Fire: the Triangle, the Classes, and Choosing the Right Extinguisher

The fire you start soldering rarely announces itself while you are there. It smoulders inside a joist and shows up in the evening.

The short answer

Fire needs heat, fuel and oxygen — remove any one and it goes out. The classes run A solids, B liquids, C gases, D metals, with electrical apparatus as a separate consideration. Prevent fires with good housekeeping, storage of flammables and removal of waste materials.

Why this is our subject more than most trades'

Plumbers carry a naked flame into other people's buildings, routinely, and use it against pipework that runs through timber, felt, insulation and dust that has been collecting since the house was built.

The classic plumbing fire does not flare up while you are there. Heat conducts along the pipe and into the joist or the wall plate behind it, and something smoulders for hours before it finds enough air. The van is long gone when it does.

That is why the controls are about what you do before and after, not during: clear the combustibles, use a heat mat, and check the area again before you leave. A soldering fire is not a lapse of attention — it is a failure to look afterwards.

Hot works permits and the fire watch

On commercial and industrial sites, soldering needs a hot works permit. It typically requires the area cleared, an extinguisher to hand, the work finished by a stated time, and a fire watch — somebody checking the area for a period after the flame goes out, often an hour.

The fire watch is the part that looks like theatre and is the part that works. It exists precisely because of the delay above.

None of this applies in a domestic house, where nobody issues you anything. The habit has to come with you: last thing before you pack up, go back and put a hand near where you were working.

Press fit, and why it is taking over

A large part of the trade has moved to press fit jointing, and the reason is mostly this. No flame means no hot works permit, no fire watch, and no smouldering joist. On occupied buildings, listed buildings and anywhere with a serious fire risk it is often the only method allowed.

The tooling is expensive and the fittings cost more than solder ones, which is why small firms are slower to move. You will meet both, and you should learn both.

Choosing the extinguisher, and the one that matters

All extinguishers are red; the label colour tells you the type. Water is red-labelled and for class A only. Foam is cream, for A and B. Dry powder is blue and covers A, B, C and electrical. CO₂ is black, for B and electrical. Wet chemical is yellow, for cooking oils.

The one to have straight: never water or foam near anything live, because both conduct back to you. CO₂ or dry powder.

To use one, PASS: pull the pin, aim at the base, squeeze, sweep. Aiming at the flames rather than the base is the commonest mistake, and it does nothing at all.

Knowing when to walk away

Deciding not to fight a fire is a correct decision and not a failure. Leave it if it is bigger than about a waste bin, if it is spreading, if the room is filling with smoke, if you are unsure which extinguisher to use, or if you have no clear way out behind you.

Smoke is what kills people, not flame, and it does it far faster than anyone expects. Raise the alarm and get out.

What comes next

At Level 2, hot works and safe isolation become part of how you plan a job rather than a list to recite — and press fit becomes a jointing method you are assessed on.