Heating season is when CO risk peaks — and when most households discover their detector is expired, misplaced, or missing entirely. Here is the winter check that takes ten minutes.
Carbon monoxide earns its "silent killer" reputation honestly. It is colorless, odorless, and produced by exactly the equipment your home relies on most in winter: furnaces, water heaters, gas ranges, fireplaces, and — during outages — portable generators. You cannot smell it, see it, or taste it. The only reliable warning is a working CO alarm, and winter is precisely when that alarm matters most and gets checked least.
This article covers where CO comes from, where detectors belong, how to test them properly, and how to tell whether the unit on your wall is still protecting you at all.
Where carbon monoxide comes from
CO forms whenever fuel burns incompletely. In a typical home, the sources are:
- Furnaces, boilers, and gas water heaters — especially with cracked heat exchangers or blocked vents
- Gas ranges and ovens used for heating (never do this)
- Fireplaces and wood stoves with chimney or flue problems
- Portable generators run in a garage, basement, or too close to windows during an outage
- Charcoal grills used in enclosed spaces
- A vehicle warming up in an attached garage
A venting problem can turn a normally safe appliance into a CO source without any visible sign, which is why detection — not inspection alone — is the safety net.
One misconception worth clearing up first, because it shapes where people place detectors: CO does not pool at the floor or rise to the ceiling like a layer of oil or steam. It mixes evenly with room air. If you want the longer explanation, this guide on whether carbon monoxide rises or falls goes through the physics and what it means for placement height.
Where CO detectors belong
Following NFPA and CDC guidance, the baseline for any home with fuel-burning appliances or an attached garage is:
- At least one detector on every level
- One outside each sleeping area, close enough that the alarm will wake you — within about 15 feet of bedrooms is the common guidance
- Near fuel-burning appliances, but not on top of them: keep detectors outside the 5-foot zone around a furnace, water heater, stove, or fireplace; 15 feet is better for avoiding nuisance alarms
Avoid placing a detector directly in front of vents, drafts, ceiling fans, or in direct sunlight, all of which can dilute readings. Kitchens and bathrooms are covered best from just outside the doorway rather than inside, where steam and humidity shorten sensor life.
Height, given that CO mixes with air, matters less than location and audibility: plug-in units go directly into a wall outlet (not a power strip), battery units can sit on a tabletop or mount on a wall — what matters is that the alarm can be heard and the display or indicator can be checked during your weekly test.
Plug-in or portable battery: which fits which room
The two common formats solve different problems:
- Plug-in with battery backup suits permanent locations with a free outlet. The battery backup matters more than it sounds: power outages are exactly when generators and backup heating come out, so a CO alarm that dies with the power fails at the moment of highest risk.
- Portable battery-powered units — especially those with an LCD showing live CO readings in ppm — suit rooms without a convenient outlet, rentals where you cannot modify wiring, and anyone who wants to watch levels trend after ventilating a space.
Both use electrochemical sensors; the real decision is power source, display, and mounting. Current examples of both formats are compared on this carbon monoxide detectors page, including which models show live readings.
Testing: the weekly ten seconds most people skip
A CO alarm's test button is not decorative. Pressing it verifies the sensor circuit, the sounder, and the battery in one action.
- Test weekly. Press TEST/SILENCE until the alarm sounds, following your model's pattern.
- Clean monthly. Vacuum the air openings gently with a soft brush; dust and cobwebs in the sensing chamber cause both false alarms and missed ones.
- Know the difference between a test and a reading. A weekly test confirms the unit works; it does not measure the air. For a fuller walkthrough of test methods — including what the alarm patterns mean — see this step-by-step on how to test a carbon monoxide detector.
If the unit fails its test, does not respond as the manual describes, or chirps a pattern you cannot identify — replace it. Do not silence it and move on.
The expiration date nobody reads
Electrochemical CO sensors degrade with age. Every detector has a finite service life — commonly 10 years for current models — after which it must be replaced even if it still appears to work. The date is printed on the unit's label. When a detector reaches end of life, it typically chirps a distinct pattern; the correct response is to replace the whole detector, not just the battery, because the sensor is the worn-out part.
A practical winter ritual: when you set your clocks back and swap HVAC filters in the fall, check the manufacture date on every CO and smoke alarm in the house. Any unit nearing its decade goes on the replacement list while heating season is still young.
Battery notes for the formats above: plug-in models carry a long-life lithium backup (rated up to 10 years on current models), while portable units on AA batteries typically need fresh batteries about every three years — check your manual for the specific interval.
If the alarm sounds
Treat every CO alarm as real until professionals say otherwise:
- Move everyone — including pets — outside to fresh air.
- Call 911 or the local fire department from outside.
- Do not re-enter until emergency services confirm it is safe.
- Do not open windows and "air it out" as a substitute for steps 1–3, and do not dismiss the alarm as faulty because nobody feels sick yet. Symptoms lag exposure, and the alarm exists precisely because your senses cannot judge this gas.
The bottom line
Winter concentrates every CO risk factor at once: sealed houses, hard-working furnaces, fireplaces, outages, and generators. Ten minutes with a test button and a date label will tell you whether your home is actually covered. Detector on every level, one outside the bedrooms, tested weekly, replaced at end of life — that is the whole system, and it only works if all four parts happen.
Disclosure: The author works with SITERWELL, a manufacturer of smoke and carbon monoxide alarms. The safety guidance above follows NFPA and CDC recommendations and applies to any brand of CO alarm.
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