Fire services generally reckon on around three minutes to get everyone out of a burning house. That number comes from how quickly a modern room fire develops once flaming combustion begins, and it is unforgivingly short. Three minutes to wake, orient, gather children, and get to a door.

The uncomfortable part is that the three minutes starts when the fire flashes over, not when the alarm sounds. So the entire question becomes how much warning you get before that point, and in a lot of New Zealand houses the answer depends on something most people have never considered: whether the alarms are connected to each other.

The problem with a standalone alarm

A standalone alarm does exactly one thing. It detects smoke in its immediate vicinity and it sounds locally. That is fine if the fire starts in the same room as the alarm and everyone is awake in that room.

Now run the more realistic scenario. It is winter, it is two in the morning, the fire starts in a downstairs laundry from a dryer choked with lint, and the family is asleep upstairs with bedroom doors shut. The laundry alarm detects the smoke and starts screaming. Downstairs. Behind a closed laundry door. Two floors and three closed doors from anyone who needs to hear it.

Closed doors are the issue, and they are doing two contradictory things at once. They hold smoke back, which buys survival time, and they block sound, which costs warning time. An 85 decibel alarm measured at three metres in open air loses a great deal of its effectiveness through a closed door, along a hallway, up a stairwell, and through a second closed door to a sleeping adult.

By the time enough smoke has migrated upstairs to trigger a second standalone unit, the downstairs fire may have been developing for many minutes. That is the warning time you have lost.

What interconnection changes

Interconnected alarms are linked, either by wiring or by radio frequency. When any one unit detects smoke, every unit in the system sounds at the same moment.

The practical effect is that detection point and notification point are decoupled. The fire is found at its origin, in the laundry, at the earliest possible moment, and the alarm goes off in the bedroom where the person who needs to act is sleeping. Those minutes of migration delay simply disappear.

That is not a marginal improvement. In a two storey house it is frequently the difference between waking to a smoke alarm and waking to a smoke filled landing.

Where the gain is largest

  • Two storey and split level homes, where vertical distance and a stairwell separate the likely origin from the sleeping areas.

  • Long single level homes, particularly older houses with bedrooms at one end and a laundry, garage or kitchen at the other.

  • Properties with sleepouts, cabins or converted garages, where an occupant may be entirely out of earshot of the main house.

  • Homes with anyone who is a heavy sleeper, elderly, hearing impaired or medicated, where a distant alarm may not rouse them at all.

  • Households with young children, who frequently sleep through alarms that would wake an adult and need someone else to reach them.

Wireless removes the excuse

For years the objection to interconnection in existing homes was the wiring. Linking alarms across a finished house meant running cable through ceilings and wall cavities, which in an older bungalow with sarking and a low pitch roof is genuinely difficult and expensive.

Radio frequency interconnection resolves that. Battery powered units communicate wirelessly, so a whole house system can be retrofitted in an afternoon with a drill and a ladder, no cable and no electrician required for the battery range. Systems typically pair with a short button sequence and then operate as a single unit.

It is now required for new work

Interconnection is no longer just a good idea. The updated Building Code Acceptable Solutions C/AS1 and C/AS2, effective November 2024, require interconnected systems referencing NZS 4514:2021 for new residential builds and consented alterations. Existing owner occupied homes are not compelled to upgrade, but the reasoning behind the requirement applies equally to them.

Specifying a system

Interconnection only works within a compatible range, so the whole system should come from one manufacturer. Beyond that, look for photoelectric sensing on the smoke units, sealed ten year lithium batteries, heat alarms rather than smoke units for the kitchen, laundry and garage, and a hush function so nuisance activations can be silenced without disabling anything.

In New Zealand, CAVIUS, Watchman and SmartSense product is distributed by On Point Distribution through more than 700 retailers and electrical wholesalers nationwide. Their range covers battery and mains powered options plus wireless accessories such as remote controls and wall controllers, so putting together a matched interconnected smoke alarm system for a whole house is reasonably simple.

Three minutes

You cannot make a fire develop more slowly. The only variable you control is how early you find out, and in a house with more than one room that is largely a question of whether your alarms talk to each other.

Buy back the minutes you are currently losing to a closed door.