EN 12845 Wet Pipe Installations: Clause 11.1 and the Limits of Table 17

Source: TS EN 12845+A2:2026 (EN 12845:2015+A2:2026), Clause 11.1, EN p. 66-67. Every figure and every reference in this article is taken directly from the text of the standard; no value that is not in the standard is given here.

The wet pipe installation is the default installation type of EN 12845. The definition in Clause 11.1.1 is simple: "Except as covered by 11.1.2, wet pipe installations are permanently charged with water under pressure." The limits the standard places on this installation type are fewer than most people assume, and most site arguments arise from numbers that are attributed to the standard without being in it.

Clause 11.1.1: the condition of use is two sentences

Clause 11.1.1 (EN p. 66) sets only three provisions for wet pipe installations:

1. The installation is permanently charged with water under pressure.
2. Wet pipe installations are installed only in areas where there is no possibility of freezing damage to the installation and where the ambient temperature will not exceed 95 °C.
3. For grid and loop systems only wet pipe installations are used.

The point to watch here is this: there is no 4 °C lower limit in 11.1.1. The standard says "no possibility of freezing damage" and gives no figure. The value of 4 °C appears in two separate places:

• Clause 8.1.3 (EN p. 42): "The supply pipe and the control valve set shall be maintained at a minimum temperature of 4 °C."
• Clause 11.1.2.3 (EN p. 66): each trace heating element shall be able to maintain the pipe at not less than 4 °C.

The grid and loop restriction has to be read together with Clause 13.1 (EN p. 82): in gridded or looped arrangements the pipe sizes are always determined by full hydraulic calculation, and the pre-calculated route cannot be used.

Clause 11.1.2: three routes for sections at risk of freezing

Clause 11.1.2.1 (EN p. 66) lists three options for installation sections that may be exposed to freezing: antifreeze liquid, electrical trace heating, or a subsidiary dry pipe or alternate extension (see Clause 11.5).

Antifreeze (Clause 11.1.2.2)

Electrical trace heating (Clause 11.1.2.3)

This is the most detailed list in the standard (EN p. 66-67):

• The system is monitored for power supply failure and for failure of the heating element or sensor (Annex I).
• The pipework is provided with Euroclass A1 or A2 insulation.
• Duplicate heating elements are fitted on unheated pipework; each element alone shall be able to maintain the pipe at not less than 4 °C.
• Each trace heating circuit is electrically monitored and switched by separate circuits.
• Trace heating tapes shall not cross over one another.
• The tape is affixed on the opposite side of the pipe to the sprinkler heads.
• The tape terminates within 25 mm from the pipe ends.
• All trace heated pipework is lagged with Euroclass A1 or A2 insulating material at least 25 mm thick and a waterproof covering; all ends are sealed to prevent the ingress of water.
• The maximum rating of the tape is 10 W/m.

Clause 11.1.3 and Table 17: area per alarm valve

Table 17 (Clause 11.1.3, EN p. 67) gives the maximum protected area that a single wet alarm valve (and a pre-action alarm valve) may control. Sprinklers on subsidiary extensions are included in that area.

Where a subsidiary extension is installed, Clause 11.5.2 (EN p. 69) adds a separate numerical limit: the number of sprinklers on any subsidiary extension shall not exceed 100; where a single control valve set controls more than two subsidiary extensions, the total number of sprinklers shall not exceed 250.

The privileges of the wet installation in other clauses

The standard sets the wet installation apart explicitly in several places:

• Copper pipe: Clause 17.1.9 (EN p. 113) permits copper pipe only in wet pipe systems, only for LH, OH1, OH2 and OH3, and only downstream of steel pipework.
• Flow switch: Clause 16.2.2 (EN p. 110) states that water flow alarm switches are used only in wet installations.
• Slope: Clause 17.1.8 (EN p. 112) requires the minimum fall of 0.4 % for range pipes and 0.2 % for distribution pipes only for dry, alternate and pre-action installations. In a wet system a fall is recommended at NOTE level only, for cold climates where severe freezing is possible.
• Sprinkler orientation: Clause 12.1.3 (EN p. 70) requires sprinklers in dry, alternate and pre-action installations to be upright (except where dry pendent sprinklers are used). There is no such restriction for wet installations.
• Water delivery time: the maximum water delivery time in Table 18 (LH 90 s; OH and HH 60 s, EN p. 67) belongs to dry and alternate installations; no time requirement is defined for a wet installation.

Duration of supply and design criteria

The wet installation takes its design density and area of operation from Clause 7. Table 3 (EN p. 36) gives the following values for wet or pre-action systems: LH 2.25 mm/min × 84 m²; OH1 5.0 mm/min × 72 m²; OH2 5.0 mm/min × 144 m²; OH3 5.0 mm/min × 216 m²; OH4 5.0 mm/min × 360 m²; HHP1 7.5 mm/min × 260 m²; HHP2 10.0 mm/min × 260 m²; HHP3 12.5 mm/min × 260 m². Dry or alternate installations are not permitted in class LH; the OH1 criteria are used instead.

Duration of supply, Clause 8.1.1 (EN p. 42): LH 30 min, OH 60 min, HHP 90 min, HHS 90 min.

Commissioning and trapped air

Clause 19.1.1.2 (EN p. 117): all installation pipework is tested hydrostatically for at least 2 hours at not less than 15 bar or 1.5 times the maximum pressure to which the system will be subjected, whichever is the greater; both values are measured at the installation control valves.

Clause 15.6 (EN p. 109), while dealing with flushing connections, adds two warnings specific to wet installations: fully water filled pipework can be damaged by the pressure rise caused by a temperature increase; and where a gridded arrangement has flushing connections at the ends, complete expulsion of the air is likely, so the fitting of a pressure relief valve is to be considered. The NOTE at the end of the clause states that excessive trapped air can lead to unacceptable alarm operating times and to hydraulic imbalance in gridded arrangements.

Field note: taken together, these two provisions mean that air venting at the end points of wet gridded installations belongs on the commissioning check list. The standard gives no numerical acceptance criterion for that check.

Frequently Asked Questions

In which areas does EN 12845 require a wet pipe installation?

TS EN 12845+A2:2026 Clause 11.1.1 (EN p. 66): a wet pipe installation is permanently charged with water under pressure and is installed only in areas where there is no possibility of freezing damage to the installation and where the ambient temperature will not exceed 95 °C. The same clause permits only wet pipe installations for grid and loop systems. There is no 4 °C lower limit in the text of 11.1.1; the 4 °C requirement is given for the supply pipe and the control valve set in Clause 8.1.3 (EN p. 42) and for electrical trace heating in Clause 11.1.2.3 (EN p. 66).

What is the largest area a single wet alarm valve may control?

Clause 11.1.3 and Table 17 (EN p. 67): the maximum protected area controlled by a single wet alarm valve is 10 000 m² for LH, 12 000 m² for OH (except as allowed in Annexes D and F) and 9 000 m² for HH. That area includes any sprinklers on subsidiary extensions connected to the installation.

How many sprinklers may be protected by antifreeze?

Clause 11.1.2.2 (EN p. 66): the number of sprinklers in any pipe section protected by antifreeze liquid shall not exceed 20. Where a single control valve set controls more than two antifreeze sections, the total number of sprinklers in those sections shall not exceed 100. The freezing point of the solution shall be below the expected minimum temperature of the area, the specific gravity shall be checked with a suitable hydrometer, and a backflow prevention device shall be fitted to prevent contamination of the water.

On what conditions is electrical trace heating accepted?

Clause 11.1.2.3 (EN p. 66-67): the heating system is monitored for power supply failure and for failure of the heating element or sensor; duplicate heating elements are fitted on unheated pipework and each element alone shall be able to maintain the pipe at not less than 4 °C. Each circuit is monitored and switched by separate circuits, tapes shall not cross over one another, the tape is affixed on the opposite side of the pipe to the sprinkler heads and terminates within 25 mm from the pipe ends. All trace heated pipework is lagged with Euroclass A1 or A2 material at least 25 mm thick and a waterproof covering, all ends are sealed, and the maximum rating of the tape is 10 W/m.

What is the commissioning pressure test for a wet installation?

Clause 19.1.1.2 (EN p. 117): all installation pipework is tested hydrostatically for at least 2 hours at not less than 15 bar or 1.5 times the maximum pressure to which the system will be subjected, whichever is the greater, both measured at the installation control valves. Defects such as permanent deformation, rupture or leakage are corrected and the test is repeated.

Why is trapped air a problem in a wet installation?

The NOTE at the end of Clause 15.6 (EN p. 109): excessive trapped air in a wet pipe installation can lead to unacceptable alarm operating times and can create hydraulic imbalance in gridded pipe arrangements; measures to reduce the volume of trapped air in the installation may be provided. The same clause states that fully water filled pipework can be damaged by the pressure rise caused by a temperature increase, and that in such cases the fitting of a pressure relief valve is to be considered.

References

• TS EN 12845+A2:2026 Clause 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, Table 17 (EN p. 67)
• TS EN 12845+A2:2026 Clause 11.2.2, Table 18 (EN p. 67)
• TS EN 12845+A2:2026 Clause 11.5.2 (EN p. 69)
• TS EN 12845+A2:2026 Clause 8.1.1, 8.1.3 (EN p. 42)
• TS EN 12845+A2:2026 Clause 7.1, Table 3 (EN p. 35-36)
• TS EN 12845+A2:2026 Clause 12.1.3 (EN p. 70)
• TS EN 12845+A2:2026 Clause 13.1 (EN p. 82)
• TS EN 12845+A2:2026 Clause 15.6 and its closing NOTE (EN p. 109)
• TS EN 12845+A2:2026 Clause 16.2.2 (EN p. 110)
• TS EN 12845+A2:2026 Clause 17.1.8, 17.1.9 (EN p. 112-113)
• TS EN 12845+A2:2026 Clause 19.1.1.2 (EN p. 117)

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