Wet, Dry, Alternate and Pre-Action Systems Compared

There are four installation types, and the choice starts with one question: does this space freeze? If not, wet. If it does, you have to decide between dry, alternate and pre-action — and each affects the design area, the water delay and the cost differently.

Four types in one table

Feature	Wet	Dry	Alternate	Pre-action
Medium in the pipe	Water	Compressed air / nitrogen	Water in summer, air in winter	Air plus independent detection
Where used	No freezing risk	Cold stores, car parks, roof voids	Seasonal freezing	Data centres, archives, museums
Water delay	None (pipe full of water)	EN 12845: up to 90 s LH, 60 s OH/HH	As dry, in dry mode	Detection plus pipe fill time
Design area	Base	EN 12845: plus 25 percent; NFPA 13: plus 30 percent	Plus 25 percent for dry mode (EN)	EN 12845: same as wet; NFPA 13 double interlock: plus 30 percent
Gridded or looped layout	Permitted	Not permitted	Not permitted	EN 12845: not permitted; NFPA 13: limited
Accidental discharge risk	Present	Present	Present	Lowest (EN Type A, NFPA single/double interlock)
Corrosion risk	Moderate (MIC)	High (humid air)	Highest	High
Maintenance burden	Low	Moderate (compressor, accelerator)	High (seasonal changeover)	High (detection loop)
High hazard storage (HHS)	Suitable	Strongly discouraged	Strongly discouraged	Same column as wet in EN 12845 tables

The area increases come from EN 12845 Table 3 (OH1 72 → 90 m², OH2 144 → 180 m², OH3 216 → 270 m², HHP 260 → 325 m²) and the notes to Tables 4 and 5 for HHS, and from NFPA 13-2025 19.2.3.2.6 (dry and double interlock pre-action: plus 30 percent). EN 12845 does not allow dry or alternate systems for LH or OH4 (OH1 and HHP1 are used instead). For grid and loop systems EN 12845 11.1.1 allows wet installations only; NFPA 13 prohibits gridded dry systems (8.2.1) and gridded double interlock pre-action (8.3.2.7.1), and prohibits gridding single and non-interlock pre-action only in high-piled storage (8.3.2.7.2).

Water delivery time: the heart of a dry system

EN 12845 limits the time between a single sprinkler opening and water discharging: 90 seconds for LH and 60 seconds for OH and HH (11.2.2, Table 18). The test is carried out at the remote test valve specified in 15.5.2. A note in the standard says that with an accelerator, a pipe volume of no more than 4 m³ for LH or OH and 3 m³ for HH is expected to meet the limit. If the limit is exceeded there are three routes: fit an accelerator, split the pipe volume into subsidiary installations, or revisit the pipe sizes.

Compressor capacity is selected from the internal pipe volume, not the sprinkler count: under NFPA 13-2025 8.2.7.3 the air supply must be able to restore normal system pressure within 30 minutes (60 minutes in refrigerated spaces held below −15 °C, 8.2.7.4).

Alternate systems: two decisions a year

An alternate installation runs wet in summer and dry in winter. The changeover is driven by temperature, not the calendar; the practical reference is when the coldest night temperature crosses 4 °C. In a typical Anatolian climate that means switching to dry in late October and back to wet in late March. Returning to wet early removes the water delay and cuts compressor running hours; the hydraulic design, however, must always be made for the dry mode (the 25 percent larger area).

Pre-action: EN 12845 Type A and Type B, NFPA 13 interlock types

• Type A (EN 12845 11.4.1.2): the control valve set is activated by an automatic fire detection system only, not by the operation of the sprinklers. The air pressure in the pipework is monitored at all times, and at least one quick-opening manual valve allows the pre-action valve to be opened in an emergency. If the detection system develops a fault, the installation shall operate as an ordinary dry pipe system. The standard recommends Type A only where an accidental discharge could cause considerable damage. Its NFPA 13 counterpart is the single interlock system (8.3.2.1(1)).
• Type B (EN 12845 11.4.1.3): the control valve set is activated either by the automatic fire detection system or by the operation of the sprinklers; a pressure drop in the pipework opens the alarm valve independently of the detectors. It may be used wherever a dry system is called for and rapid fire spread is expected, in place of a dry system with or without an accelerator. Its NFPA 13 counterpart is the non-interlock system (8.3.2.1(2)).
• Double interlock (NFPA 13 8.3.2.1(3)): water enters the piping only when detection and sprinkler operation both occur. EN 12845 has no equivalent. NFPA 13 increases the design area by 30 percent (19.2.3.2.6), prohibits gridding (8.3.2.7.1) and sizes the system under 8.3.2.3.1 (a system of no more than 500 gal (1900 L) has no water delivery requirement; another route is water at the test connection within 60 seconds).

For water-sensitive spaces such as data centres and archives, the type EN 12845 recommends is Type A; Type B gives no more protection against accidental discharge than a dry system. With double interlock, an unnoticed detection fault means no water even when a sprinkler operates, so the detection system needs regular inspection and its fault signal must go to a constantly monitored point.

Decision flow

1. Is the ambient continuously between 4 and 70 °C? → Wet. Discussion over.
2. Does only a small section freeze? → Keep the main installation wet and make that section a subsidiary dry extension (EN 12845 11.5.2: 100 sprinklers maximum per extension; 250 in total where more than two extensions share one control valve set).
3. Does the building freeze seasonally? → Alternate.
4. Is it permanently cold (a cold store)? → Dry or pre-action, with the valve in a heated space.
5. Are the contents water sensitive (servers, archives, museums)? → Pre-action (Type A under EN 12845; single or double interlock under NFPA 13).
6. Is it high hazard storage? → Avoid dry and alternate systems (EN 12845 11.2.2); look for another solution such as heating the space.

Frequently Asked Questions

Why is the design area enlarged on a dry system?

During the delay while the air vents, the fire grows and more sprinklers open. EN 12845 increases the area of operation by 25 percent on dry and alternate systems (Table 3; notes to Tables 4 and 5 for HHS), and NFPA 13 increases it by 30 percent on dry and double interlock pre-action systems (19.2.3.2.6).

What is the maximum water delivery time?

Under EN 12845 Table 18, 90 seconds for LH and 60 seconds for OH and HH. The test is carried out at the remote test valve.

What is the difference between Type A and Type B pre-action?

Under EN 12845, Type A is opened by the fire detection system only; sprinkler operation does not open the valve. Type B is opened either by the detection system or by sprinkler operation. Type A is the one recommended for water-sensitive areas. The double interlock system, which needs both detection and sprinkler operation, is defined in NFPA 13.

Is a dry system used in high hazard storage?

EN 12845 11.2.2 strongly recommends against it: the delay in water reaching the first operating sprinklers could seriously impair the effectiveness of the system. If it is nonetheless necessary, the area of operation is increased by 25 percent.

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