How do I test smoke and carbon monoxide alarms?

How often, how to test properly, where they should be, and what the different beeps mean.

Difficulty: beginner
Time: 15 min
Read: 2 min
Safety: warning

Short answer

Press the test button on every alarm once a month and replace batteries yearly. Whole alarms need replacing every ten years — they have an expiry date printed on them and most people never look.

Alarms are the single highest-value maintenance task in a house, and almost everyone does it less than they intend. A monthly button press takes two minutes for the whole house.

Safety

If a carbon monoxide alarm sounds, get everyone out into fresh air immediately, do not go back in, and call the emergency gas number from outside. Do not try to find the source. Carbon monoxide is odourless and colourless, and symptoms — headache, dizziness, nausea — are easily mistaken for flu.

Step by step

Press the test button on each alarm monthly.Hold it until it sounds. This checks the battery, the sounder and the circuit. It does not check the sensor itself, which is why expiry dates matter.
Check the expiry date on every alarm.Printed on the back or side. Smoke alarms are typically good for ten years and carbon monoxide alarms for seven to ten. The sensor degrades whether or not it has ever been triggered.
Replace batteries at least yearly.Unless they are sealed ten-year units. A fixed date — a clock change, or a birthday — is easier to remember than an interval.
Check you have enough, in the right places.At minimum one smoke alarm per floor, in hallways and landings. A heat alarm rather than a smoke alarm in the kitchen. Carbon monoxide alarms in any room with a fuel-burning appliance.
Learn what the different sounds mean.A single chirp every minute usually means low battery or end of life. Continuous sounding means an actual alarm. They are easy to confuse at three in the morning.
Vacuum them occasionally.Dust and insects cause false alarms and can prevent detection. A quick pass with the vacuum brush every few months.
Do not remove the battery to stop a false alarm.The commonest and most dangerous habit. Fan the alarm, or move it further from the kitchen. A disabled alarm protects nobody.
Make an escape plan and tell everyone.Two ways out, a meeting point, and where the keys are kept. The alarm buys time; the plan is what uses it.

Tips

Sealed ten-year battery alarms remove the annual battery job and the temptation to borrow the battery for something else.
Interlinked alarms sound together, so an alarm downstairs wakes people upstairs. Increasingly required and worth fitting regardless.
If you rent, alarms are usually the landlord's responsibility to provide and test at the start of a tenancy. Report a faulty one in writing.

Common mistakes

Removing the battery after a cooking false alarm — It is almost never put back. A disabled alarm is the most common factor in fatal house fires.
Never checking the expiry date — The sensors degrade over ten years regardless of testing. An alarm that beeps when tested can still fail to detect smoke.

Questions people ask

How often should I test smoke alarms?

Press the test button monthly, change batteries yearly unless sealed, and replace the whole alarm every ten years or by its printed expiry date.

Where do carbon monoxide alarms go?

In any room with a fuel-burning appliance — boiler, gas fire, stove, open fire — and in bedrooms if an appliance is nearby. Follow the manufacturer's height guidance.