EN 12845 Water Flow Alarm and Water Motor Gong

If the Pelton wheel does not turn, the project is not handed over: the water motor gong, the retard chamber, pressure switches and the field errors around them.

During commissioning at a logistics warehouse we opened the wet alarm valve and waited for the noise — not a single click came from the gong. The panel LED lit, the BMS signal arrived, but the cast iron gong on the outside wall stayed silent. Stripping it revealed a limescale blockage in the Pelton wheel nozzle and a dried wasp nest stuck to a vane. The water motor alarm clause exists precisely to prevent that scene: even if the power is off, the panel has failed and the BMS is down, an audible alarm must sound outside when water flows.

What the clause requires

The standard compresses this into three sub-clauses:

• General: every control valve set needs a water motor alarm to EN 12259-4 and an electrical device for remote alarm, both as close to the alarm valve as practicable. Several wet alarm valves in the same valve room may share one gong, provided each valve carries an indicator showing which installation is sounding.
• Water motor and gong: the gong goes on an external wall, with its centre no more than 6 m above the alarm valve connection point. A removable strainer sits between the alarm valve outlet and the motor nozzle for cleaning, and the water discharge must be visible.
• Gong pipework: 20 mm galvanised steel or non-ferrous metal, with an equivalent length of no more than 25 m, each change of direction counted as 2 m. A stop valve inside the building on the line, and a permanent drain at the end through a stainless or non-ferrous orifice no larger than 3 mm.

The installation number is marked legibly on the gong — essential in a multi-valve room to know which system is sounding.

How a Pelton gong works

The mechanics are school physics: when the alarm valve opens, the clapper lifts and mains-pressure water travels through the 20 mm pipe to the nozzle in the motor body outside. The nozzle directs a jet tangentially onto a Pelton (bucket) wheel. The wheel turns, and a hammer on its shaft strikes the inside of the gong bell. One moving part, one supply: independent of the panel, the batteries and the mains during a fire. That autonomy is why the standard requires the gong alongside the pressure switch.

Nozzle diameter is typically 3–4 mm, giving a flow of 25–40 L/min, which discharges visibly after passing the wheel. The requirement for a visible discharge both simplifies testing and reveals blockage early.

Position: facade, 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 an external wall, with rain protection.
• It must be clearly audible and visible to the fire service and site personnel. Placing it in an internal courtyard or on a blind elevation is a breach.
• Vertical distance from the alarm valve connection to the gong centre: 6 m. That limit guarantees enough flow pressure for the Pelton wheel to produce the required torque; beyond it, the jet energy needed to turn the wheel may fall short.
• The stop valve on the line stays inside the building, so it cannot be closed by vandalism or accident from outside.

Pipework: 20 mm, 25 m equivalent, drain orifice

The pipework rules keep the hydraulic balance:

Parameter	EN 12845 value	Site note
Pipe size	20 mm	Galvanised or non-ferrous; carbon steel blocks with corrosion
Max. equivalent length	25 m	Each elbow counts 2 m; two 90° elbows plus 18 m of straight pipe is at the limit
Stop valve	Inside the building, on the gong line	Needed for maintenance; kept sealed and labelled open
Permanent drain orifice	≤ 3 mm, stainless or non-ferrous	Empties the gong pipe after a test, preventing freezing
Strainer	Between valve and nozzle	Strip and wash twice a year
Height	Max. 6 m above the alarm valve	Exceeding it reduces Pelton torque

A 3 mm orifice looks trivial but creates a permanent trickle in the gong pipe, both reducing freezing risk and confirming visually on site that the line really is pressurised. Making the orifice from a carbon steel plate and letting corrosion enlarge it produces false signals from the alarm valve port and a gong that rings "by itself" — which is why the standard requires stainless or non-ferrous.

Pressure switches: complementary, not alternative

The clause says "and": a water motor gong and an electrical remote alarm device. Two types of electrical device are accepted:

• Water flow switch (paddle): wet systems only. A paddle in the pipe senses flow and sends a contact to the alarm panel. A test valve downstream is mandatory, with an orifice equivalent to the smallest sprinkler in the system.
• Pressure switch: senses the pressure rise at the alarm port — the only electrical option on dry, alternate and pre-action systems.

In a typical installation, a retard chamber 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: 5–90 seconds

The term "retard chamber" is not itself mandatory in the EN 12845 text; but wet alarm valve manufacturers supply one as standard, to filter the false alarms caused by sudden fluctuations in town main pressure. How it works:

1. When the alarm port opens, water begins to fill the retard chamber.
2. If the flow is real, the chamber fills within its set time (adjustable between 5 and 90 s) and passes pressure through the overflow port to the gong line.
3. If it is a momentary pressure spike, a calibrated drain hole beneath the chamber empties it and the gong does not ring.

In district warehouses where town main pressure swings by a bar through the day, a system without a retard chamber rings once a week in the middle of the night, and the fire service issues false alarm penalties. Setting the time too long (60 s or more) delays a real fire; designers generally settle on 20–30 s. On pre-action and dry valves the delay is already built into the accelerator or exhauster mechanism, and no separate retard is used.

Field errors

The four errors I see most often in commissioning reports:

• A wasp nest inside the gong. Classic 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 (5 mm aperture) over the discharge.
• A strainer that has never been cleaned. Limescale partially blocks the nozzle; the gong rings but weakly. The standard defines the strainer as readily accessible for cleaning — so it is fitted with a flange or union and stripped after every test.
• A carbon steel drain orifice enlarged by corrosion. A 3 mm hole opened out to 6–7 mm keeps the line permanently empty, and the gong stays silent in a real alarm. Measure it 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 20 m line rings weakly or not at all. The requirement is unambiguous: 20 mm.

Systems above 12 bar

In tall buildings, where pressure at the alarm valve exceeds 12 bar, the standard Pelton wheel and nozzle wear out early. The annex offers two solutions: fit a pressure reducing valve in the gong line, or feed the gong separately from the town main — connecting a diaphragm valve to the alarm port so that the town main feeds the gong when the diaphragm opens. The second is more common, because it avoids maintaining a pressure reducing valve as well.

Comparison with NFPA 13

That both standards describe almost identical hardware is no coincidence — the Pelton gong is a shared inheritance dating from the nineteenth century. The differences that matter on site:

Item	EN 12845	NFPA 13
Pipe size	20 mm fixed	3/4 in (about 20 mm) minimum
Max. length	25 m equivalent	75 ft (about 22.8 m) in most cases
Max. height	6 m	20 ft (about 6.1 m)
Drain orifice	≤ 3 mm	Max. 1/8 in (about 3.2 mm)
Strainer	Mandatory, removable	Mandatory
Electrical alarm	Always additionally required	Always additionally required
Delay	Manufacturer or site preference	Retard chamber, 60–90 s standard on wet systems

The numerical limits are practically identical; a designer can move between the two standards without changing the gong hardware.

Turkish context

BYKHY 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". Both BYKHY and EN 12845 require them together; a pressure switch alone is not enough, and a missing gong is a frequent rejection in building inspection reports.
• Mounting the gong inside the valve room. It must be audible from outside for the fire service. Where the valve room is in a basement, the gong must be run out to the external wall, or commissioning will not be accepted.

The annual maintenance form should include the gong test as its own item — open the alarm test valve, direct water to the Pelton wheel, 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 Pelton water wheel is entirely mechanical; water flow alone turns the bell. That is exactly why the alarm must remain audible during a power cut.

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

No. The gong centre must be no more than 6 m above the alarm valve connection, and the 20 mm galvanised or non-ferrous pipe must not exceed 25 m of equivalent length, with each change of direction counted as 2 m.

Is a retard chamber mandatory?

Not directly in the EN 12845 text. But where town main pressure fluctuations cause false alarms, a retard chamber (5–90 s) is added with the wet alarm valve manufacturer's manual. Pre-action and dry systems have the delay built into the valve.

Can a pressure switch replace the gong?

No. Every control valve set requires both a water motor gong and an electrical remote alarm device, as close to the alarm valve as practicable. Each backs the other up.

How is the gong protected above 12 bar?

Either through a pressure reducing valve in the gong line, or by feeding the gong separately from the town main through a diaphragm valve — otherwise the Pelton wheel and nozzle wear rapidly.

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