 to 15 minutes. Alarms are intentionally designed not to sound at very low levels, so a detector staying silent doesn’t guarantee zero carbon monoxide in the home.
This matters because some sources, like a slightly maladjusted gas stove running at 30 ppm, may never trigger an alarm yet still cause chronic low-level symptoms such as headaches, dizziness, and fatigue over weeks of exposure. Placing detectors on every level of the home and near sleeping areas catches the dangerous scenarios, but annual appliance inspections catch the slow, subtle ones.
Preventing CO Buildup
An annual inspection by an HVAC technician is the single most effective step. During a proper check, the technician drills a small hole in the furnace flue and inserts a combustion analyzer probe to measure CO concentrations directly. They also inspect burners, heat exchangers, draft hoods, and flue pipes for corrosion, soot buildup, melted wires, and moisture, all of which signal incomplete combustion or venting failure. Horizontal vent pipes should angle slightly upward toward the exterior, and any duct joints or patches should be professionally sealed rather than taped by hand.
Beyond the furnace, the same technician can check your water heater, gas dryer, and any other fuel-burning equipment in the home. Chimney flues for fireplaces and wood stoves should be inspected and cleaned annually as well. Keeping exhaust vents and dryer ducts clear, replacing furnace filters on schedule, and ensuring combustion appliances have an adequate supply of fresh air all reduce the risk that normal household equipment becomes a source of poisoning.