EN 12845 Alternate (Seasonal) Installations: Wet in Summer, Dry in Winter

For spaces that carry a risk of freezing but stay mild for most of the year, TS EN 12845+A2:2026 (EN 12845:2015+A2:2026) recognises a third type of installation: the alternate installation. All the provisions, figures and table values in this article are taken from the text of that standard; each is given together with its clause or table number and EN page.

1. Definition (Clause 11.3.1, EN p.68)

The definition in the standard reads as follows: 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 charged with compressed air or inert gas, and the rest of the system upstream of the alarm valve is charged with pressurized water. During the rest of the year the installation operates as a wet pipe installation.

For comparison: Clause 11.1.1 (EN p.66) defines a wet pipe installation as being permanently charged with pressurized water and states that it shall be installed only in premises where there is no possibility of freezing damage and where the ambient temperature will not exceed 95 °C. Clause 11.2.1 (EN p.67) states that dry pipe installations shall be installed only where there is a possibility of freezing damage or where the temperature exceeds 70 °C (for example in drying ovens).

2. Water delivery time and installation size

Clause 11.3.2 (EN p.68): the maximum time between the opening of a single sprinkler and the discharge of water shall not exceed the value in Table 18.

Table 18 (Clause 11.2.2, EN p.67) - Maximum water delivery time, dry and alternate installations:

The test is carried out with the remote test valve specified in Clause 15.5.2 (EN p.109): a test facility shall be provided at the hydraulically most remote position, connected to a distribution pipe, giving a flow equivalent to the discharge of a single sprinkler.

The same Clause 11.2.2 contains two further important pieces of information:

• Warning: it is strongly recommended that dry and alternate installations not be used in 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 occupancies is expected to meet the requirements of 11.2.2.

Pre-action installations have a similar time: Clause 11.4.1.4 b) (EN p.69) requires that the time between the simultaneous activation of more than one pre-action installation and the discharge of water from any remote test valve shall not exceed 60 s.

Installation size: Table 17 (Clause 11.1.3, EN p.67) gives the maximum protected area controlled by a single wet alarm valve: LH 10 000 m², OH (including LH sprinklers within it) 12 000 m² (other than those permitted in Annexes D and F), HH (including OH and LH sprinklers within it) 9 000 m².

Subsidiary extensions (Clause 11.5, EN p.69): subsidiary dry pipe or alternate extensions shall comply with 11.2 and 11.3, but shall be of limited extent and shall form an extension of normal wet installations. They are installed only in these two cases: (a) as a dry pipe or alternate extension to a wet pipe installation in small areas of an adequately heated building where there is a possibility of freezing damage; (b) as a dry pipe extension to a wet pipe or alternate installation in cold stores and in high temperature ovens or furnaces. Clause 11.5.2: the number of sprinklers in any subsidiary extension shall not exceed 100; where more than two subsidiary extensions are controlled by one control valve set, the total number of sprinklers in the subsidiary extensions shall not exceed 250.

3. Design criteria: the area grows

Table 3 (Clause 7.1, EN p.36) - Design criteria for LH, OH and HHP:

The density stays the same and the area of operation grows by 25%. In two classes dry or alternate operation is not permitted at all: LH (where OH1 is used) and OH4 (where HHP1 is used).

On the HHS side, the design criteria are given by Table 4 (roof/ceiling protection only, EN p.38) and Table 5 (with in-rack sprinklers, EN p.39). The NOTE to both tables is the same: dry and alternate systems should be avoided in high hazard storage, particularly with more combustible commodities (higher categories) and higher storage; where a dry or alternate system nevertheless has to be installed, the area of operation should be increased by 25%.

4. Pressure and flow: the pump grows too

The minimum values that the water supply shall provide at each control valve set in pre-calculated LH and OH systems are given by Table 6 (Clause 7.3.1, EN p.39). The rows of the table are paired; alternate operation means moving up one row:

NOTE: ps is the static head loss (bar) arising from the height of the highest sprinkler in the relevant array above gauge 'C' of the control valve set.

In other words, while an OH3 wet installation requires 1 100 l/min @ 1.7+ps and a maximum of 1 350 l/min @ 1.4+ps, the same area as OH3 alternate requires 1 800 l/min @ 2.0+ps and a maximum of 2 100 l/min @ 1.5+ps. Both the flow and the pressure increase.

The pump characteristic is matched by Table 16 (Clause 10.7, EN p.62). For OH3 dry or alternate (and OH4 wet or pre-action) with h ≤ 15 m: nominal data 1.9 bar / 2 650 l/min; characteristic points 3.0 bar / 2 100 l/min and 3.5 bar / 1 800 l/min.

5. Water volume

Table 9 (Clause 9.3.2.2, EN p.47) - Minimum water volume for pre-calculated LH and OH systems (m³):

NOTE: sprinklers in the sprinkler valve room are excluded. h is the height of the highest sprinkler above the lowest sprinkler.

In an OH3 area with h ≤ 15 m, a wet system requires 135 m³ and an alternate system 160 m³.

For reduced capacity tanks, Table 11 (Clause 9.3.4, EN p.49) gives the minimum effective capacity: LH 5 m³; OH1 wet 10 m³; OH1 dry/alternate and OH2 wet 20 m³; OH2 dry/alternate and OH3 wet 30 m³; OH3 dry/alternate and OH4 wet 50 m³; HHP and HHS 70 m³ (in no case less than 10% of the full capacity).

For fully calculated systems, Clause 9.3.2.3 (EN p.48) defines the minimum effective water volume as the product of the maximum demand flow rate (Qmax) and the duration in Clause 8.1.1. The durations in Clause 8.1.1 (EN p.42): LH 30 min, OH 60 min, HHP 90 min, HHS 90 min.

6. Air pressure and frost protection

Clause 11.2.1 (EN p.67): a permanent source of air/inert gas shall be installed to maintain the pressure in the pipework. The installation shall be pressurized so as to fall within the pressure range recommended by the alarm valve supplier. The standard gives no numerical target pressure here.

For frost protection the standard recognises three routes (Clause 11.1.2.1, EN p.66): antifreeze fluid, electrical trace heating, or subsidiary dry pipe or alternate extensions.

• Antifreeze (11.1.2.2, EN p.66): the number of sprinklers in any section of pipework protected by antifreeze fluid shall not exceed 20; where more than two antifreeze sections are controlled by one control valve set, the total shall not exceed 100. The freezing point of the solution shall be below the expected minimum temperature of the area. A backflow prevention device shall be fitted.
• Trace heating (11.1.2.3, EN p.66-67): the system shall be monitored for power supply failure and for failure of the heating element or sensor. Duplicate heating elements shall be provided on unheated pipework, each capable of maintaining the pipe at at least 4 °C. Each trace heating circuit shall be electrically monitored and switched by separate circuits. The tape shall be attached to the face opposite that on which the sprinkler heads are located and shall terminate within 25 mm of the pipe ends. The trace heating tape shall be rated at not more than 10 W/m. All trace heated pipework shall be covered with Euroclass A1 or A2 insulation material at least 25 mm thick with a waterproof covering.

Fixed temperature requirements are given in two places:

• Clause 8.1.3 (EN p.42): the supply pipework and the control valve set shall be maintained at a temperature of at least 4 °C.
• Clause 10.3.3 (EN p.58): the pump set room shall be maintained at at least 4 °C for electric pump sets and at least 10 °C for diesel pump sets.

On the monitoring side, Annex H.2.7 (EN p.154) requires the minimum temperature of the sprinkler valve room and the pump room to be monitored: an indication shall be given if the temperature falls below the required minimum level. H.2.5 (EN p.153) requires pressures to be monitored at the water supplies and downstream of all dry and alternate control valve sets.

7. Pipework and sprinkler orientation

• Clause 17.1.2 (EN p.111): galvanized steel should preferably be used in dry, alternate or pre-action installations.
• Clause 17.1.8 (EN p.112): provision shall be made so that all pipework can be drained; where this cannot be done with the drain valve at the control valve set, additional valves complying with Clause 15.4 shall be fitted. In dry, alternate and pre-action installations, range pipes shall fall at least 0.4% towards the distribution pipe and distribution pipes at least 0.2% towards the relevant drain valve. NOTE: in cold climates where severe freezing conditions are possible, it may also be necessary to provide a fall in wet systems and to increase the fall in dry systems. Range pipes shall be connected to distribution pipes only from the side or from the top.
• Table 39 (Clause 15.4, EN p.108) gives the minimum diameters of drain valves: LH installation 40 mm; OH, HHP or HHS installation 50 mm; subsidiary installation 50 mm; a zone 50 mm; trapped distribution pipes 25 mm for a diameter ≤80 and 40 mm for a diameter >80; trapped range pipes 25 mm; trapped pipework between a dry or subsidiary alarm valve and a subsidiary stop valve installed for test purposes 15 mm.
• Clause 12.1.3 (EN p.70): except where dry pendent type 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: upright sprinklers may be less prone to mechanical damage and to the accumulation of foreign matter in the sprinkler connections, and they facilitate complete drainage of water from the waterways.
• Table 38 (Clause 14.5.1, EN p.106): on sprinkler sensitivities: standard 'A' and special response sprinklers may be used in dry systems and in Type A pre-action; quick response sprinklers are not used in dry systems.

8. Alarms and monitoring

• Clause 16.2.3 (EN p.111): each installation shall be fitted with a low air/gas pressure alarm giving visual and audible warning in accordance with Annex I.
• Annex I Table I.1 (EN p.155): in dry pipe and pre-action systems the low pressure alarm is classified as type B (technical alarm); the water flow alarm is type A. Clause I.2 (EN p.156): signals from water flow alarms and/or alarm valves shall be indicated as a fire alarm (level A); technical faults such as a power failure that could prevent the system operating correctly in a fire shall be indicated as a fault alarm (level B).
• Clause 15.5.1 (EN p.108): 15 mm test valves shall be fitted for testing the hydraulic alarm and the electrical alarm pressure switch: drawing water immediately downstream of the alternate alarm valve and drawing water downstream of the main water supply stop valve and upstream of the alternate alarm valve.

9. Operation and maintenance

Weekly routine (Clause 20.2.2.2, EN p.119): all water and air pressure gauge readings on installations, trunk mains and pressure tanks shall be checked and recorded. The same clause gives a numerical criterion for dry, alternate and pre-action installations: the pressure in the pipework should not fall faster than 1.0 bar per week.

Clause 20.2.2.6 (EN p.120): heating systems that prevent freezing in the sprinkler system shall be checked for correct operation (as part of the weekly routine).

Half-yearly routine (Clause 20.3.3.2, EN p.121): the moving parts of dry alarm valves in dry pipe installations and subsidiary extensions, together with the accelerators and exhausters, shall be exercised in accordance with the supplier's instructions. NOTE: alternate installations do not need to be tested in this way, because they are operated twice a year through the change from wet to dry operation and back again.

Quarterly routine (Clause 20.3.2, EN p.120-121): hazard review (20.3.2.2), cleaning and where necessary replacement of sprinklers (20.3.2.3), corrosion check of pipework and hangers (20.3.2.4), testing of each water supply with each control valve set (20.3.2.5), operation of all stop valves (20.3.2.7) and checking of the flow alarms (20.3.2.8).

Annual routine (Clause 20.3.4, EN p.121-122): testing each pump set under full load conditions (20.3.4.2.1), testing the diesel engine failure to start alarm (20.3.4.3), checking the ball float valves in tanks (20.3.4.4), inspection of pump suction chambers and screens (20.3.4.5).

In addition, Clause 21 (EN p.123): the sprinkler system shall be inspected periodically at least once a year by a competent person; the inspection report shall assess the conformity of the system with this standard in terms of maintenance, operation and adequacy for the risk concerned, and a list of deviations shall be issued.

Field note: An alternate installation does not combine freeze risk with the comfort of a wet system; for part of the year it is a fully dry system and the design is made for that dry period. The decision point is usually a comparison between the cost of keeping the space above 4 °C with local heating and the cost of the larger pump, tank and pipework.

Frequently Asked Questions

What is an alternate (seasonal) installation?

TS EN 12845+A2:2026 Clause 11.3.1, EN p.68: 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 charged with compressed air or inert gas, and the rest of the system upstream of the alarm valve is charged with pressurized water. During the rest of the year the installation operates as a wet pipe installation.

What is the water delivery time in an alternate installation?

Clause 11.3.2 and Table 18, EN p.67-68: the maximum time between the opening of a single sprinkler and the discharge of water shall not exceed 90 s in LH and 60 s in OH and HH. The test is carried out with the remote test valve specified in Clause 15.5.2.

How does dry or alternate operation change the design area?

Table 3, Clause 7.1, EN p.36: the density stays at 5.0 mm/min and the area of operation grows. OH1 goes from 72 m² to 90 m², OH2 from 144 to 180 m² and OH3 from 216 to 270 m². Dry or alternate operation is not permitted for LH, where OH1 is used, nor for OH4, where HHP1 is used. In HHS, the NOTES to Tables 4 and 5 recommend increasing the area of operation by 25% where a dry or alternate system is unavoidable.

By how much does an alternate installation increase the water tank volume?

Table 9, Clause 9.3.2.2, EN p.47: for a height h ≤ 15 m of the highest sprinkler above the lowest sprinkler, the minimum water volume is 135 m³ for an OH3 wet or pre-action system and 160 m³ for an OH3 dry or alternate system. For OH2 at the same height it is 105 m³ wet and 135 m³ dry or alternate.

Which pipework and sprinklers are used in an alternate installation?

Clause 17.1.2, EN p.111: galvanized steel should preferably be used in dry, alternate or pre-action installations. Clause 12.1.3, EN p.70: except where dry pendent sprinklers are used, sprinklers in dry pipe, alternate and pre-action installations shall be upright. Table 38, Clause 14.5.1, EN p.106: quick response sprinklers are not used in dry systems.

References

• TS EN 12845+A2:2026 (EN 12845:2015+A2:2026), Clause 7.1 and Table 3 (EN p.35-36)
• TS EN 12845+A2:2026, Table 4 and NOTE (EN p.38), Table 5 and NOTE 2 (EN p.39)
• TS EN 12845+A2:2026, Clause 7.3.1 and Table 6 (EN p.39)
• TS EN 12845+A2:2026, Clauses 8.1.1 and 8.1.3 (EN p.42)
• TS EN 12845+A2:2026, Clause 9.3.2.2 and Table 9 (EN p.47), Clause 9.3.2.3 (EN p.48)
• TS EN 12845+A2:2026, Clause 9.3.4 and Table 11 (EN p.49)
• TS EN 12845+A2:2026, Clause 10.3.3 (EN p.58), Table 16 (EN p.62)
• TS EN 12845+A2:2026, Clauses 11.1.1, 11.1.2.1, 11.1.2.2, 11.1.2.3 (EN p.66-67)
• TS EN 12845+A2:2026, Clause 11.1.3 and Table 17, Clauses 11.2.1, 11.2.2 and Table 18 (EN p.67)
• TS EN 12845+A2:2026, Clauses 11.3.1 and 11.3.2 (EN p.68)
• TS EN 12845+A2:2026, Clauses 11.4.1.4 and 11.5.1, 11.5.2 (EN p.69)
• TS EN 12845+A2:2026, Clause 12.1.3 (EN p.70)
• TS EN 12845+A2:2026, Clause 14.5.1 and Table 38 (EN p.106)
• TS EN 12845+A2:2026, Clause 15.4 and Table 39, Clause 15.5.1 (EN p.108), Clause 15.5.2 (EN p.109)
• TS EN 12845+A2:2026, Clauses 16.2.3 and 17.1.2 (EN p.111), Clause 17.1.8 (EN p.112)
• TS EN 12845+A2:2026, Clause 20.2.2.2 (EN p.119), 20.2.2.6 and 20.3.2 (EN p.120), 20.3.3.2 and 20.3.4 (EN p.121-122), Clause 21 (EN p.123)
• TS EN 12845+A2:2026, Annex H.2.5 and H.2.7 (EN p.153-154), Annex I Table I.1 and I.2 (EN p.155-156)

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