Technical·7 min read
How Do Sprinkler Systems Work? Types, Design and How Many m² per Head?
Unlike in the movies, a fire does not set off every sprinkler at once — only the head reached by the heat operates. We explain the working principle of sprinklers, the wet/dry/pre-action/deluge system types, how many m² one head covers by hazard class, and the NFPA 13 / EN 12845 design criteria — from an engineering perspective.
A-Pro Engineering
The sprinkler is the most widespread and most proven automatic suppression method in the world; designed correctly, it brings a fire under control while it is still small, with no need for human intervention. Yet how it works is often misunderstood — starting with the belief that “a fire sets off every head at once.” In this article we look at the sprinkler system from an engineering perspective: how it really works, what types exist, how many m² one head covers by hazard class, and what criteria the design rests on.
How does a sprinkler really work?
The most common misconception comes from films: people imagine that when one head opens, every sprinkler on the ceiling gushes water at once. In standard systems the opposite is true. Each sprinkler head behaves like an independent heat detector and opens only when the temperature above it reaches the trigger value.
The mechanism is simple and reliable: the head’s orifice is sealed by a small liquid-filled glass bulb (or a soldered metal link). When a fire starts and hot smoke rises to the ceiling, the liquid in the bulb expands; at the set temperature (typically 68°C) the bulb shatters, the plug drops out and that head distributes water over the area in a defined pattern. As a result:
- Usually only one or two heads directly over the fire operate.
- Water does not discharge where there is no fire; water damage stays limited.
- The system starts responding before anyone notices and before the panel is alerted.
The head’s opening temperature is chosen well above the space’s normal ceiling temperature; this value is colour-coded by bulb: orange 57°C, red 68°C, yellow 79°C, green 93°C, blue 141°C. In addition, “quick response” heads use a thinner bulb to open earlier and are preferred in life-safety spaces such as hotels and hospitals.
The only exception to this independent-operation logic is the deluge system — covered below.
System types: wet, dry, pre-action, deluge
Sprinklers fall into four main types according to “how the water waits in the pipes.” The right type is chosen by the space’s temperature and by how sensitive its contents are to water damage:
| System type | What is in the pipes? | How it works | Where it is used |
|---|---|---|---|
| Wet-pipe | Water at all times | Head opens, water discharges instantly | All heated spaces (most common) |
| Dry-pipe | Pressurised air | Head opens, air escapes, valve opens, water arrives | Freeze-prone areas: car parks, cold stores |
| Pre-action | Pressurised air | Water enters only when both detection and a head trigger | Water-sensitive areas: data centres, archives, museums |
| Deluge | Empty (open-nozzle heads) | Detection triggers, all heads discharge at once | High hazard: transformers, hangars, chemicals |
- The wet-pipe system is the fastest and simplest; the instant a head opens, water is ready. Its only condition is a heated environment where the pipes will not freeze.
- In a dry-pipe system the pipes are full of pressurised air; when a head opens, the air escapes first, the dry alarm valve opens and water fills the pipes. Because the water would risk freezing, it does not wait in the pipe directly — at the cost of a few seconds’ delay before water arrives.
- The pre-action system is a step safer than the dry system: water usually requires two conditions together to enter the pipe — an independent detection system seeing the fire and a head opening. This way, a single accidentally opened head or a physical knock does not discharge water; it is ideal for spaces holding valuable equipment.
- In a deluge system the heads have no bulb; they are all open-nozzle. When the detection system sees a fire, the deluge valve opens and water discharges over the whole area at once. It is used for high-hazard spaces where fire can spread very fast. For facilities with high storage stacks, a specialised solution is ESFR sprinklers (early suppression, fast response).
Hazard classes: not every space gets the same sprinkler
The first and most decisive step in sprinkler design is determining the space’s hazard class. An office and a chemical store have very different fire loads — and therefore very different required water densities and head spacings. NFPA 13 and EN 12845 group spaces into three main categories:
- Light Hazard — low fire load, slow spread: offices, schools, hotel rooms, hospitals.
- Ordinary Hazard — medium fire load: shops, restaurants, manufacturing areas, car parks (subdivided into OH1–OH2).
- Extra/High Hazard — high fire load, fast spread: areas handling flammable liquids, high-rack storage, paint shops.
As the hazard class rises, the required water density (flow/m²) increases and the heads get closer together. This is exactly where the answer to “how many m² per head” begins.
How many m² does one sprinkler head cover?
This is the most frequently asked design question. Although the exact number is settled by hydraulic calculation, NFPA 13 sets the maximum coverage area and maximum head spacing for standard spray heads by hazard class:
| Hazard class | Max coverage per head | Max head spacing |
|---|---|---|
| Light Hazard | ≈20.9 m² (225 ft²) | 4.6 m |
| Ordinary Hazard | ≈12.1 m² (130 ft²) | 4.6 m |
| Extra Hazard | ≈9.3 m² (100 ft²) | 3.7 m |
These values are an upper limit — heads may be placed closer, but not sparser than this. In practice:
- In a light-hazard office one head can protect ≈20 m², whereas
- in an extra-hazard area the same head protects only ≈9 m²; that is, nearly twice as many heads are needed for the same area.
There is also a lower limit: two heads must be at least 1.8 m apart. Otherwise the cold water from an opened head can wet the neighbouring head’s bulb and delay its operation (“cold soldering”). Heads are also positioned at set distances from walls, beams, lighting and ventilation elements so the spray pattern is not blocked by an obstruction.
In short, “how many m² per head” is not a single number; it is a result given jointly by hazard class + ceiling geometry + hydraulic calculation.
The other design drivers: density, design area and water supply
Head layout is the tip of the iceberg. A sprinkler system is actually sized by three values:
- Design density (mm/min or L/min·m²) — the amount of water that must fall on a unit area. It rises as the hazard class rises.
- Design area — the most unfavourable area assumed to operate at once (e.g. the farthest/highest corner). The system is calculated for the scenario in which all heads in that area open together.
- Water-supply duration and flow — Turkish Fire Code (BYKHY) Article 92 sets the fire water tank by hazard class: low hazard 30 min, ordinary hazard 60 min, high hazard 90 min. The machine that provides this flow and pressure is the fire pump — see our NFPA 20 and redundancy article for the detail.
These three values meet in the hydraulic calculation together with the selected head type and layout; the calculation proves that the required flow and pressure are delivered even at the most unfavourable head.
Standards: NFPA 13 and EN 12845
Sprinkler design is not free engineering; it rests on mature standards:
- NFPA 13 — The core American standard for the installation of sprinkler systems; it defines hazard classes, design densities, coverage areas and layout rules.
- EN 12845 — The European (and Turkish) standard for fixed sprinkler systems; it gives similar criteria within the EN framework.
- BYKHY — Determines in which buildings sprinklers are mandatory and the water-tank durations (Article 92). You can find which structure requires which system in our general guide.
BYKHY answers “is it required,” while NFPA 13 / EN 12845 answer “how is it sized.” Good design combines both with the facility’s real conditions.
Summary
A sprinkler is not a system in which all heads open at once; each head is independent and opens only where the heat reaches it (typically 68°C), directing water to the point that needs it — which limits water damage. The right system type (wet/dry/pre-action/deluge) is chosen by the space’s temperature and water sensitivity; how many m² per head depends on the hazard class (light ≈20.9 m², ordinary ≈12.1 m², extra ≈9.3 m²) and is settled by hydraulic calculation. At A-Pro we engineer water-based suppression systems in line with NFPA 13 and EN 12845, according to your facility’s real hazard class and geometry. Explore our sprinkler systems page, and contact us for a facility-specific design and hydraulic calculation with a free site survey within Ankara.
This content is for informational purposes. A binding, facility-specific assessment requires project-based work with a fire engineer.
© 2026 A-Pro Mühendislik. This is original technical content produced by A-Pro Engineering; all rights reserved. Copying, reproducing or republishing it in whole or in part without attribution and permission is prohibited.
Frequently asked questions
Do all sprinklers open at the same time in a fire?+
No — this is a common misconception (it comes from films). In standard sprinkler systems each head is independent and opens only when the temperature above it reaches the trigger value (typically 68°C). So usually just one or two heads directly over the fire operate, water discharges only where it is needed, and water damage stays limited. The exception is 'deluge' systems: there the heads are open-nozzle and all discharge together when the detection system is triggered.
At what temperature does a sprinkler head open?+
The most common head opens at 68°C (red bulb). The temperature rating is colour-coded by bulb: orange 57°C, red 68°C, yellow 79°C, green 93°C, blue 141°C. The value is chosen well above the space's normal ceiling temperature, so the head opens only in a real fire, not on ordinary warming. The liquid inside the glass bulb expands with heat, shatters the bulb and releases the water.
What is the difference between wet, dry, pre-action and deluge sprinklers?+
In a wet-pipe system the pipes are permanently full of water; it is the fastest and most common solution, for heated spaces. In a dry-pipe system the pipes are full of pressurised air and the water is held at the valve for freeze-prone areas (car parks, cold stores). In a pre-action system water does not enter the pipe until both detection and a head are triggered; it is for water-sensitive areas (data centres, archives, museums). In a deluge system the heads are open-nozzle and, once detection triggers, water discharges over the whole area at once; it is for high-hazard areas (transformers, hangars).
How many square metres does one sprinkler head cover?+
It depends on the hazard class. Under NFPA 13, for a standard spray head the maximum coverage of one head is ≈20.9 m² in light hazard (heads no more than 4.6 m apart), ≈12.1 m² in ordinary hazard (max 4.6 m apart) and ≈9.3 m² in extra hazard (max 3.7 m apart). Heads must also be at least 1.8 m apart so a neighbouring head is not delayed. The exact figure is finalised by hydraulic calculation and ceiling/obstruction geometry.
To which standard is a sprinkler system designed?+
The two main references widely used in Türkiye are NFPA 13 (American) and EN 12845 (European); the Turkish Fire Code (BYKHY) also sets where sprinklers are mandatory and the water-supply durations (Article 92: 30/60/90 min). Design starts by determining the space's hazard class; then the design density, design area, head type and layout are sized by hydraulic calculation. Good design is built by combining the standard's criteria with the facility's real load and geometry.
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