Site, test and replace smoke and CO alarms

Why this page is in the power section

Because everything else in this domain raises the risk these alarms exist to catch.

The moment a household starts burning things indoors for heat and light, running a generator, charging batteries, cooking on stored fuel and lighting with flame, the two hazards that alarms detect stop being background risks and become the most likely way someone in the house dies. The alarms matter more in this scenario than in normal life, and they are also the thing most likely to have quietly expired in a cupboard.

Alarms expire

• Smoke alarms: replace at 10 years. This is the NFCC position and it aligns with BS 5839-6.
• CO alarms: 5 to 10 years by model. The sensor is a consumable and it degrades whether or not it has ever seen carbon monoxide. Sealed 10 year units give an end of life warning signal, typically a distinct chirp pattern, which people routinely mistake for a low battery and ignore.
• Write the replace-by date on the front of the casing in marker when you fit it. Then it is visible from the floor instead of requiring a ladder and a torch to establish.

Smoke alarms: where they go

The corners of a room are dead air. Smoke rises and then spreads across the ceiling, and the 300 mm clearance keeps the alarm out of the still pocket where the ceiling meets the wall. An alarm tucked into a corner because that was where the fixing was easy is a slower alarm.

Minimum sensible coverage in a house:

• One on every storey, in the circulation space, which means the hall and the landing.
• In every room where a fire could start while people sleep. In this scenario that list grows: any room with a stove, a heater, a battery bank, candles or a lamp.
• At the top of every staircase, because a staircase is the route smoke takes to the bedrooms.

Interlinked alarms are considerably better than standalone ones, because a fire starting downstairs sounds the alarm upstairs where the people are. Radio-interlinked battery models exist and do not need wiring, and this is one of the genuinely worthwhile upgrades.

Do not put a smoke alarm in a bathroom. Steam sets it off, and an alarm people have learnt to ignore is worse than no alarm.

The kitchen problem

The kitchen gets a heat alarm, not a smoke alarm, and the reason is behavioural rather than technical.

A smoke alarm near a kitchen goes off when someone makes toast. People then remove the battery, or take the alarm down, and intend to put it back. Nuisance alarms are the main reason working alarms end up disabled, so the standard deals with the problem by not putting a smoke alarm there in the first place.

A heat alarm responds to a rapid rise in temperature instead, so it ignores cooking smoke and still catches a chip pan.

This matters more, not less, in the scenario this app is written for, because cooking is likely to move to flame, to a stove, or to improvised arrangements that smoke considerably more than an electric hob.

CO alarms: where they go

BS EN 50292 places them 1 metre minimum to 3 metres maximum from the potential source. Closer than a metre gives false alarms from normal appliance behaviour; further than three and you lose the early warning.

• In every room containing a fuel-burning appliance. Stove, boiler, gas fire, paraffin heater, anything with a flue.
• In every room where anyone sleeps, which is a separate requirement and not covered by the one near the stove.
• At breathing height or on the ceiling, and here the practical guidance is to follow the manufacturer's instruction for that specific unit. CO mixes fairly evenly with air rather than rising like smoke or sinking like a heavy gas, which is why placement is more forgiving than for smoke, and why the "put it low because CO sinks" advice you may have heard is wrong.
• Not in a cupboard, not behind furniture, not in dead air, and not directly above a heat source.
• Look for BS EN 50291 on the box. Unmarked units bought cheaply online are not a controlled product.

A CO alarm is not a substitute for ventilation. Ventilating indoor burning covers why: the alarm is a backstop calibrated to prevent death, not to indicate clean air, and its response times are far slower than people assume at the concentrations that make you unwell.

What the test button actually tests

Only the electronics, the sounder and the battery. Not the sensor.

This is the crucial limitation and it follows directly from the expiry problem. A pressed test button confirms the alarm can make a noise if it decides to. It confirms nothing about whether it can still detect smoke or carbon monoxide.

To test the sensor:

• Smoke alarms: aerosol smoke alarm test spray, available cheaply, sprayed briefly from the distance on the can. This is the only home test that exercises the actual detection path.
• CO alarms: CO test gas exists but is specialist. In practice most households rely on the test button plus replacing on date, and that is a reasonable position provided the replacement date is genuinely observed.

Never test a CO alarm with a car exhaust, a cigarette, or a candle flame. It is uncontrolled, it can contaminate the sensor, and it tells you nothing useful about the response threshold.

The testing routine

Realistic rather than aspirational, because a routine nobody follows protects nobody.

• Weekly: press the test button on one alarm, rotating through them. A month covers the house.
• Monthly: test all of them, and vacuum the outside of each with a brush attachment. Dust and cobwebs genuinely reduce sensitivity, and insects inside an alarm are a common cause of both failure and false alarms.
• Annually: replace batteries in anything that is not a sealed unit, check the manufacture date on each alarm, and test the smoke alarms with spray.
• Every 10 years: replace, on date, whether or not it appears to work.

Attach it to something you already do. The clocks changing is the traditional hook for battery replacement and it works because it happens without being remembered.

In this scenario, add one: test every alarm the first time you light a stove, run a generator, or bring any new burning thing into the house for the season. The alarm matters most on the first cold night, which is exactly when nobody has thought about it since spring.

Buying, and what to stock

• Sealed 10 year lithium units where you can. No batteries to change, no batteries to borrow, and the whole unit is replaced at expiry, which forces the decision you would otherwise skip.
• Interlinked for anything covering sleeping areas.
• A digital display on at least one CO alarm. The number is genuinely useful: it tells you whether a reading is a transient from lighting a stove or a rising trend, and it shows the peak recorded, which is the single most useful piece of information after an incident.
• Spares in the store. Alarms are cheap, they have a long shelf life in a sealed packet, and they are exactly the sort of thing that will not be purchasable when it matters. Buy two spare CO alarms and two smoke alarms and note their manufacture dates, because they age on the shelf as well as on the wall.
• Batteries for anything not sealed, stored somewhere findable in the dark.

When one goes off

Smoke alarm: get everyone out, then investigate. In that order, not the reverse. A closed door buys minutes.

CO alarm:

1. Open doors and windows.
2. Get everyone outside, including pets, and including anyone who says they feel fine.
3. Turn off the appliance if you can do it safely on the way out. Do not go looking for it.
4. Do not go back in until the alarm has stopped and you know why it sounded.
5. Get medical help for anyone with symptoms, and see carbon monoxide poisoning. Headache, nausea and confusion in more than one person at once, or in the pets, is carbon monoxide until proved otherwise.

Do not assume a CO alarm is faulty because you feel well. The alarm responds at concentrations below the ones that make a healthy adult obviously ill, which is the entire point of it, and the person least able to judge their own condition is the one already being poisoned.