Smoke detectors are among the most common devices in New Zealand homes and among the least understood. The result is a set of beliefs that sound sensible, get repeated confidently, and quietly undermine the thing they are meant to support.

Here are ten of them.

1. If it beeps when I press the button, it works

The test button checks the sounder, the battery and the circuitry. It does not check the sensing chamber, which is the part that detects smoke.

Chambers accumulate dust, cooking residue and insects over years, and sensitivity drifts. A unit can beep perfectly on the test button and be substantially degraded as a detector. This is the single most consequential misunderstanding on the list.

2. Smoke detectors last forever

They have a ten year service life from the date of manufacture printed on the back of the unit, not from the date you installed it. After that they should be replaced regardless of how healthy they seem.

Twist each unit off its mount and read the date. In a house that has changed hands, it is very common to find units from two owners ago still on the ceiling.

3. All smoke detectors are basically the same

There are two sensing technologies and they behave very differently. Ionisation units detect the fine particles of fast flaming fire. Photoelectric units detect by light scattering and respond much earlier to the large particles of smouldering combustion.

Smouldering fires are what kill people at night. International testing has repeatedly found photoelectric units responding many minutes ahead of ionisation units on those fires. New Zealand rental rules specify photoelectric for this reason.

4. One in the hallway is enough

A single hallway unit is the bare minimum and it assumes bedroom doors are open. In a New Zealand winter they are usually shut, and a closed door blocks sound substantially while the person who needs to wake is behind it.

Coverage should extend to every hallway serving bedrooms, inside sleeping rooms where possible, every level of a multi level house, and any sleepout or converted garage. Sleepouts are the most commonly missed position in the country.

5. The alarm will wake the children

Sleep research has repeatedly found that children under about ten frequently sleep through a standard alarm sounding in the same room. They spend much more of the night in deep slow wave sleep than adults do.

This is why a family escape plan needs a named adult assigned to each child's room, agreed in advance rather than improvised.

6. My dog will wake me

Sometimes. Often not, particularly with a smouldering fire producing carbon monoxide, which affects animals the same way it affects people. Cats typically hide rather than alert. Pets are not a detection layer.

7. My smoke detector will warn me about carbon monoxide

It will not. Smoke alarms detect airborne particles. Carbon monoxide is a colourless, odourless gas, and there is nothing for a particle sensor to see.

Carbon monoxide detection requires a separate device with an electrochemical sensor, or a combination unit. If your home has gas appliances, an unflued gas heater, a wood burner or an open fire, you need both types.

8. Mains powered units do not need batteries

Mains connected alarms require battery backup, and that backup is the point. House fires and power interruptions are correlated, and a mains unit without working backup stops functioning at exactly the moment it is needed.

9. Interconnection means rewiring the house

This belief costs New Zealand homeowners real money on renovation projects. Interconnection can be achieved over radio frequency between battery powered units, with no cable at all.

Wireless linking satisfies the Building Code requirement that applies to new builds and consented alterations since November 2024, and it makes retrofitting an existing villa or bungalow an afternoon's work rather than a wiring project.

10. A unit going off from cooking means it is too sensitive

It means it is in the wrong place, or it is the wrong type of sensor for that room. A smoke detector within three metres of a hob will trigger on toast and steam, and the response is almost never to move it. It is to disable it, which is how fatal fires happen.

Kitchens, laundries and garages want heat alarms, which respond to rapid temperature rise rather than airborne particles and therefore ignore steam, cooking smoke and vehicle exhaust while still reacting quickly to a real fire.

A bonus one: it is fine to take the battery out until I get a new one

Fire services worldwide report attending fatal house fires where an alarm sat on the ceiling and its battery sat in a kitchen drawer. The interval between removing a battery and replacing it is almost always far longer than intended.

If you remove one at 3am to stop a chirp, leave a note on the kitchen bench so it gets dealt with before the weekend disappears. Better still, fit sealed ten year lithium units so there is no battery to remove and no annual change to forget.

What to do with all of this

Read the manufacture dates on every unit in your house this weekend. Replace anything ten years old or undated. Check whether what you have is photoelectric, and if the unit does not say, assume it is not.

When you replace, the three features that matter are photoelectric sensing, a sealed ten year battery, and interconnection, with heat alarms in the kitchen, laundry and garage. On Point Distribution supplies CAVIUS, Watchman and SmartSense through more than 700 retailers and electrical wholesalers across New Zealand, so comparing smoke detectors nz on those features rather than on price is a short job.

Ten minutes of checking undoes most of the beliefs above.