Wet pipe, dry pipe, pre-action and deluge. They differ in one thing: what is in the pipe before the fire. That single difference decides how fast water arrives, where each can be used, and what a service visit has to include.
The great majority of UK buildings
The pipework is full of water at all times. A head opens and water comes out of that head immediately. There is nothing else to happen.
Used in. Anywhere that never drops below freezing: offices, shops, warehouses, flats.
Watch for. It cannot be used anywhere that freezes. A wet system in an unheated loading bay or an external canopy will split, and the first anyone knows is the thaw.
Cold stores, loading bays, canopies
The pipework holds compressed air. A head opens, the air escapes, the pressure drop trips a dry alarm valve at the control set and water then travels up the pipe to the open head.
Used in. Unheated spaces, and only where the temperature genuinely justifies it.
Watch for. There is a delay between the head opening and water arriving, so the standard limits how much pipe volume one valve may serve. It is also the system most damaged by trapped water, which is why the pipework has to be laid to fall and drained properly.
Data centres, archives, galleries
Two things have to happen. A fire detection system has to detect, AND a sprinkler head has to open. Only then does water enter the pipework. A single event does nothing.
Used in. Rooms where an accidental discharge would cost more than the fire.
Watch for. It is two systems that both have to be maintained, and the interlock between them is the part that gets forgotten. A pre-action system with a detection head out of service is not a pre-action system.
Process risks, transformers, flammable storage
The heads have no heat element at all. They are simply open nozzles. A detection system opens a deluge valve and every head in the zone discharges together.
Used in. High hazard, where a fire will spread faster than head-by-head operation can control it.
Watch for. Water volumes are very large and the drainage has to be designed for it. This is a process engineering decision, not a building services one.
How a head actually triggers, and why only the heads over the fire open, is in the guide on how a sprinkler system works.
The four types above appear in both, but a commercial system and a residential one are designed to different standards for different purposes, and they are maintained differently as a result.
A property protection standard. The building is classified by hazard, the design density and area of operation follow from that, and the water supply has to be proved for a required flow and duration. Warehouses, retail, offices, basement car parks and plant rooms.
A life safety standard. Get the occupants out, using a smaller supply often taken off the domestic main, with concealed heads that do not look industrial. Blocks of flats, student accommodation, care homes.
A single building can hold both: flats to BS 9251 above a car park to BS EN 12845, on separate designs and separate service regimes. Where that line falls is in our guide to residential sprinklers.
The head is chosen for the ceiling and the risk, not for looks, and the wrong one in the wrong place is a defect rather than a preference.
Hangs down below the ceiling. The default in a suspended ceiling, and the one most people picture.
Sits above the pipe, spraying up into a deflector. Used on exposed pipework under a deck or in a roof void, where a pendent would collect dust and damage.
Fires horizontally from a wall. Used in corridors and small rooms where running pipework across the ceiling is not worth it.
A pendent behind a flat cover plate that drops away with heat. Chosen where the ceiling matters: receptions, hotels, flats. The cover plate is part of the listed assembly and must not be painted.
Early Suppression Fast Response. A large-orifice, fast-response roof-level head for warehousing, designed to drive water through a rack fire plume and suppress it rather than control it.
The liquid in the glass bulb is colour coded to its operating temperature. It is the quickest way to tell whether the head above you is the right one for the room it is in.
Red and yellow cover most ordinary occupancies. A higher rating goes near roof lights, kitchens, plant rooms and anywhere the ambient temperature would otherwise trip a standard head.
Look at the control valve set rather than the ceiling. A wet system has one main gauge above and below the alarm valve and no compressor. A dry system has an air compressor and a second, lower pressure reading on the air side. A pre-action set has a detection panel wired into it. If in doubt, the commissioning certificate names the system type.
It is the operating temperature of that head. Orange is 57°C, red 68°C, yellow 79°C, green 93°C, blue 141°C and mauve 182°C. Red and yellow cover most ordinary occupancies. A higher rating is used near heat sources such as roof lights, kitchens and plant so the head does not operate on ambient heat.
No. The cover plate is a listed part of the assembly and it is designed to drop away at a set temperature. Painting it changes that, and a painted cover plate is a defect on any inspection. The same applies to the head itself: a painted sprinkler head must be replaced, not cleaned.
Not for fire. It is safer for the contents against an accidental discharge, which is why data centres and archives use it. For fire performance a wet pipe system is the fastest of the four, because there is nothing between the head opening and water arriving.
The intervals are the same under BS EN 12845, but the work is not. A dry or pre-action set adds air pressure checks, compressor checks and a trip test of the dry valve, and a pre-action system adds proving the interlock with the detection system. A quotation that prices a dry system as though it were wet has missed several items.
Not sure which you have? The commissioning certificate names it, and if nobody can find one an engineer can tell from the valve set in about a minute.
Which type of system a building needs, and what it has to achieve once installed, is set by these.
The standard commercial and industrial sprinkler systems are designed, installed and maintained to. It sets the weekly, monthly, quarterly and annual routines.
BSI
The residential equivalent: blocks of flats, care homes, student accommodation and houses.
BSI
Statutory guidance to Part B of the Building Regulations. Volume 1 covers dwellings, Volume 2 everything else, and it is where the height thresholds that trigger sprinklers and risers actually live.
UK Government
British Standards are copyright documents and most are bought rather than read free. We hold the ones we work to. If you need to know what one of them says about your building, ask us rather than buying it.
No survey fee to find out where you stand. Describe the site and we will tell you which intervals apply and what the visit involves.
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