Most people's mental model of a house fire comes from film, where flames spread visibly and characters have time to gather things. The reality is faster, darker and considerably more toxic, and understanding the actual sequence explains why every piece of standard fire safety advice exists.

Here is the timeline, and where detection sits in it.

Phase one: incipient, and this is where the time is

A fire begins with something heating up rather than igniting. A phone charging under a duvet, a cigarette in an armchair, an overloaded multibox behind a bookshelf, wiring in a wall cavity. Material heats, begins to pyrolyse, and releases smoke and carbon monoxide without any visible flame.

This phase can last a very long time. Twenty minutes, an hour, sometimes longer. It produces large, cool smoke particles and a rising concentration of carbon monoxide, and it is almost entirely silent.

This is also the entire window in which detection is easy and evacuation is calm. Everything that matters about smoke alarm choice comes down to how much of this phase you capture.

Why sensor type decides that

Photoelectric sensing uses light scattering and is most sensitive to the large particles that smouldering combustion produces. Ionisation sensing detects the very fine particles of fast flaming fire and is markedly slower on smouldering.

Repeated testing internationally has recorded photoelectric units responding to smouldering fires many minutes ahead of ionisation units in identical conditions, with gaps of fifteen to twenty minutes in some scenarios. Those minutes come out of the incipient phase, which is the only part of the timeline where minutes are cheap.

This is why New Zealand's rental property rules specify photoelectric alarms and why Fire and Emergency New Zealand recommends them for homes generally.

Phase two: growth, and the room turns against you

Once flaming combustion starts, the fire begins doubling. A plume rises, hits the ceiling, and spreads horizontally as a hot smoke layer that gets thicker and lower by the second.

Several things happen at once, and none of them are what people expect.

·         The room goes black. Not smoky, black. Visibility drops to nothing, and a familiar hallway becomes unnavigable.

·         Carbon monoxide reaches incapacitating levels well before heat does. Most fire deaths are from smoke inhalation, not burns.

·         Temperature stratifies violently. Floor level might be survivable while head height is already lethal, which is exactly why the advice is to stay low.

·         Modern furnishings burn faster and produce more toxic smoke than the timber and wool of older houses, so the timeline has shortened over the decades rather than lengthened.

Phase three: flashover

At some point the hot smoke layer radiates enough heat downward that everything combustible in the room reaches ignition temperature simultaneously. The entire room goes up at once. This is flashover, and it can occur within three to five minutes of flaming combustion beginning.

Flashover is not survivable in the room, and it is the point at which the fire starts pushing into the rest of the house. Everything after this is the fire service's problem, not yours.

Where the three minutes comes from

The commonly quoted three minutes to escape is measured from flaming combustion, not from the moment the alarm sounds. Which means the whole question is how far back in the timeline your detection sits.

A photoelectric alarm inside the room detects during the incipient phase, potentially twenty minutes ahead of flashover. An ionisation alarm in a distant hallway may not respond until the fire is well into growth, leaving perhaps ninety seconds.

Those are the same house, the same fire, and two entirely different outcomes.

Closed doors and why interconnection matters

A closed door does two opposing things. It holds smoke back, buying survival time in the room behind it, which is why closing bedroom doors at night is genuinely good advice. It also blocks sound, which means an alarm sounding elsewhere in the house may not wake the person behind it.

Interconnected alarms resolve the contradiction. When any unit detects, every unit sounds, so the fire is found at its origin at the earliest possible moment and the alarm goes off in the room where the sleeping person actually is. The door keeps its protective benefit and loses its cost.

What this means for what you buy

Photoelectric sensing, because it moves your detection point deep into the incipient phase. Detection inside sleeping rooms, not just hallways, because closed doors block sound. Interconnection so origin and notification are decoupled. Heat alarms in kitchens, laundries and garages, because a smoke sensor there will nuisance trigger until somebody disables it and a disabled alarm detects nothing at all.

In New Zealand, CAVIUS, Watchman and SmartSense are distributed by On Point Distribution through more than 700 retailers and electrical wholesalers nationwide, so comparing smoke alarms nz on sensing type, sealed battery life and interconnection is straightforward.

And the plan

Because the room goes black and the clock is short, the plan cannot be improvised. Two ways out of every room, agreed in advance. Stay low. Get out and stay out. A specific meeting point.

The fire follows the same sequence every time. The only variable you control is how early you enter the story.