EN 12845 Water Flow Alarm and Water Motor Gong

If the water motor gong does not sound, the installation is not handed over: the water motor gong, pressure switches, the retard chamber and the field errors around them.

During commissioning at a logistics warehouse we opened the wet alarm valve and waited for the gong — not a single click came from it. The panel LED lit and the building management system received the signal, but the cast iron gong on the outside wall stayed silent. Stripping it revealed a limescale blockage in the motor nozzle and a dried wasp nest stuck to a vane of the wheel. EN 12845 Clause 16.1 requires, in addition to the electrical remote alarm, a mechanical alarm driven by the water itself; this article covers that clause and what it means on site.

What the clause requires

The standard sets this out in three sub-clauses (TS EN 12845+A2:2026, TSE p.113 / EN p.110):

• 16.1.1 General: each control valve set shall be provided with a water motor alarm in accordance with EN 12259-4 and an electrical device for remote alarm indication, both located as close as possible to the alarm valve. A single alarm motor and gong may be common to a group of wet alarm valves in the same valve room, provided an indicator is fitted to each alarm valve to show when it is operating. Each gong shall be prominently marked with the number of the installation.
• 16.1.2 Water motor and gong: the water motor shall be installed so that the gong is on the outside of an exterior wall, with its centre line not higher than 6 m above the point of connection to the alarm valve. A strainer, readily accessible for cleaning, shall be fitted between the motor nozzle and the alarm valve connection, and the water outlet shall be arranged so that any flow can be seen.
• 16.1.3 Piping to the water motor: 20 mm galvanised steel or non-ferrous metal pipe, with an equivalent length between the alarm valve and the water motor of no more than 25 m, each change of direction counted as 2 m. The pipe shall have a stop valve within the premises and a permanent drain through an orifice of no more than 3 mm diameter; the orifice plate shall be stainless steel or non-ferrous.

The installation number on the gong tells anyone outside which installation is sounding — essential in a valve room with several valves.

How a water motor gong works

When the alarm valve opens, its clapper lifts and pressurised water from the alarm port travels through the 20 mm pipe to the nozzle in the motor body outside. The nozzle directs a jet tangentially onto a bucket (Pelton type) wheel; the wheel turns, and a hammer on its shaft strikes the inside of the gong. The gong runs on the energy of the water alone and does not depend on the panel, batteries or mains power. EN 12845 requires this mechanical alarm in addition to the electrical remote alarm device (16.1.1).

The water leaving the motor discharges visibly; the requirement of Clause 16.1.2 that any flow can be seen both simplifies testing and reveals a blocked nozzle early. Nozzle size and motor flow depend on EN 12259-4 and manufacturer data; EN 12845 gives no values for them.

Position: exterior wall, visibility and the 6 m rule

Hanging the gong inside the valve room and saying "you'll hear it anyway" is common on site. EN 12845 does not allow it:

• The gong always goes on the outside of an exterior wall (16.1.2).
• Choose an elevation where the fire service and site personnel can clearly hear and see it; the standard gives no further criterion for the choice of elevation.
• Vertical distance from the alarm valve connection to the gong centre: no more than 6 m. The standard gives no rationale; the higher the gong, the lower the pressure at the nozzle.
• The stop valve on the line stays within the premises (16.1.3), so it cannot be tampered with or closed by accident from outside.

Pipework: 20 mm, 25 m equivalent, drain orifice

Clause 16.1.3 sets the rules for the gong line:

Parameter	EN 12845 value	Site note
Pipe size	20 mm	Galvanised steel or non-ferrous metal; black steel blocks with corrosion
Max. equivalent length	25 m	Each change of direction counts 2 m; for example four elbows (4 × 2 m) plus 17 m of straight pipe is exactly at the limit
Stop valve	Within the premises, on the gong line	Needed for maintenance; kept sealed and labelled open
Permanent drain orifice	≤ 3 mm, stainless steel or non-ferrous	Empties the line after an alarm, reducing the freezing risk
Strainer	Between nozzle and alarm valve connection, readily accessible for cleaning (16.1.2)	Fitted with a union or flange so it can be stripped and cleaned after each test
Height	Max. 6 m above the alarm valve connection	Exceeding it lowers the pressure at the nozzle

With the alarm valve closed, the gong pipe is not under pressure. The 3 mm orifice drains the water left in the line after an alarm or test, so the pipe normally stays empty and the freezing risk falls. The standard requires the orifice plate to be stainless steel or non-ferrous; a black steel plate enlarged by corrosion lets too much of the alarm water escape through the drain, and the gong sounds weakly.

Pressure switches: complementary, not alternative

Clause 16.1.1 says "and": a water motor alarm and an electrical remote alarm device. Clause 16.2.1 accepts two kinds of electrical device: water flow switches conforming to EN 12259-5, or pressure switches.

• Water flow switch: wet installations only (16.2.2). A test connection with a drain is fitted downstream of each switch to simulate the operation of a single sprinkler; the pressure/flow characteristic of the fully opened test valve and draw-off pipe shall equal that of the smallest nominal bore sprinkler supplied through the switch. The draw-off pipe is galvanised steel or copper.
• Pressure switch: senses the pressure rise at the alarm port of the alarm valve; since flow switches may only be used in wet installations, this is the electrical option on dry, alternate and pre-action installations.

In a typical installation, a retard chamber (where fitted) and then a pressure switch connect to the wet alarm valve's alarm port, with the gong pipe fed from the same line. The gong is mechanical, the pressure switch electrical: if one fails, the other still signals.

The retard chamber

The EN 12845 text contains no requirement for a retard chamber. Wet alarm valve manufacturers supply one to filter the false alarms caused by sudden fluctuations in supply pressure. How it works:

1. When the alarm port opens, water first begins to fill the retard chamber.
2. If the flow is real, the chamber fills and water passes through the overflow outlet to the gong and pressure switch line.
3. If it is a momentary pressure surge, a calibrated drain hole beneath the chamber empties it and no alarm is given.

The delay depends on the manufacturer and the model; a delay that is too long holds back the alarm in a real fire. NFPA 13-2025 16.11.4 requires a retarding device on each alarm check valve used under conditions of variable water pressure; EN 12845 has no equivalent provision.

Field errors

The four errors I see most often in commissioning reports:

• A wasp nest inside the gong. Common where the discharge opening is not meshed; at the annual test the wheel does not turn, or barely does. The fix: a stainless wire mesh over the discharge.
• A strainer that has never been cleaned. Limescale and debris partially block the nozzle; the gong rings, but weakly. The standard requires the strainer to be readily accessible for cleaning (16.1.2) — which is why a union or flange connection is practical.
• A black steel drain orifice enlarged by corrosion. If the 3 mm hole opens out, part of the alarm water escapes through the drain and the gong sounds weakly. Check the diameter with a gauge at annual maintenance.
• 15 mm pipe instead of 20 mm. When the installer runs half-inch galvanised because it is cheaper, a long line rings weakly or not at all. The requirement is unambiguous: 20 mm.

Systems above 12 bar: Annex E.2.5

In high rise systems (Annex E) pressures can exceed 12 bar. Annex E.2.5 first requires pipework, fittings, valves and other equipment to withstand the maximum pressure likely to be encountered; to overcome the problem of pressures above 12 bar it then offers two routes: hydraulic alarm gongs may be driven (a) via a pressure reducing valve, or (b) from a secondary water supply such as a town main, controlled by a diaphragm valve connected to the alarm port of the main installation control valve.

Comparison with NFPA 13

Both standards describe the water motor gong alongside an electrical alarm. The differences that can be verified in the NFPA 13-2025 text available to us:

Item	EN 12845 Clause 16.1	NFPA 13-2025 Section 16.11
Piping	20 mm, galvanised steel or non-ferrous metal	At least 3/4 in (20 mm); galvanised steel, brass, copper or other approved corrosion-resistant metal (16.11.1.4)
Max. equivalent length	25 m	No numerical limit found in NFPA 13
Max. height	6 m	No numerical limit found in NFPA 13
Drain orifice	≤ 3 mm	No numerical limit found in NFPA 13
Strainer	Mandatory, accessible for cleaning	Listed 3/4 in (20 mm) strainer at the alarm outlet of the waterflow detecting device (16.11.8.1)
Retarding device	No provision	Mandatory on each alarm check valve under variable water pressure (16.11.4)

The hardware is largely common, but the length, height and orifice limits are specific to EN 12845 and must be checked separately when moving between the two standards.

Turkish context

BYKHY Article 96(5) requires sprinkler design to TS EN 12845, which covers the alarm valve and its accessories. Two mistakes are common in practice:

• "The panel gets the signal, so we don't need a gong". BYKHY Article 96(1) lists alarm bells and flow indicators among the components of a sprinkler system, and because design follows TS EN 12845 under Article 96(5) the alarm arrangement must meet that standard; a pressure switch alone is not enough.
• Mounting the gong inside the valve room. Clause 16.1.2 requires the gong on the outside of an exterior wall. Where the valve room is in a basement, the gong must be brought out to the facade while staying within the 6 m and 25 m limits.

The annual maintenance form should include the gong test as its own item — open the alarm test valve, send water to the water motor, confirm the sound on site and observe the discharge from the drain orifice. A single line reading "alarm valve tested" is not sufficient.

Frequently asked questions

Does a water motor gong need power?

No. The bucket (Pelton type) water wheel is entirely mechanical; the flow of water rings the gong, so it also works during a power cut. EN 12845 Clause 16.1.1 requires this mechanical alarm in addition to the electrical remote alarm device.

Is the distance from the alarm valve to the gong unlimited?

No. EN 12845 Clause 16.1.2 requires the gong centre line to be no more than 6 m above the alarm valve connection, and Clause 16.1.3 requires the 20 mm galvanised steel or non-ferrous pipe not to exceed 25 m of equivalent length, with each change of direction counted as 2 m.

Is a retard chamber mandatory?

The EN 12845 text contains no requirement for a retard chamber. Where supply pressure fluctuations cause false alarms, the wet alarm valve manufacturer's retard chamber is used; the delay depends on the manufacturer. NFPA 13-2025 16.11.4, by contrast, makes a retarding device mandatory under variable pressure.

Can a pressure switch replace the gong?

No. Clause 16.1.1 requires every control valve set to have both a water motor alarm and an electrical remote alarm device, as close to the alarm valve as possible. Each backs the other up.

How is the gong fed in systems above 12 bar?

Annex E.2.5 states that, to overcome the problem of pressures above 12 bar, hydraulic alarm gongs may be driven via a pressure reducing valve, or from a secondary water supply such as a town main, controlled by a diaphragm valve connected to the alarm port of the main control valve.

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