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 (5–10 s) | Up to 60 s | Up to 60 s in dry mode | Detection plus fill time |
| Design area | Base | Plus 25 percent | Plus 25 percent in dry mode | Plus 25 percent (as dry) |
| Gridded or looped layout | Permitted | Not permitted | Not permitted | Limited |
| Accidental discharge risk | Present | Present | Present | Near zero with 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 | Suitable | Not recommended | Not recommended | Not recommended |
Water delivery time: the heart of a dry system
EN 12845 limits the time between one sprinkler opening and the first continuous discharge: 90 seconds for LH and 60 seconds for OH and HH. The measurement is taken with a stopwatch at the hydraulically most remote test valve. 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: it must fill the whole system to working pressure within 30 minutes.
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 gives back the 25 percent area penalty and reduces compressor hours.
The most common alternate system error: the water left above the alarm valve. If the valve room is not held above 4 °C, that small volume freezes even in dry mode. The pipework side is full of compressed air and does not freeze; the risk always sits in the valve room.
Pre-action: Type A and Type B
- Type A (single interlock): only the detector signal opens the valve. A sprinkler operating does not bring water on its own, but if the detector is faulty the system behaves like a normal dry pipe system and the sprinkler still brings water.
- Type B (double interlock): both the detector signal and a pressure drop in the pipework are required. The accidental discharge risk is lowest; but if a detector fault goes unnoticed, water never arrives even when a sprinkler operates.
Type B is the standard choice in a data centre. The price of that choice is weekly supervision of the detection loop and transmission of the fault alarm to a monitored point.
Decision flow
- Is the ambient continuously between 4 and 70 °C? → Wet. Discussion over.
- Does only a small section freeze? → Keep the main installation wet and make that section a subsidiary dry extension (100 sprinklers maximum).
- Does the building freeze seasonally? → Alternate.
- Is it permanently cold (a cold store)? → Dry or pre-action, with the valve in a heated space.
- Are the contents water sensitive (servers, archives, museums)? → Pre-action, Type B.
- Is it high hazard storage? → Avoid dry and alternate; look for heating or another solution.
Frequently Asked Questions
Why is the design area enlarged on a dry system?
Because other sprinkler heads can open during the air venting delay, EN 12845 increases the area of operation by at least 25 percent on dry and alternate systems.
What is the maximum water delivery time?
90 seconds for LH and 60 seconds for OH and HH. The measurement is taken at the hydraulically most remote test valve.
What is the difference between Type A and Type B pre-action?
Type A opens on the detector signal alone. Type B requires both the detector signal and a pressure drop in the pipework, and carries the lowest accidental discharge risk.
Is a dry system used in high hazard storage?
The standard advises strongly against it. The water delay seriously reduces the suppression effectiveness of the first heads to open.
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Download MEP Calc on the App StoreBS EN 12845:2015+A1:2019 installation types and water delivery time tables. NFPA 13 (2025) system chapters. EN 12259-2 and EN 12259-3 (alarm valve assemblies). Values are for information only.