NFPA 13 groups sprinkler systems by what the pipes normally contain and how water gets to the sprinklers. Choosing the system type is a balance between freezing risk, sensitivity to water damage, response time and maintenance capability, and it directly changes the hydraulic design. This article is part of the NFPA 13 Clause Guide series and summarises the points that matter for each type.
Quick comparison
| System | Pipes normally contain | How water is released | Typical use | Design impact |
|---|---|---|---|---|
| Wet | Water under pressure | Immediately when a sprinkler opens | Any area without freezing risk | Reference system |
| Dry | Compressed air or nitrogen | Sprinkler opens, pressure falls, dry pipe valve trips | Unheated warehouses, car parks, cold areas | Design area +30%; water delivery limits |
| Pre-action | Air (supervised) or atmospheric | Detection opens the valve (some types also need a sprinkler to open) | Data centres, archives, museums, freezers | Double interlock: design area +30% |
| Deluge | Empty, open sprinklers | Detection opens the valve; all sprinklers discharge together | Transformers, loading docks, fast-developing hazards | All sprinklers calculated flowing |
| Antifreeze | Listed antifreeze solution | Immediately when a sprinkler opens | Small portions exposed to freezing | Listed solutions only |
Wet pipe systems
The simplest, most reliable and fastest-responding system, and the standard's default: the other types exist for situations where a wet system cannot be used. Its main limit is freezing; the protected area must be maintained at 4 °C (40 °F) or above. Small areas with occasional freezing risk are handled with heating, antifreeze or dry pendent/sidewall sprinklers. For a detailed comparison read wet vs dry pipe sprinkler systems.
Dry pipe systems
The pipes are filled with compressed air or nitrogen. When a sprinkler opens, pressure falls, the dry pipe valve trips and water travels through the pipework to the open sprinklers. That delay is the system's fundamental weakness, and the standard compensates in two ways:
- The design area is increased by 30% (density stays the same), on the assumption that the fire grows and more sprinklers open during the delay.
- System volume and water delivery time are limited. Up to 1,893 L (500 gal) is allowed without a quick-opening device (accelerator or exhauster), and up to 2,839 L (750 gal) with one. Larger systems must show by calculation or test that water delivery stays within the maximum time set for the hazard, for example 60 seconds for light hazard and 15 seconds for dwelling units.
Pipe pitch for drainage, low-point drains and the quality of the air supply are critical for corrosion and freezing. Nitrogen markedly reduces internal corrosion. For typical corrosion patterns in dry systems see sprinkler corrosion case studies.
Pre-action systems
A pre-action system is fed through a valve controlled by a separate detection system. There are three variants:
- Non-interlock: either detection or a sprinkler opening trips the valve.
- Single interlock: detection trips the valve; no water discharges until a sprinkler opens, so pipe damage alone does not release water.
- Double interlock: both detection and a sprinkler opening (loss of pipe pressure) are needed to trip the valve. It gives the best protection against accidental discharge, but behaves like a dry system, so the 30% area increase and water delivery considerations apply.
Pre-action systems above a certain number of sprinklers must keep the pipework under supervised air pressure so that damage is detected. More in pre-action sprinklers in detail.
Deluge systems
In a deluge system every sprinkler or nozzle is open. When detection trips the deluge valve, all sprinklers in the protected area discharge at once. The hydraulic calculation is therefore not based on an area of operation but on every open sprinkler in the system flowing together. Water demand is usually very high and the supply must be sized to suit. Deluge suits fast-spreading fires and equipment that needs area cooling. See deluge system design.
Antifreeze systems
Antifreeze systems are used for small parts of a wet system exposed to freezing. The rules changed significantly after it became clear that high-concentration antifreeze could contribute to a fire. Current editions allow only listed, factory pre-mixed antifreeze solutions in new antifreeze systems. In existing systems, solution type and concentration must be checked regularly under NFPA 25. Backflow protection and expansion provisions are also required where the antifreeze section connects to the wet system.
A practical selection approach
- No freezing risk: wet.
- Large unheated area: dry (with nitrogen where possible).
- Small area exposed to freezing: dry sprinklers or listed antifreeze.
- High sensitivity to water damage: pre-action (double interlock where accidental discharge is critical).
- Very fast fire growth: deluge.
To see all options in one table, read wet, dry, alternate and pre-action compared.
What happens if the system type changes?
If heating is removed from an area in service, or a wet section is converted to dry, it is not just a valve swap. The design area rises by 30%, the hydraulic calculation is redone, pipes need drainage pitch and low-point drains, and upright or listed dry sprinklers must be used. The air supply and dry valve room must be protected from freezing. Converting dry to wet also needs checks on volume, internal corrosion and the suitability of the existing sprinklers.
Common mistakes
- Forgetting the 30% area increase on dry systems.
- Not calculating or testing water delivery time on large dry systems.
- Calculating a double interlock pre-action system as if it were wet.
- Using old-style, unlisted antifreeze solutions in a new system.
- Missing pitch and low-point drains on dry systems, leading to winter freeze-ups.
Frequently Asked Questions
Why is the design area increased for dry systems?
Water reaches the sprinklers later, and in that time the fire grows and opens more sprinklers. The standard compensates by increasing the design area by 30%; the density stays the same.
What is the maximum volume of a dry pipe system?
1,893 L (500 gal) without a quick-opening device and 2,839 L (750 gal) with one. Larger systems must demonstrate water delivery within the time limit for the hazard.
When is a double interlock pre-action system chosen?
Where accidental discharge is unacceptable, such as freezers and some data spaces. It is designed like a dry system, including the area increase.
Can antifreeze be used in a new system?
Yes, but only with listed, factory pre-mixed antifreeze solutions. Site-mixed glycol or glycerine solutions of the old type are not accepted in new systems.
SprinkCalc — Fire Sprinkler Design Across Three Standards
SprinkCalc covers hazard classification, design density and area, K-factor selection, water demand and hydraulic calculations for NFPA 13, FM Global and BS EN 12845 in a single iOS app, and exports a professional PDF report.
Download SprinkCalc on the App StoreNFPA 13, Standard for the Installation of Sprinkler Systems (current edition), system types; NFPA 25 (antifreeze solution and dry system maintenance). Confirm values against the edition in force and with the AHJ. The findings here are typical defect patterns, not an account of events at any particular site.