If you own or manage a commercial building, you may have more than one type of sprinkler system on site without realizing it. A warehouse might have a wet system in the heated office area and a dry system over an unheated loading dock. A server room might have a pre-action system. Knowing which type protects each area helps you plan maintenance, repairs, and renovations.
NFPA 13, the national installation standard for sprinkler systems, recognizes four main types: wet pipe, dry pipe, pre-action, and deluge. Here’s how each one works.
Wet pipe systems
A wet pipe system is the most common type. The piping is filled with water at all times. Each sprinkler head is individually sealed by a heat-sensitive element, usually a glass bulb or fusible link. When heat from a fire reaches a particular head, that head opens and water discharges immediately. Only the heads exposed to enough heat operate; the rest stay closed.
Where it fits: Heated spaces such as offices, retail, apartments, schools, and most of the interior of commercial buildings.
What owners should know:
- Wet systems are relatively simple, with fewer components to maintain than the other types.
- Because the pipes hold water, they must not be installed where they can freeze. Unheated stairwells, attics, loading docks, and parking garages are common problem areas.
- A frozen pipe can crack fittings, cause water damage, and leave part of the building unprotected.
Dry pipe systems
A dry pipe system is designed for spaces that can freeze. Instead of water, the piping is filled with pressurized air or nitrogen. Water is held back at a dry pipe valve, usually located in a heated room. When a sprinkler head opens, the air pressure drops, the dry pipe valve trips, and water flows into the piping and out through the open head.
Where it fits: Unheated warehouses, parking garages, loading docks, and attics.
What owners should know:
- There is a short delay between a head opening and water arriving, because the air has to escape first. The system is designed with this delay in mind.
- Dry systems have more parts: the dry pipe valve, an air compressor or nitrogen supply, and sometimes an accelerator. These need regular attention.
- Water left in the piping after a trip test or activation, along with condensation, can collect at low points. Draining low points before cold weather is an important part of upkeep.
- The mix of oxygen and moisture inside dry piping can contribute to corrosion over time, which is one reason some owners use nitrogen.
Pre-action systems
A pre-action system is similar to a dry system, but water is held back by a valve that opens only after a separate detection system, such as smoke or heat detectors, senses a fire. Depending on the design, the system may also require a sprinkler head to open before water discharges.
There are three common variations:
- Non-interlock: water enters the piping when either the detection system or a sprinkler head operates.
- Single-interlock: water enters the piping when the detection system operates. Water discharges only when a head opens.
- Double-interlock: water enters the piping only when both the detection system and a sprinkler head have operated.
Where it fits: Areas where accidental water discharge would be especially costly, such as data centers, server rooms, museums, archives, and some freezer spaces.
What owners should know:
- The extra steps reduce the chance of water being released because of a damaged head or pipe.
- Pre-action systems are more complex, so they involve more testing and coordination between the sprinkler side and the detection side.
Deluge systems
A deluge system uses open sprinklers or nozzles, meaning there’s no heat-sensitive element holding each one closed. The piping is normally dry. When a detection system activates, the deluge valve opens and water flows out of every sprinkler or nozzle on that system at the same time.
Where it fits: High-hazard spaces where a fire could spread very quickly, such as certain industrial processes, flammable liquid areas, and aircraft hangars.
What owners should know:
- A deluge system releases a large volume of water over a wide area at once, so drainage and water supply are key design considerations.
Quick comparison
| System | Piping normally contains | What releases water | Typical use |
|---|---|---|---|
| Wet pipe | Water | Individual heads open from heat | Heated, general spaces |
| Dry pipe | Pressurized air or nitrogen | Head opens, air pressure drops, valve trips | Spaces that can freeze |
| Pre-action | Air (often supervised) | Detection system, sometimes plus a head opening | Water-sensitive areas |
| Deluge | Nothing (open heads) | Detection system opens deluge valve | High-hazard areas |
Choosing or changing a system
The right system type depends on the building’s use, temperature conditions, the hazard being protected, and the value of what’s inside. Changes in how a space is used, such as turning a heated office into unheated storage or adding high-piled racks, can mean the existing system is no longer the right fit. Review sprinkler coverage early when planning a renovation, tenant change, or addition.
All four types also need ongoing inspection, testing, and maintenance under NFPA 25. Dry, pre-action, and deluge systems have extra valves and components that add to that list, so it helps to know exactly what you have.
Getting help with your sprinkler system
Whether you’re planning a new system, adapting an existing one to a changed space, or dealing with a leak, frozen pipe, or failed component, JT Fire & Safety in Somerville, NJ can help. Learn more about fire sprinkler installation and fire sprinkler repair, or call (908) 333-2590 to talk through your building.