Where should carbon monoxide detectors be placed?

On every level of the home, outside each sleeping area, and per the alarm's instructions for height and distance from appliances. Bedrooms with fuel-burning appliances or attached garages raise the standard, not lower it.

What do I do if the CO alarm goes off?

Take it as real: get everyone, pets included, to fresh air, call emergency services or the fire department from outside, and let them clear the house and find the source. Anyone with symptoms, headache, dizziness, nausea, confusion, needs medical evaluation.

How long do CO detectors last?

Five to ten years depending on model; the sensor expires even if the test button still chirps happily. The replace-by date is printed on the unit, and an expired alarm is a decoration.

Can a furnace cause carbon monoxide without failing completely?

Yes. Backdrafting flues, blocked vents, cracked heat exchangers, and burners running dirty all produce exposures from equipment that still heats the house. Working alarms plus periodic combustion service cover the gap.

Carbon Monoxide and Combustion Appliances: Safety Basics

Page last reviewed: August 2026 · Reviewed by Harold R. Bennett, retired home inspector

Carbon monoxide is combustion’s signature hazard: colorless, odorless, produced whenever fuel burns incompletely, and responsible for hundreds of accidental deaths and tens of thousands of ER visits in the United States every year, concentrated in heating season. Carbon monoxide furnace safety, and the wider combustion-appliance version of it, rests on two independent layers: alarms that detect, and maintenance plus venting that prevent. This reference covers both.

Where CO comes from in a house

Every fuel-burning device is a candidate when combustion or venting goes wrong: furnaces and boilers, gas water heaters, ranges and ovens, fireplaces and wood stoves, and the seasonal outsiders, generators, grills, and vehicles, whose exhaust indoors or in attached garages produces the fastest and most lethal exposures on record. The residential failure modes worth knowing by name: backdrafting, where exhaust spills indoors instead of rising, when natural-draft flues lose to house depressurization from exhaust fans and tight construction; blocked or failed venting, nests, collapse, disconnections, and snow at terminations, the whole subject of the chimney and venting reference; cracked heat exchangers, the furnace-specific breach covered in depth here; and dirty combustion, misadjusted burners, lazy yellow flames on equipment meant to burn blue, and appliances starved of combustion air by the storage boxes stacked around them. Unvented use rounds out the list: ovens as space heaters and unvented heaters run beyond their ratings are exposures by instruction manual.

The alarm layer

Alarms are the non-negotiable minimum because CO gives no sensory warning and its early symptoms, headache, fatigue, nausea, impersonate the flu, famously across whole households at once, easing when everyone leaves the house. Placement: every level including basements, outside each sleeping area, and in bedrooms where the jurisdiction or the appliance layout suggests it, mounted per the unit’s instructions, with combination smoke/CO units acceptable where listed. Lifespan is the neglected detail: sensors expire at five to ten years regardless of the test button’s optimism, the replace-by date is on the housing, and battery discipline follows the same clock-change ritual as smoke alarms, per the detector placement reference. Response is unambiguous: an alarm means out, everyone, to fresh air, with the call to emergency services made from outside and symptomatic people seen medically; the source hunt belongs to responders with meters, not to a resident with a headache walking room to room. Low-level chronic exposures, alarms that chirp intermittently, unexplained recurring symptoms, earn a professional low-level CO investigation, since standard alarms deliberately ignore concentrations that still matter over months.

The prevention layer

Alarms detect failures; maintenance prevents them. The rhythm: professional combustion service on the multi-year schedule for every fuel appliance, furnace, boiler, and water heater alike, including combustion analysis, venting inspection, and heat-exchanger checks; homeowner eyes between visits on flame color (blue is health, persistent yellow on gas is a service call), vent pipes (connected, sloped, uncorroded), and terminations after storms and nesting season; combustion air kept unblocked around appliance closets and mechanical rooms; and the depressurization awareness that modern tight houses demand, since a new high-power range hood or a sealed-up envelope can flip a marginal natural-draft flue into a backdrafting one, the interaction that upgrades duct sealing and appliance venting from efficiency topics to safety ones. The absolutes stand apart: never a generator, grill, or engine indoors or in a garage attached or not, never an oven for heat, and never a vehicle idling in an attached garage even with the door open.

The bottom line

CO is the failure product of every flame in the house, arriving through backdrafts, blockages, breaches, and bad burns. Alarms on every level and outside every bedroom, replaced on their printed date, are the detection floor; combustion service, flame and vent vigilance, and respect for depressurization are the prevention ceiling; and the generator stays outside, always, full stop.