EN 12845 Dry Pipe Installations

The design logic of an air-charged sprinkler installation, water transit time, and the compressor error we see most often on site.

A single-storey logistics facility had an open car park canopy; because a wet pipe system would be at risk of freezing in cold weather, that part had to be separated as a dry pipe extension. At commissioning, opening the remote test valve produced water at the end after 78 seconds. That is above the limit for OH2, and the system was rejected. The cause is one we see constantly: the compressor had been sized on sprinkler count rather than on internal pipe volume.

This article brings the field logic of the dry pipe clause, the water transit time calculation, the 25 % area increase and compressor selection into one frame. The subject looks small, but design area, tank volume and the commissioning test all derive from it.

What a dry pipe installation is

The definition is clear: pressurised water upstream of the dry alarm valve, and pressurised air or inert gas downstream of it on the sprinkler side. When a sprinkler opens, the air in the pipe escapes; the pressure drop trips the dry alarm valve and water fills the system and discharges from the open head. There is therefore a delay — the dry side must vent its air first.

The standard permits this system in only two circumstances:

• Freezing risk — spaces where ambient temperature can fall below zero (open car parks, cold stores, unheated detached stores, external canopies).
• High temperature — spaces where ambient exceeds 70 °C, such as drying oven galleries, where holding water in a head whose thermal element has not yet operated is hazardous.

Outside those two conditions a dry system may not be selected. Making a "heated but long" corridor dry is outside the standard, because the primary purpose of the clause is freeze and high-temperature protection, not hydraulic convenience.

Water transit time: the 60/90 s threshold

The one concrete performance criterion for the dry side is the time from a single sprinkler opening to water discharge:

Hazard class	Max. water transit time	Test point
LH	90 s	Remote test valve
OH (1–4)	60 s	Remote test valve
HH	60 s	Remote test valve

That time depends on four things: dry alarm valve response, system internal volume (pipe size × length), air pressure, and whether an accelerator or exhauster is fitted. The standard notes that with an accelerator, the threshold is achievable in practice provided the system volume stays under 4 m³ for LH and OH, or 3 m³ for HH. A quick rule for estimating volume:

DN	Litres per metre of pipe
DN 25	0.55
DN 32	0.90
DN 40	1.38
DN 50	2.15
DN 65	3.80
DN 80	5.30
DN 100	8.60
DN 150	19.60

Example: in a 1200 m² OH2 area with 90 m of DN100 main, 220 m of DN65 distribution, 380 m of DN50 branches and 540 m of DN32 range pipes, total internal volume is about 0.86 + 0.84 + 0.82 + 0.49 = 3.01 m³ — below the 4 m³ threshold, so the 60 s OH limit is achievable with an accelerator. Once volume reaches 5–6 m³, subdividing with a second dry alarm valve becomes mandatory.

Design area: the 25 % increase

The most critical effect on site is that the area of operation for a dry system must be increased by at least 25 % over the wet equivalent. The logic: during the air venting delay the fire continues to grow and further heads open, so by the time water arrives, more than the first head has already operated. The hydraulic calculation must cover that enlarged area.

Read directly from the table:

Class	Density (mm/min)	Wet area (m²)	Dry / alternate area (m²)
LH	2.25	84	84 (no increase required for LH)
OH1	5.0	72	90
OH2	5.0	144	180
OH3	5.0	216	270
OH4	5.0	360	450
HHP1	7.5	260	325
HHP2	10.0	260	325

Because tank volume derives from that area, a dry OH3 system typically needs 25–35 m³ more storage than its wet equivalent. Signing off a dry decision without running that cost means the project later collides with the plant room layout.

The high-hazard storage warning

The notes are clear: dry and alternate systems are strongly discouraged for high-hazard storage. Again, the reason is transit delay. Fire growth in high racking is rapid, and a 30–60 s delay eliminates the suppression advantage of the first ESFR or CMSA heads to open. For cold store HHS projects the alternatives are:

• Converting the area to a heated compartment and using a wet system — usually the most economical.
• A glycol-filled wet system, for small extensions.
• A wet main installation with a subsidiary dry pipe extension — up to 100 heads, or 250 across several extensions.

Where dry is unavoidable in HHS, the 25 % area increase is not negotiable and the reasoning should be written into the report.

The air compressor: the number one field error

The standard requires the system to be pressurised within the alarm valve supplier's pressure band, with a permanent air or inert gas supply maintaining it continuously. That word "permanent" is frequently represented on site by a small 24 L garage compressor — which is wrong.

Practical rules for compressor selection:

• It must fill the system's internal volume to the working pressure (typically 2.5–3.5 bar) within 30 minutes.
• It must run from an automatic pressure switch; manual starting is not permitted.
• Size it with a receiver so that it does not start more than about six times an hour — frequent starting burns out the motor.
• The compressor and receiver room must be warm and dry. Running a compressor in a cold space carries moisture into the system, where the air freezes and the valve sticks.

When transit time exceeds the limit at commissioning, the dry alarm valve is usually blamed first; in reality about 70 % of cases start with low air pressure from an undersized compressor, or an air leak. A 24-hour pressure log before the test exposes it immediately: any 24-hour period losing more than 0.2 bar means a leak or an inadequate compressor.

Common field errors

• Pipe fall omitted. All mains in a dry system must be pitched for drainage — a minimum of 0.4 % for dry. Without fall, condensate collects, freezes in autumn, and ice blocks the pipe when a head opens.
• Wet and dry pipework mixed. Treating the end branches of a wet installation as dry once they enter a cold space is outside the subsidiary extension rules; that main either stays wet with freeze protection, or connects to an independent dry alarm valve.
• Design area not increased. Carrying an OH3 wet calculation (216 m²) straight into a dry system invalidates the hydraulic report; it must be recalculated at 270 m².
• Accelerator omitted. Above 4 m³ of system volume, the 60 s limit will not be met without an accelerator or exhauster.
• No remote test point. The remote test valve must be at the hydraulically most remote point; a "test" performed beside the dry alarm valve does not measure real transit time.

Comparison with NFPA

NFPA 13 expresses the equivalents differently but on the same logic: a 60 s water delivery time for dry pipe systems above about 1900 L of volume, and a 30 % design area increase. EN 12845 is slightly more relaxed at 60 s and 25 % for OH and HH, and permits 90 s for LH, which does not map exactly onto the NFPA light hazard definition. On mixed projects — multinational logistics brands, facilities insured through international programmes — establish which standard governs in advance and calculate to those thresholds. Picking the more lenient clause from each standard and writing that into the report will be rejected at audit.

Link to Turkish regulation

BYKHY requires sprinkler systems to be designed to standard and accepts TS EN 12845. Projects specifying "wet" in cold stores, open car parks and external canopies commonly need a revision within six months because of freezing. Checking room temperatures against the 4 °C rule during design is the first filter for the dry-versus-wet decision. Where glycol is expected in a cold store, remember that concentration is capped at about 50 %, giving protection to around −25 °C; projects seeing −30 °C make a dry system unavoidable.

Frequently asked questions

Where may a dry pipe installation be used?

Only in two situations: spaces at risk of freezing (open car parks, cold stores, external canopies, unheated stores), or spaces where ambient temperature exceeds 70 °C, such as drying ovens.

What is the maximum water transit time?

From a single sprinkler opening to discharge: no more than 90 s for LH and 60 s for OH and HH, measured at the remote test valve.

Why is the design area increased?

Because further sprinklers may open during the air venting delay. The standard requires the area of operation to be increased by at least 25 % for dry and alternate systems — OH1 goes from 72 to 90 m².

How is compressor capacity selected?

The compressor must fill the entire internal volume to the supplier's working pressure within 30 minutes. Internal pipe volume governs, not the number of sprinklers.

Is a dry system suitable for high-hazard storage?

No. The standard strongly discourages dry and alternate systems there, because water delay severely reduces the effectiveness of the first heads to open. Where unavoidable, the design area is still increased by 25 %.

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