From below, a sprinkler system looks like nothing more than pipework under the ceiling. The principle is simple. The pipes are kept under pressure, a heat-sensitive element breaks at a set temperature, and water is released only at that point. No electricity, software or human action is needed. That simplicity is the main reason sprinklers have been trusted for well over a century.
Below we walk through the system from the head to the valve station, and then deal with the misconceptions we hear most often on site.
The sprinkler head
In a standard head the water outlet is closed by a cap. The cap is held in place by a small glass bulb filled with liquid. As the surrounding air heats up, the liquid expands and the bulb shatters at a defined temperature. The cap falls away and water strikes the deflector, which spreads it over the design area.
The bulb colour indicates the operating temperature:
| Bulb colour | Nominal operating temperature | Typical use |
|---|---|---|
| Orange | 57 °C | Cool, controlled interiors |
| Red | 68 °C | Most general areas: offices, hotels, warehouses |
| Yellow | 79 °C | Roof voids, moderately warm areas |
| Green | 93 °C | Near heaters, warm process areas |
| Blue | 141 °C | Around ovens and boilers |
| Mauve | 182 °C | High-temperature process areas |
| Black | 204-260 °C | Special applications |
As a rule, the operating temperature is chosen roughly 30 °C above the highest normal ambient temperature. Fitting red bulbs in a roof void that reaches 50 °C in summer invites unwanted operation.
Bulb diameter matters too. Quick response heads use a thinner bulb that picks up heat faster, and they are the usual choice in light hazard occupancies such as offices and residential buildings. Some heads use a fusible solder link instead of a bulb. The principle is the same.
The alarm valve station
Water reaches the sprinkler pipework through a main control valve and an alarm valve station. On a wet system the core of the station is a clapper (a check valve) with an alarm line beside it.
Normally the clapper is closed and pressures on both sides are balanced. When a head opens, pressure above the clapper drops, the clapper lifts and part of the flow is diverted into the alarm line. That line carries:
- A retard chamber that prevents false alarms from mains pressure surges
- A water motor gong that rings without any electrical supply
- A pressure switch that signals the fire alarm panel and the pump controller
On larger installations, subsidiary zone valves and flow switches are fitted on each floor or zone so the panel can show where water is flowing.
System types
Wet pipe
The pipes are permanently filled with water, so discharge starts the moment a head opens. It is the simplest, cheapest and most reliable arrangement and the first choice wherever there is no risk of freezing.
Dry pipe
Used in unheated warehouses, open car parks and cold stores. The pipes are filled with compressed air or nitrogen. When a head opens the air escapes, the dry pipe valve trips and water fills the network. This creates a delay, which is why dry systems are limited in volume, larger ones need an accelerator, and the design area is increased compared with a wet system. Pipework must also be pitched with drain points, otherwise trapped water freezes.
Pre-action
The pipes are normally dry, and water only enters after the fire detection system has given a signal. In a single interlock system the detection signal opens the valve and fills the pipes, and water discharges only when a head operates. A double interlock system requires both the detection signal and the pressure drop from an operated head.
Pre-action is typical for archives, server rooms and museum stores, where accidental discharge would do serious damage.
Deluge
The heads are open, with no bulbs. When the deluge valve is opened by the detection system or manually, every head in the protected zone discharges at once. Deluge is used where fire can spread very fast: transformers, fuel loading areas, aircraft hangars. It is often combined with foam.
Common myths
“When one head goes off, they all go off”
That is what films show. In wet, dry and pre-action systems each head responds only to the heat at its own location. Most fires are controlled by between one and four heads. Deluge is the only type where all heads discharge together, and that is a deliberate design choice.
“Cigarette smoke will set off the sprinklers”
Heads respond to heat, not smoke. The bulb has to reach its operating temperature. A smoke detector going into alarm and a sprinkler head opening are independent events.
“Sprinkler water does more damage than the fire”
A single open head typically discharges somewhere between 60 and 200 litres per minute. Fire brigade hose streams use far more water, and they usually arrive once the fire has grown. Early, local suppression keeps total water damage down.
“Once installed, it needs no maintenance”
Sprinklers look passive, but valves must stay open, pumps must start, and heads must not be painted over or covered. A head coated in paint or dust responds late. We cover maintenance in a separate article.
Final note
The right system type depends on how the space is used, the freezing risk and the consequences of accidental discharge. If the use of an existing building has changed, for example offices turned into storage or a warehouse into a cold store, the system needs to be reassessed. ESFA can survey the site and report whether the existing installation still suits the new use.