EN 12845 Subsidiary Dry and Alternate Extensions

Small freeze-prone branches fed from a wet main: how the subsidiary extension clause reads on site, and where you cannot afford a mistake.

We were revising the fire installation at a logistics centre. The main hall was heated to +18 °C on a standard OH3 wet system. But the loading dock had a 6 m deep canopy, and at the end of it a small +2 °C freezer lobby. The canopy was partly open and dropped to −8 °C in winter; the lobby doors opened and closed constantly. Thirty-eight sprinklers in total. Converting the whole main system to alternate made no sense, and antifreeze was contentious next to a food store. EN 12845 gives the right answer here: a subsidiary dry or alternate extension.

What the clause says

The standard defines the subsidiary dry or alternate extension and the two conditions under which it may be used:

• (a) where a small area of an otherwise adequately heated building is at risk of frost damage, as a dry or alternate extension off the wet system;
• (b) in cold stores and high-temperature oven or stove areas, as a dry extension off a wet or alternate main system.

All other conditions match those for dry or alternate installations: air or inert gas pressure, an alarm valve set, upright sprinklers, drainage falls and a remote test valve. The only difference is the size limits, which the standard deliberately keeps tight — the purpose of a subsidiary extension is not to be a fully equipped dry installation, but to handle one small freezing branch of a wet system.

Size limits

The numerical framework is unambiguous:

Parameter	Limit
Sprinklers on a single subsidiary extension	≤ 100
Total sprinklers where more than two extensions share one control valve set	≤ 250
Dry or alternate pipe volume (LH or OH, with accelerator)	≤ 4 m³
Dry or alternate pipe volume (HH, with accelerator)	≤ 3 m³
Water delivery time, LH	≤ 90 s
Water delivery time, OH and HH	≤ 60 s

In practice, an accelerator plus a 4 m³ pipe volume is the working recipe for LH and OH — a volume that corresponds roughly to the 100-sprinkler limit and keeps dry system behaviour predictable. Anything larger requires either an independent full dry installation, or splitting the extension in two on separate valves.

Valves, air and drainage — all independent

The subsidiary extension connects to the wet main at a single point, but from there it must carry its own miniature system:

• Control valve set: either a dry alarm valve or a composite alternate valve set, with an isolating valve between it and the wet main so the extension can be drained on its own for maintenance.
• Air or inert gas supply: a permanent supply maintaining the pressure range the valve manufacturer specifies. A small extension does not exempt you from the compressor; a small riser-mounted compressor or a nitrogen cylinder is still required.
• Drainage: all dry branches pitched back toward the valve, with drum drips at low points and a main drain at the valve set. At cold store entrances the water must clear before it can freeze, so the discharge line runs to a warm area.
• Test valve: a remote test valve at the most remote sprinkler position. Water delivery time is measured from it.
• Sprinkler orientation: upright. If pendent heads are wanted, a dry pendent type is mandatory — body in the warm zone, barrel extending into the cold.

The cold store entrance scenario

In the loading dock and freezer lobby example, the typical arrangement is:

1. The wet main control valve set (OH3) sits in the heated plant room, feeding the warm zone through the distribution main.
2. At the warm/cold boundary, on the warm side of the internal wall, the subsidiary extension valve set is mounted — a dry alarm valve plus accelerator.
3. From there all pipework runs into the cold zone with upright sprinklers. Pipe volume is kept under 4 m³ and the sprinkler count under 100.
4. The air supply is in the warm zone — a compressor or small nitrogen cylinder, with the pressure switch monitored at the control panel.
5. The drain line runs from the cold zone to a gully in the warm zone where water will not freeze.
6. A remote test valve is fitted at the most remote head, and the 60 s water delivery time for OH is verified at commissioning.

The same logic stops working once you move into the main volume of a cold store — a large cold store is no longer a subsidiary extension but requires either a completely independent dry or alternate installation, or a bespoke solution. Once the wet main plus the cold volume exceeds 100 sprinklers, it falls outside the subsidiary extension scope.

The high-hazard storage warning

The standard carries an important note: dry and alternate systems are strongly discouraged in high-hazard storage applications, because the delay in getting water to the first sprinklers to open can seriously weaken the system's effect. A subsidiary extension is no exception. Where a cold store carries a high-hazard storage scenario — palletised storage, for instance — the alternatives are:

• Trace heating plus a wet system: pipework heated by electrical cable, with A1/A2 insulation and a duplicate circuit.
• An antifreeze section: a maximum of 20 sprinklers per section and 100 per control valve set, with a backflow preventer.
• Building the whole system dry or alternate and reconsidering the protection strategy, including the storage arrangement itself.

Capacity accounting on the wet main

A subsidiary extension is counted within the area protected by the wet alarm valve set. One wet valve set can cover up to 12 000 m² for OH, and the sprinklers on subsidiary extensions and the area they cover come out of that budget. A small 38-sprinkler extension therefore takes its share. If the area of operation falls inside the extension, the accelerator delay and dry system delivery time must still sit inside the 60 s OH requirement.

Common field errors

• Forgetting the drainage fall. A horizontal branch on the cold side laid without fall traps the first return water, which freezes; the next thaw brings corrosion and possibly a split. This is exactly why the standard requires upright sprinklers and correct drainage.
• Running the air supply from a single mains compressor. A power cut drops the pressure and raises a spurious alarm. A small nitrogen cylinder plus a pressure switch is a practical backup.
• Fitting 120 sprinklers on one extension. The limit is 100. Exceeding it puts you outside the standard; splitting into two extensions then brings the 250 total into play.
• Using pendent heads. Dry branches require upright sprinklers; if pendent is needed, a dry pendent type is mandatory.
• Omitting the remote test valve. Water delivery time is measured at commissioning; without the valve, the acceptance test cannot be carried out.

The NFPA equivalent

NFPA 13 has the same concept as an auxiliary dry pipe system. The logic is identical: a small freezing branch fed from a wet system, with a separate dry alarm valve, an air supply and a water delivery time limit. NFPA keeps delivery time around 60 s depending on system type, and the sprinkler count limit is again around 100. EN 12845 runs directly parallel to NFPA 13 here in field practice; the differences emerge in design density, K-factor and hose allowance.

Turkish context

BYKHY references EN 12845 or NFPA 13 for sprinkler design. Subsidiary dry extensions have become common practice in Turkey over the last five to ten years at cold store entrances, frozen food logistics loading docks and supermarket freezer room corridors. Insurers — whether international or local reinsurers — generally require a commissioning report recording the water delivery time measurement; systems handed over without a remote test valve are not accepted.

Frequently asked questions

How many sprinklers can a subsidiary dry extension have?

A maximum of 100 per extension. Where more than two extensions share one control valve set, the total across them cannot exceed 250.

What is water delivery time?

The time for water to reach a single opened sprinkler on a dry or alternate system: a maximum of 90 s for LH and 60 s for OH and HH. It is measured at the remote test valve.

Does a subsidiary extension need its own valve and drainage?

Yes. It must operate with its own alarm valve (dry or alternate), its own air or nitrogen supply, its own drainage and its own test connections, and it must be isolatable independently of the wet main.

Is a subsidiary extension suitable for a cold store entrance?

Yes. The standard specifically permits subsidiary dry extensions in cold stores and high-temperature oven areas. Small freezing entrances, loading dock canopies and freezer lobbies are the typical applications.

Are they recommended in high-hazard storage?

No. The standard strongly discourages dry and alternate systems in high-hazard storage, because the delay in delivering water to the first sprinklers seriously weakens protection. Trace heating or an antifreeze section should be considered instead.

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