Smoke alarm types — ionisation vs photoelectric, and why Victoria's rules changed

There are two fundamentally different technologies in residential smoke alarms, and they don't detect fires the same way. For most of the history of domestic smoke alarms, both types were on the market and the regulations didn't distinguish between them. That changed. Victoria now requires photoelectric smoke alarms in residential properties, and the reasoning is straightforward once you understand how each type works.

How ionisation alarms work

An ionisation smoke alarm contains a small amount of radioactive material — typically Americium-241 — that ionises the air in a small detection chamber. This creates a tiny, continuous electrical current between two charged plates. When smoke enters the chamber, it disrupts that ionisation current, and the alarm triggers.

Ionisation alarms are fast at detecting small particle, fast-burning fires — the kind that produce little smoke initially and a lot of flame. A paper fire in an open room is the classic example. The technology responds quickly to combustion products that include many small particles.

What ionisation alarms are notoriously slow at detecting is slow, smouldering fires — the kind that produce dense, large-particle smoke before breaking into open flame. A mattress fire, a fire starting in a wall cavity, a fire that begins in upholstered furniture — these fires produce exactly the kind of smoke that ionisation alarms are slow to respond to. In some laboratory tests, ionisation alarms have taken ten minutes or more to trigger during a smouldering fire scenario that had already filled a room with smoke.

The other well-known problem with ionisation alarms is false triggering from cooking. They respond to the combustion particles produced by normal cooking — which is why so many of them end up with their batteries removed, making them useless for any fire.

How photoelectric alarms work

A photoelectric smoke alarm uses a light source — typically an LED — aimed at an angle into a detection chamber. Under normal conditions, the light doesn't reach the sensor. When smoke particles enter the chamber, they scatter light in all directions, and some of that scattered light reaches the sensor. The alarm triggers.

Photoelectric alarms are better at detecting large-particle, slow-smouldering smoke — the dense, visible smoke produced by upholstered furniture fires, mattress fires, and fires that begin in wall cavities or behind furniture. These are the fires that kill people in their sleep: slow-developing, filling a dwelling with toxic smoke before the fire becomes visually obvious.

Photoelectric alarms are also generally less prone to false triggering from cooking, because they don't respond to the submicron combustion particles that cooking produces.

For residential occupancies — particularly bedrooms, where people are asleep and may have no warning until a fire is well-developed — the research consistently shows that photoelectric technology provides better protection against the fires that are statistically most likely to cause residential fatalities.

What AS 3786 requires

AS 3786 is the Australian standard that covers smoke alarms. The 2014 edition removed the references to ionisation alarms as an acceptable alarm type for new installations. The standard still describes ionisation technology but specifies that smoke alarms for installation in residential occupancies should be photoelectric (Type A or Type D under the standard's classification).

The standard also covers alarm sensitivity, alarm sound levels, power source requirements, and reliability testing. An alarm sold in Australia for residential installation should carry the appropriate marking showing it complies with AS 3786.

Victoria's legislative requirements

Victoria's smoke alarm requirements are set out in the Building Regulations and the Residential Tenancies Act, with the specific technical requirements flowing from the Building Code of Australia (now the National Construction Code or NCC).

The NCC has required photoelectric smoke alarms in new residential construction since 2014. Victorian Building Regulations require that any smoke alarm installed as a replacement in an existing dwelling must also be photoelectric. Installing an ionisation alarm as a replacement in a Victorian home is not compliant.

Interconnection: The NCC requires that smoke alarms be interconnected — when one triggers, they all trigger. In new construction, this is achieved by hardwiring alarms together. In existing homes where hardwired interconnection is impractical, alarms can be interconnected wirelessly via radio frequency, provided they're designed for that purpose. A single standalone alarm in a bedroom, with no interconnection to the rest of the house, does not meet the interconnection requirement for new installations.

Location: Alarms are required in each bedroom, in corridors serving bedrooms, and on each storey of the dwelling. For a typical two-storey eastern suburbs home this means an alarm inside each bedroom, one in the upstairs corridor or hallway, and at least one on the ground floor.

Hardwired vs battery: In new residential construction, alarms must be hardwired to the mains with battery backup. In existing homes, replacement alarms can be 10-year sealed battery alarms (no mains connection required) provided they're photoelectric and meet AS 3786.

Rental properties — what landlords need to know

Victorian rental property obligations around smoke alarms have been clarified through the Residential Tenancies Act amendments that came into effect in March 2021. The obligations on landlords:

• Smoke alarms must be present on each storey of the rental property and in each bedroom
• Alarms must be in working order at the start of each tenancy and tested annually
• Any alarm that needs replacement must be replaced with a photoelectric alarm compliant with AS 3786
• New rental properties (new builds) must have hardwired photoelectric alarms; existing properties can use 10-year sealed battery alarms as replacements

Consumer Affairs Victoria's guidance makes clear that landlords bear responsibility for smoke alarm compliance. A rental property with ionisation alarms that haven't been replaced is not compliant, regardless of when those alarms were installed.

For landlords managing properties in the eastern suburbs — particularly older stock in areas like Blackburn, Vermont, Croydon, and Mooroolbark where the housing stock includes a lot of 1970s-1990s construction — many properties will still have ionisation alarms from the original installation or from a replacement done before the photoelectric mandate took effect. These need to be updated.

Service life and replacement dates

Photoelectric smoke alarms have a service life. The manufacture date is printed on the alarm body (look on the back or in the battery compartment). Most manufacturers rate their alarms for 10 years from manufacture date, not from installation date. An alarm manufactured in 2014 that was installed in 2016 is already past its recommended replacement date.

Landlords who haven't checked the manufacture dates on their rental property alarms should do so. An alarm that's past its service life should be replaced, regardless of whether it still triggers when you test it — the sensing chamber degrades over time and may not respond reliably to actual smoke even if the alarm sounds when you press the test button.

Dual-sensor alarms

There are alarms on the market that incorporate both ionisation and photoelectric sensors in a single unit, marketed as responding more quickly to both fast-flaming and slow-smouldering fires. These dual-sensor alarms do exist and they do comply with Victorian requirements (the photoelectric sensor satisfies the requirement).

The case for fitting dual-sensor alarms in residential properties isn't as strong as it's sometimes marketed. The evidence for the combination providing meaningfully better protection than photoelectric-only in residential applications is not compelling, and the cost premium is real. A quality photoelectric alarm compliant with AS 3786 is the standard recommendation.

What we do on smoke alarm installations

When we install or replace smoke alarms in an eastern suburbs property, we assess the existing installation: how many alarms, where they are, what type they are, how old they are, and whether they're interconnected. We then specify and install photoelectric alarms in the correct locations, interconnect them (hardwired where the installation suits it, RF-interconnected where hardwiring isn't practical), and test the complete installation before we leave.

If you're a landlord preparing a property for a new tenancy, or a homeowner who isn't sure what type of alarms you have or whether they're due for replacement, we can assess and sort it out in a single visit.

Talk to us about smoke alarm installation and compliance.