How Does an Alarm Check Valve Work? — Working Principle, Function & Water Motor Alarm Gong

What Is an Alarm Check Valve and What Does It Do?

An alarm check valve (also called an alarm check valve or sprinkler alarm valve) is the master control component installed at the base of a wet-pipe automatic fire sprinkler system riser — between the fire water supply main and the distribution pipework that feeds the sprinkler heads throughout the building.

Its three core functions are:

CA-FIRE manufactures six types of alarm check valves to cover all installation requirements — from the standard flanged wet alarm valve for heated buildings to the dry alarm valve for freeze-risk environments and the pre-action alarm valve for data centres and museums. All types share the same fundamental working principle described below. See the full alarm check valve range →

Stage 1 — Pressurised Standby: How the Clapper Stays Closed

In standby (normal, no-fire condition), the alarm check valve clapper is held closed by a pressure balance. The supply water pressure acts on the underside of the clapper from the water supply main. The system water pressure (the water trapped in the distribution pipework above the valve) acts on the upper face of the clapper.

Because both pressures are equal — and the clapper’s own weight adds a small closing force — the clapper remains firmly seated on its EPDM rubber seal. No water flows through the valve. The water motor alarm gong is silent. Both the supply pressure gauge and the system pressure gauge show the same reading.

Stage 2 — Sprinkler Activation: How the Clapper Opens

When a fire occurs and the ambient temperature at a sprinkler head reaches its activation temperature (typically 57–93°C depending on the head’s temperature rating), the fusible element or glass bulb in the sprinkler head fails and the head opens. Water begins to discharge from the open head.

This discharge causes the system-side pressure to drop — water is flowing out of the distribution pipework through the open head, reducing the pressure acting on the upper face of the clapper. The supply-side pressure remains unchanged (the water supply main maintains constant pressure).

The result is a pressure differential across the clapper: supply pressure pushing up from below now exceeds system pressure pushing down from above. This net upward force lifts the EPDM clapper off its seat — opening the waterway fully. Water flows freely from the supply main through the alarm check valve body into the distribution network, and from there to the open sprinkler head.

Stage 3 — Alarm Port Activation: How the Alarm Is Triggered

As the clapper lifts, an alarm port — a tapped groove or channel machined into the valve body seat ring — is uncovered. This alarm port connects via a small-bore pipe to the retard chamber, which in turn connects to the water motor alarm gong and alarm pressure switch.

Water simultaneously flows through the alarm port into the retard chamber. Because this is sustained flow from a genuine fire activation, the retard chamber fills faster than its calibrated drain orifice can empty. Pressure builds in the alarm circuit, and two things happen in parallel:

How the Water Motor Alarm Gong Works

The water motor alarm gong — also called the hydraulic alarm gong or water-driven bell — is one of the most elegant engineering solutions in fire protection. It delivers a loud, reliable alarm signal using nothing but water pressure, with no dependency on electrical power, batteries, or electronic controls.

Why the Water Motor Gong Is Mandatory — Not Optional

Both NFPA 13 and GB 5135.6 / GB 50084 require a water motor alarm gong on every wet-pipe alarm valve station — not as an optional accessory, but as a mandatory listed component. The reason is reliability:

• Power-independent alarm: Fires frequently disrupt electrical supplies. An electrical-only alarm system can fail precisely when it is most needed. The water motor gong sounds as long as water flows — independent of any electrical system.
• Local audible alert: The gong provides an immediate audible alarm at the building exterior — alerting building occupants and the fire brigade even before the fire alarm panel has been acknowledged or the monitoring centre has been notified.
• Self-confirming: If the gong is ringing, water is flowing. This is an immediate physical confirmation that the system has activated — not a panel indicator that might represent an electrical fault.

The Retard Chamber — Why the Alarm Doesn’t False-Trip

Without the retard chamber, the alarm check valve would generate false alarms every time a water pressure surge momentarily lifts the clapper — which is a regular occurrence in any pressurised water system connected to a fire pump or municipal supply. The retard chamber prevents this by introducing a time-delay filter between the alarm port and the alarm devices.

Complete Working Sequence — Standby to Reset

The full working cycle of an alarm check valve — from standby through activation to post-fire reset — consists of seven stages:

Alarm Check Valve vs Standard Check Valve — Key Difference

A common question from specifiers and contractors new to sprinkler systems is: why can’t you use a standard check valve instead of a dedicated alarm check valve on a sprinkler riser? The answer is the alarm port.

Installing a plain check valve in place of an alarm check valve on a sprinkler riser would allow water to flow to open sprinkler heads during a fire — but would generate no alarm signal whatsoever. The fire would be suppressed, but no one would know it had happened until the water damage was discovered. Both NFPA 13 and GB 5135.6 specifically prohibit the use of standard check valves as a substitute for alarm check valves on sprinkler risers.

Working Principle Differences: Wet, Dry & Pre-Action Systems

The working principle described above applies to wet-pipe alarm valve systems — where the distribution pipework is always full of water. Dry-pipe and pre-action systems use the same alarm check valve concept but with important differences in how the clapper opens: