EN 12845 Dry Pipe Installations

A dry pipe installation is a trade-off devised for the cases where a wet installation would be exposed to frost: the pipework does not freeze, but the water arrives late. TS EN 12845+A2:2026 tightens that trade-off in three places: where it may be installed, how quickly it has to deliver water, and by how much the design area grows. Source: TS EN 12845+A2:2026, Clause 11.2, Table 18, Table 3 and Clause 17.1.8.

Definition and condition of use

Clause 3.36 (TS p.21 / EN p.18): a dry (pipe) installation is an installation in which the pipework is filled with compressed air or inert gas. Clause 11.2.1 (TS p.70 / EN p.67) expands on this: dry pipe installations are normally filled with compressed air or inert gas downstream of the dry alarm valve and with pressurized water upstream of it.

The same clause sets two requirements:

• A permanent air/inert gas supply shall be provided to maintain the pressure in the pipework. The installation shall be pressurized so that it falls within the pressure range recommended by the alarm valve supplier.
• Dry pipe installations shall be installed only where there is a possibility of frost damage or where the temperature exceeds 70 °C, for example in drying ovens.

For comparison, Clause 11.1.1 (TS p.69 / EN p.66): wet installations are installed only where there is no possibility of frost damage and where the ambient temperature will not exceed 95 °C; on gridded and looped systems only wet installations may be used.

The text of the standard contains no provision requiring the compressor to fill the system within a given number of minutes, no mandatory automatic pressure switch and no hourly cycle count; the only requirements are the permanent supply and the supplier's pressure band.

What limits the size of the installation: Table 18

On wet and pre-action installations the size limit is given by the protected area in Table 17 (LH 10 000 m², OH 12 000 m², HH 9 000 m²). On a dry pipe installation the limit is not area but time.

Clause 11.2.2 (TS p.70 / 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 shall be carried out with the remote test valve in Clause 15.5.2.

Table 18 applies equally to alternate installations (Clause 11.3.2). Clause 11.2.2 also strongly recommends that dry and alternate installations not be used for HHS applications; the delayed arrival of water at the first operating sprinklers can seriously impair the effectiveness of the system.

The NOTE to the clause gives a practical size figure: experience has shown that, with an accelerator, volumes not exceeding 4 m³ for LH or OH and 3 m³ for HH hazards are expected to meet the requirement of Clause 11.2.2. That volume is the only numerical size measure the standard itself gives; there are no tables in the standard derived from litre/metre pipe calculations.

Design criteria: an area of operation 25 per cent larger

Table 3 (Clause 7.1, TS p.39 / EN p.36) gives the design density and area of operation for LH, OH and HHP, separated into wet/pre-action and dry/alternate:

The rate of increase is constant: 72 x 1.25 = 90; 144 x 1.25 = 180; 216 x 1.25 = 270; 260 x 1.25 = 325. Two rows prohibit dry installations outright: there is no dry or alternate installation in LH (move to OH1) and there is no dry or alternate installation in OH4 (move to HHP1).

On the HHS side, the NOTES to Table 4 and Table 5 (TS p.41-42 / EN p.38-39): dry and alternate systems should be avoided in high hazard storage, particularly for the more flammable products and high stacks; where such a system nevertheless has to be installed, the area of operation should be increased by 25 per cent.

Water supply: flow, pressure and volume

Table 6 (Clause 7.3.1, TS p.42 / EN p.39) gives the flow and pressure to be provided at the control valve set for pre-calculated LH and OH systems. The table consists of two column pairs: (flow / pressure) and (maximum demand flow rate / pressure). The dry and alternate classes are grouped in the same row as the next wet class up:

NOTE: ps is the static head loss, in bar, due to the height of the highest sprinkler in the relevant arrangement above the 'C' gauge of the control valve set. There is no OH4 dry/alternate row in Table 6, because Table 3 already prohibits that case.

Water volume, Table 9 (TS p.50 / EN p.47): for h ≤ 15 m, 105 m³ for the OH1 dry/alternate and OH2 wet group; 135 m³ for the OH2 dry/alternate and OH3 wet group; 160 m³ for the OH3 dry/alternate and OH4 wet group. For OH4 dry/alternate the table says "use HH protection".

HHP/HHS, Table 10 (TS p.51 / EN p.48): at 7.5 mm/min, 225 m³ for a wet system against 280 m³ for a dry system; at 30.0 mm/min, 1 090 m³ instead of 875 m³.

Reduced capacity tank, Table 11 (TS p.52 / EN p.49): OH1 dry/alternate 20 m³, OH2 dry/alternate 30 m³, OH3 dry/alternate 50 m³.

Equipment rules

• Sprinkler orientation, Clause 12.1.3 (TS p.73 / EN p.70): except where dry pendent type sprinklers are used, sprinklers on dry pipe, alternate and pre-action installations shall be upright. Upright sprinklers are fitted with their yoke arms parallel to the pipe. The NOTE to the clause states that the upright orientation facilitates the complete drainage of water from the sprinkler waterways.
• Sensitivity, Table 38 (Clause 14.5.1, TS p.109 / EN p.106): the column for dry systems and type A pre-action systems reads "No" for Quick response; Standard 'A' and Special are used in that column.
• Pipe material, Clause 17.1.2 (TS p.114 / EN p.111): galvanized steel should preferably be used for dry, alternate or pre-action installations.
• Slope, Clause 17.1.8 (TS p.115 / EN p.112): on dry, alternate and pre-action installations, range pipes shall slope at least 0.4 per cent towards the distribution pipe and distribution pipes at least 0.2 per cent towards the relevant drain valve. Range pipes are connected to distribution pipes only from the side or from the top. The NOTE to the clause adds that these slopes may need to be increased in cold climates where severe frost conditions are possible.
• Pressure gauge, Clause 15.7.3 (TS p.112-113 / EN p.109-110): the 'B' gauge on dry alarm valves shall be provided with an indicator showing the maximum pressure reached.
• Test valve, Clause 8.5.3 (TS p.48 / EN p.45): an additional flow test valve arrangement facilitating informal supply pressure tests may be fitted to dry or alternate control valve sets downstream of the main stop valve; this valve and pipe are of 40 mm nominal diameter on LH installations and 50 mm on others.
• Frost protection, Clause 8.1.3 (TS p.45 / EN p.42): the feed pipe and the control valve set shall be kept at a minimum of 4 °C. The pipework of a dry installation does not freeze, but the valve set is still on the wet side.

Subsidiary dry extensions

Rather than making the whole building dry, it is possible to leave only the cold section dry. Clause 11.5.1 (TS p.72 / EN p.69) limits these extensions to two cases:

• a) as a dry pipe or alternate extension to a wet installation in small areas where there is a possibility of frost damage in an otherwise adequately heated building;
• b) as a dry pipe extension to a wet or alternate installation in cold stores and in high temperature ovens or stoves.

Clause 11.5.2 limits the size: the number of sprinklers on any subsidiary extension shall not exceed 100. Where a control valve set controls more than two subsidiary extensions, the total number of sprinklers on the subsidiary extensions shall not exceed 250.

Commissioning, monitoring and maintenance

• Clause 19.1.1.1 (TS p.120 / EN p.117): dry pipework shall be tested pneumatically at a pressure of at least 2.5 bar for at least 24 hours. Any leak causing a pressure loss of more than 0.15 bar in 24 hours shall be corrected. Where climatic conditions do not allow the hydrostatic test of Clause 19.1.1.2 to be carried out immediately, that test should be carried out as soon as conditions permit.
• Clause 19.1.1.2: 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 the greater.
• Clause 20.2.2.2 a) (TS p.122 / EN p.119): in the weekly routine, all water and air pressure gauge readings on installations, trunk mains and pressure tanks are checked and recorded. The pressure in the pipework of dry, alternate and pre-action installations should not fall faster than 1.0 bar per week.
• Clause 20.3.3.2 (TS p.124 / EN p.121): at intervals not exceeding six months, the moving parts of the dry alarm valves on dry pipe installations and subsidiary extensions, together with any accelerators and exhausters, are exercised in accordance with the supplier's instructions. NOTE: alternate installations do not need to be tested in this way, since they are already exercised by the two wet-dry changeovers per year.
• Clause 16.2.3 (TS p.114 / EN p.111): every dry and pre-action installation is provided with a low air/gas pressure alarm giving visual and audible warning in accordance with Annex I.

Field note: the real cost of a dry installation lies less in the pipework itself than in the 25 per cent growth of the design area and the resulting chain of pump, tank and pipe diameter; where the frost risk affects only a few rooms, the subsidiary extension of Clause 11.5 usually produces a smaller system.

Frequently Asked Questions

In which cases may a dry pipe installation be used?

Clause 11.2.1 (TS p.70 / EN p.67): dry pipe installations shall be installed only where there is a possibility of frost damage or where the temperature exceeds 70 °C, for example in drying ovens. A permanent air/inert gas supply shall be provided to maintain the pressure in the pipework, and the installation shall be pressurized within the pressure range recommended by the alarm valve supplier.

How many seconds after a sprinkler opens must water arrive?

Clause 11.2.2 and Table 18 (TS p.70 / 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 with the remote test valve specified in Clause 15.5.2. The NOTE to Table 18 states that, with an accelerator, volumes not exceeding 4 m³ for LH or OH and 3 m³ for HH are expected to meet this requirement.

By how much does the area of operation grow on a dry system?

Table 3 (TS p.39 / EN p.36) gives the dry or alternate column directly: OH1 90 m² instead of 72, OH2 180 m² instead of 144, OH3 270 m² instead of 216, HHP1-HHP3 325 m² instead of 260. That is a 25 per cent increase. The LH row states 'Not permitted, use OH1' for dry/alternate and the OH4 row states 'Not permitted, use HHP1'. On the HHS side, the NOTES to Table 4 and Table 5 recommend increasing the area of operation by 25 per cent where such a system is unavoidable.

What pipe slope is required on a dry pipe installation?

Clause 17.1.8 (TS p.115 / EN p.112): on dry, alternate and pre-action installations, range pipes shall slope at least 0.4 per cent towards the distribution pipe and distribution pipes at least 0.2 per cent towards the relevant drain valve. The same clause requires range pipes to be connected to distribution pipes only from the side or from the top.

What pressure loss limits apply on a dry installation?

There are two separate provisions. Commissioning: Clause 19.1.1.1 (TS p.120 / EN p.117) requires dry pipework to be tested pneumatically at a pressure of at least 2.5 bar for at least 24 hours, and any leak causing a pressure loss of more than 0.15 bar in 24 hours to be corrected. Operation: Clause 20.2.2.2 a (TS p.122 / EN p.119) states that on dry, alternate and pre-action installations the pressure in the pipework should not fall faster than 1.0 bar per week.

How many sprinklers may a subsidiary dry extension have?

Clause 11.5.2 (TS p.72 / EN p.69): the number of sprinklers on any subsidiary extension shall not exceed 100. Where a control valve set controls more than two subsidiary extensions, the total number of sprinklers on the subsidiary extensions shall not exceed 250.

References

• TS EN 12845+A2:2026 Clause 3.36 (TS p.21 / EN p.18)
• TS EN 12845+A2:2026 Clause 7.1, Table 3 (TS p.38-39 / EN p.35-36)
• TS EN 12845+A2:2026 NOTE to Table 4 (TS p.41 / EN p.38), Table 5 NOTE 2 (TS p.42 / EN p.39)
• TS EN 12845+A2:2026 Clause 7.3.1, Table 6 (TS p.42 / EN p.39)
• TS EN 12845+A2:2026 Clause 8.1.3 (TS p.45 / EN p.42), Clause 8.5.3 (TS p.48 / EN p.45)
• TS EN 12845+A2:2026 Table 9 (TS p.50 / EN p.47), Table 10 (TS p.51 / EN p.48), Table 11 (TS p.52 / EN p.49)
• TS EN 12845+A2:2026 Clause 11.1.1, Table 17 (TS p.69-70 / EN p.66-67)
• TS EN 12845+A2:2026 Clause 11.2.1, Clause 11.2.2, Table 18 (TS p.70 / EN p.67)
• TS EN 12845+A2:2026 Clause 11.3.2 (TS p.71 / EN p.68), Clause 11.5.1, Clause 11.5.2 (TS p.72 / EN p.69)
• TS EN 12845+A2:2026 Clause 12.1.3 (TS p.73 / EN p.70)
• TS EN 12845+A2:2026 Clause 14.5.1, Table 38 (TS p.109 / EN p.106)
• TS EN 12845+A2:2026 Clause 15.5.2 (TS p.112 / EN p.109), Clause 15.7.3 (TS p.112-113 / EN p.109-110)
• TS EN 12845+A2:2026 Clause 16.2.3 (TS p.114 / EN p.111)
• TS EN 12845+A2:2026 Clause 17.1.2 (TS p.114 / EN p.111), Clause 17.1.8 (TS p.115 / EN p.112)
• TS EN 12845+A2:2026 Clause 19.1.1.1, Clause 19.1.1.2 (TS p.120 / EN p.117)
• TS EN 12845+A2:2026 Clause 20.2.2.2 (TS p.122 / EN p.119), Clause 20.3.3.2 (TS p.124 / EN p.121)

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