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Smoke and Carbon Monoxide Alarms: Where They Go and When to Replace Them
An alarm on the wrong wall, or one that is twelve years old, gives a false sense of safety. Here is how to get coverage right.
Dana Whitlock · Sep 11, 2026 · 4 min read · 4 steps
Most homes have smoke alarms. Far fewer have them in the right places, interconnected, and young enough to still work reliably. Sensors degrade over time, and a unit that chirps only when the battery dies tells you nothing about whether it can still detect smoke.
Carbon monoxide is the other half of the picture. It has no smell, and any home with a fuel-burning appliance, fireplace, or attached garage needs detection.
Where smoke alarms belong
Fire codes vary by country and region, but the common baseline is simple: one alarm on every level of the home, one outside each sleeping area, and ideally one inside every bedroom.
- Mount on the ceiling, or high on a wall, because smoke rises
- Keep alarms roughly 3 metres from cooking appliances to cut nuisance alarms
- Avoid spots next to windows, vents, and ceiling fans, where airflow can push smoke away from the sensor
- On a sloped ceiling, mount near the peak but not in the very top few centimetres, where dead air can collect
Where carbon monoxide alarms belong
Place a CO alarm on each level and near sleeping areas so it can wake people. Put one within a few metres of a boiler or furnace room, and in any room with a wood stove or gas fire. Follow the manufacturer's height guidance: CO mixes with air, so the exact height matters less than for smoke.
Testing and replacement
Press the test button monthly. That checks the horn and the battery, not the sensor itself, which is why replacement dates matter.
- Smoke alarms: replace the whole unit about every 10 years
- CO alarms: replace every 5 to 10 years, depending on the model; many now show an end-of-life warning
- Check the manufacture date printed on the back when you move into a home
- Choose interconnected alarms, wired or wireless, so one alarm sounding triggers them all
Combination units
Combined smoke and CO alarms save ceiling space and work well outside bedrooms. In kitchens, use a heat alarm instead of a smoke alarm to avoid constant false alarms from cooking.
This is general home-maintenance information, not professional advice. Building codes, permits, and safe practices vary by location — consult a licensed contractor, electrician, or plumber for work beyond your experience.