How a fire sprinkler system actually works

A sprinkler system does one thing: it puts water onto a fire, in the place the fire is, before the fire gets big enough to need the fire service. Everything else about it is arrangements for making that happen reliably after twenty years of nothing happening at all.

The head

The part people picture, and the part most misunderstood.

A sprinkler head is a plug held in place by a heat-sensitive element: either a small glass bulb filled with a liquid that expands, or a fusible link of solder that melts. The bulb colour tells you its temperature rating. Red is typically 68°C, yellow 79°C, green 93°C, and so on up.

When the air at the head reaches that temperature, the bulb shatters or the link melts, the plug drops out, and water sprays through a deflector that spreads it into a pattern over the floor.

Each head is independent. There is no wiring, no detector, no panel deciding which heads to open. A head opens because it got hot. This is why the film version, where a single flame sets off every sprinkler in the building and soaks everything, does not happen: the heads across the room never reach their operating temperature.

In recorded fires in sprinklered buildings, the overwhelming majority are controlled by four heads or fewer, and very often by one.

The pipework and the control valve set

Above the ceiling, a network of pipes runs from a control valve set, which is the assembly of gate valve, alarm valve, gauges and trim you will find in the plant room or the riser cupboard. It is the heart of the installation and it is what an annual service strips down.

The pipework is normally held at pressure. When a head opens, water moves, pressure drops, and two things happen at once.

The alarm

Water moving through the control valve set either drives a water motor gong on the outside wall, or trips a flow switch wired into the building’s fire alarm.

This is worth dwelling on, because it is the part that fails silently. A system with a seized flow switch will still put the fire out. It just will not tell anybody it has, and the first anyone knows is water coming through a ceiling. That is the whole reason BS EN 12845 requires quarterly flow switch testing, and it is one of the intervals most often skipped.

The water supply

The system is only as good as what is behind it, and this is where most of the money and most of the maintenance sits.

Town main. Simplest. The system takes water directly off the incoming main. Adequate for lighter hazards where the main can supply the required flow and pressure, and it needs proving rather than assuming.

Tank and pump. A stored water tank sized for the hazard class and the required duration, with one or more pumps to develop the pressure. This is what most commercial and industrial buildings have. The tank carries a routine of its own on top of the system’s, which is set out in sprinkler tank inspection and cleaning.

The pump set is usually an electric pump, sometimes a diesel one where the power supply is not considered reliable, and almost always a small jockey pump alongside it. The jockey’s job is to hold the system at pressure against minor leakage so the main pump does not start every time a gauge twitches. What each of those does, and why a pump that starts is not necessarily a pump that works, is covered in our guide to fire pumps.

What “it works” actually means

Here is the thing that decides whether a sprinkler system is an asset or a liability on the day.

A system is not proved by the fact it holds pressure and the gauges look right. It is proved by water arriving at the furthest and highest head, at the design density, for the design duration. That requires the water supply to deliver, the pump to reach its duty point under flow, the valves to be open, and the pipework to be clear.

Every one of those can be quietly wrong in a system that looks perfect. A valve strapped shut after a leak in 2019 and never reopened. A pump that starts beautifully and cannot make its duty because the impeller is worn. A tank a third full because the ball valve has failed.

That is what the servicing regime under BS EN 12845 is for, and it is why a pump has to be proved under flow rather than started and signed off.

Finding a fault is only half of it. What a sprinkler repair involves takes the other half: the leaks, the seized valves and the heads that have to come out.

What happens after a head operates

Rarely written down anywhere, and the part building managers are least prepared for.

The system keeps running until somebody stops it. A sprinkler head does not reset. Once the bulb has gone, that head is open and water flows until the control valve is shut, which is a decision for the fire and rescue service while they are still on site. Shutting a sprinkler system down while a fire is still burning has killed people, so the officer in charge decides, not the building.

Then it is put back. The operated head is replaced from the spares for that installation, at the correct temperature rating and orientation. The system is refilled, vented, brought back to pressure and the valve set proved, and the alarm is tested to confirm the whole chain works again. Until that is done and recorded, the building is running without the protection its fire strategy assumes.

The water is the smaller problem. A single head discharges far less than most people expect, and vastly less than a fire brigade hose. The damage that costs money is usually where the water went rather than how much of it there was: through a ceiling void into the floor below, into a riser cupboard, or into a lift shaft.

The record matters afterwards. An operated head is an event: what happened, which head, when it was replaced, and when the system came back into service. That record is the first thing an insurer asks for, and the reason it is worth having a contractor attend rather than a maintenance man with a spare head.

The five-minute version

• Heads are individual heat-triggered plugs. Only the ones over the fire open.
• Water moving triggers the alarm, so the flow switch is as important as the head.
• The water supply and the pump are where the real capability lives.
• Accidental discharge is nearly always impact damage, not the head misbehaving.
• A system that looks fine on the gauges can still be unable to deliver, which is what the annual service exists to find.

If you are not confident yours has been proved rather than glanced at, that is what sprinkler servicing is for.