Short answer: TS EN 12845+A2:2026 Clause 11.5.1 (EN p.69) defines only two uses: a) in small areas liable to frost damage within an otherwise adequately heated building, as a dry pipe or alternate extension to a wet pipe installation; b) in cold stores and in high temperature ovens or furnaces, as a dry pipe extension to a wet pipe or alternate installation.

Installing a dry system for a whole building is an expensive choice because of the delay in water reaching the sprinklers. Instead, TS EN 12845+A2:2026 allows only the small part of a wet installation that is at risk of freezing to operate dry or alternate: the subsidiary dry pipe or alternate extension. Clause 11.5 consists of two short subclauses, yet the rule set around it is broad. Source: TS EN 12845+A2:2026, Clause 11.5, EN p.69 (TSE p.72).

1. Clause 11.5.1: Definition and the two permitted uses

Subsidiary dry pipe or alternate extensions shall comply with 11.2 and 11.3, except that they are of limited extent and form extensions to normal wet installations. Clause 11.5.1, EN p.69

"Comply with 11.2 and 11.3" means that the extension carries all the technical requirements applicable to dry installations and alternate installations (air or inert gas supply, water travel time, slope, upright position).

Installation is possible in only two cases:

These two conditions form a closed list. The extension is not a means of turning a large part of an installation dry; in the words of Clause 11.5.1 itself, it is "of limited extent".

2. Clause 11.5.2: Size limit

Constraint Value
Number of sprinklers in any subsidiary extension ≤ 100
Where one control valve set controls more than two subsidiary extensions, the total number of sprinklers in the subsidiary extensions ≤ 250

Source: Clause 11.5.2, EN p.69 (TSE p.72).

There is a parallel set of figures on the antifreeze protection side (Clause 11.1.2.2, EN p.66): ≤ 20 per section and ≤ 100 in total. The antifreeze method is intended for much smaller zones than the subsidiary extension.

3. Requirements for dry installations (Clause 11.2)

Because a subsidiary extension must comply with them, Clause 11.2 is directly relevant.

Clause 11.2.1 (EN p.67): dry pipe installations are normally filled with air or inert gas under pressure downstream of the dry alarm valve and with water under pressure upstream of it. A permanent air or inert gas supply is installed to maintain the pressure in the pipework. The installation is pressurized to fall within the pressure range recommended by the alarm valve supplier. Dry pipe installations are installed only in locations where there is a possibility of frost damage or where the temperature exceeds 70 °C (for example drying ovens).

Clause 11.2.2 (EN p.67): the maximum time between the opening of a single sprinkler and the discharge of water shall not exceed the value in Table 18. The test is carried out using the remote test valve specified in Clause 15.5.2.

Maximum time for water to arrive (s)
LH 90
OH and HH 60

Source: Table 18, Clause 11.2.2, EN p.67 (TSE p.70).

The same clause contains two further important statements:

It is strongly recommended that dry and alternate installations are not used for HHS applications, because the delay in water reaching the first operating sprinklers can seriously impair the effectiveness of the system.

NOTE Experience has shown that, with an accelerator, a volume of not more than 4 m³ for LH or OH and 3 m³ for HH applications is expected to satisfy the requirements of 11.2.2.

Clause 11.3.1 (EN p.68) defines the alternate installation: alternate installations incorporate either an alternate alarm valve or a composite set consisting of a wet alarm valve and a dry alarm valve. During the winter months the installation pipework downstream of the alternate or dry alarm valve is filled with air or inert gas under pressure, and the remainder of the system upstream of the alarm valve with water under pressure. During the rest of the year the installation operates as a wet pipe installation. Clause 11.3.2 ties the size limit to Table 18.

4. Sprinkler position: the upright requirement

Clause 12.1.3 (EN p.70):

Except where dry pendent pattern sprinklers are used, sprinklers in dry pipe, alternate and pre-action installations shall be upright. An upright sprinkler shall be fitted with the yoke arms parallel to the pipe.

NOTE 1 to the clause gives the reason: upright sprinklers may be less prone to mechanical damage and to the accumulation of foreign matter in the sprinkler fittings; the upright orientation also facilitates complete drainage of water from the sprinkler waterways. NOTE 2 states that pendent sprinklers can deliver water at a higher velocity and a higher density immediately below and adjacent to the sprinkler axis, and may therefore show better fire control capability in some applications such as in-rack protection and storage area protection.

5. Drainage and slope

Clause 17.1.8 (EN p.112): "In dry, alternate and pre-action installations, range pipes shall slope at least 0.4 % towards the distribution pipe, and distribution pipes at least 0.2 % towards a suitable drain valve." The NOTE states that in cold climates where severe freezing conditions are possible it may be necessary to slope wet systems as well and to increase the slope in dry systems. Range pipes are connected only to the side or the top of the distribution pipes.

The drain valve diameters are in Table 39 (Clause 15.4, EN p.108):

Valve draining principally Minimum valve and pipe diameter (mm)
LH installation 40
OH or HHP or HHS installation 50
Subsidiary installation 50
A zone 50
Trapped distribution pipes, diameter ≤ 80 mm 25
Trapped distribution pipes, diameter > 80 mm 40
Trapped range pipes 25
Trapped pipework between a dry or subsidiary alarm valve and a subsidiary stop valve installed for testing purposes 15

Clause 15.4 (EN p.107-108) also lists the five positions at which drain valves are to be fitted; one of these concerns the subsidiary extension directly: "immediately downstream of any subsidiary alarm valve" (item b) and "between a dry pipe or subsidiary control valve set and any subsidiary stop valve installed for testing purposes" (item d). The valves are fitted at the lower end of the pipework, the outlet is not more than 3 m above floor level and is provided with a suitable plug.

6. Testing and maintenance

Subject Provision Source
Remote test valve A test valve connected at the hydraulically most remote position of a distribution pipe giving a flow equivalent to the discharge of a single sprinkler, together with the associated fittings and pipework Clause 15.5.2, EN p.109
Weekly pressure check In dry, alternate and pre-action installations the pressure in the pipework shall not fall by more than 1.0 bar per week Clause 20.2.2.2 a, EN p.119
Half yearly routine The moving parts of dry alarm valves in dry pipe installations and in subsidiary extensions, and the accelerators and exhausters, are operated in accordance with the supplier's instructions. NOTE: alternate installations need not be tested in this way, because they are already operated as a result of the changeover from wet to dry and back twice a year Clause 20.3.3.2, EN p.121
Commissioning Dry pipe installations are tested pneumatically at a pressure of at least 2.5 bar for at least 24 hours; any leakage causing a pressure loss of more than 0.15 bar in 24 hours is rectified Clause 19.1.1.1, EN p.117
Commissioning All installation pipework is tested hydrostatically for at least 2 hours at a pressure of at least 15 bar, or 1.5 times the maximum pressure to which the system will be subjected, whichever is greater Clause 19.1.1.2, EN p.117
Cold store Installations protecting cold stores with air circulation refrigeration should be dismantled for drying after every operation Annex J.4.2 (informative), EN p.158

7. Limit on installation size

The sprinklers in a subsidiary extension are included in the area of the control valve set of the wet installation to which they are connected. Clause 11.1.3 (EN p.67): "The maximum area controlled by a single wet alarm valve, including the sprinklers in a subsidiary extension, shall not exceed that shown in Table 17."

Hazard class Maximum protected area per control valve set (m²)
LH 10 000
OH (including LH sprinklers within it) 12 000, except as permitted in Annex D and Annex F
HH (including OH and LH sprinklers within it) 9 000

Source: Table 17, Clause 11.1.3, EN p.67 (TSE p.70).

8. Comparison with the alternative methods

The three tools the standard offers against the risk of freezing are intended for different scales:

Method Scale limit Additional requirement Source
Antifreeze solution ≤ 20 sprinklers per section, ≤ 100 in total Hydrometer check, backflow prevention Clause 11.1.2.2, EN p.66
Electrical trace heating No limit on numbers Duplicate heating elements, each capable of maintaining ≥ 4 °C; monitoring by separate circuits; A1/A2 insulation ≥ 25 mm; tape ≤ 10 W/m Clause 11.1.2.3, EN p.66-67
Subsidiary dry/alternate extension ≤ 100 sprinklers per extension, ≤ 250 in total Table 18 water travel time, upright sprinklers, slope Clause 11.5, EN p.69

Frequently Asked Questions

In which two situations may a subsidiary dry or alternate extension be installed?

TS EN 12845+A2:2026 Clause 11.5.1 (EN p.69) defines only two uses: a) in small areas liable to frost damage within an otherwise adequately heated building, as a dry pipe or alternate extension to a wet pipe installation; b) in cold stores and in high temperature ovens or furnaces, as a dry pipe extension to a wet pipe or alternate installation.

How many sprinklers may a subsidiary extension have?

Clause 11.5.2 (EN p.69): the number of sprinklers in any subsidiary extension shall not exceed 100. Where one control valve set controls more than two subsidiary extensions, the total number of sprinklers in the subsidiary extensions shall not exceed 250.

How quickly must water arrive in dry and alternate installations?

Clause 11.2.2 and Table 18 (EN p.67): the maximum time between the opening of a single sprinkler and the discharge of water is 90 seconds for LH and 60 seconds for OH and HH. The test is carried out using the remote test valve specified in Clause 15.5.2.

In which position are sprinklers installed in dry and alternate installations?

Clause 12.1.3 (EN p.70): except where dry pendent pattern sprinklers are used, sprinklers in dry pipe, alternate and pre-action installations shall be upright. An upright sprinkler shall be fitted with the yoke arms parallel to the pipe.

May a dry or alternate installation be used for HHS storage?

Clause 11.2.2 (EN p.67) strongly discourages it: 'It is strongly recommended that dry and alternate installations are not used for HHS applications, because the delay in water reaching the first operating sprinklers can seriously impair the effectiveness of the system.' The NOTE to Table 4 warns in the same direction and, where a dry or alternate installation has to be used nevertheless, requires the area of operation to be increased by 25 %.

What is done in a cold store after the sprinklers have operated?

Annex J.4.2 (EN p.158): installations protecting cold stores operating with air circulation refrigeration should be dismantled for drying after every operation. Annex J is informative.

References

SprinkCalc — Fire Sprinkler Design Across Three Standards

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MEP Calc — 86+ Engineering Calculators

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Standards & References

TS EN 12845+A2:2026 (EN 12845:2015+A2:2026) Fixed firefighting systems — Automatic sprinkler systems — Design, installation and maintenance; TS EN 12845-2:2025 (EN 12845-2:2024) ESFR and CMSA sprinkler systems. Every figure in this article is taken from the published standard text; clause, table and page references are listed under References. General information only, not a substitute for the standard or for a design review.

FS

Fatih Selvi

Mechanical engineer and software developer with field experience in MEP and fire protection, working actively with NFPA, FM Global and BS EN 12845 on site projects.