Douglas Krantz - Technical Writer - Describing How It Works
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What is a Waterflow Switch?

Waterflow Switch on a Sprinkler Riser
The Waterflow Switch is a mechanical-to-electrical Fire Alarm input device; it tells the fire alarm system that water is flowing in the sprinkler system.


By Douglas Krantz

The fire alarm waterflow switch, a delayed action mechanical/electrical assembly, is a conventionally wired fire alarm device.

The fire department reacts with more urgency to a waterflow switch alarm than the way they react to other alarms.

Fire Alarm

A waterflow switch is the interface between the building's fire suppression system and the building's fire alarm system. It detects water flowing in the sprinkler system and causes the fire alarm system to sound the evacuation.

When the building sprinkler system activates, the waterflow switch has three main functions:
  1. It notifies the occupants of the building to evacuate
  2. It alerts the fire department that help is needed
  3. It shows the firefighters on their arrival that water is being used, and, by sounding the outside horn, where to connect their hoses

Reaction to the Sprinkler System

The fire department considers the fire sprinkler suppression system to be a greater indicator of a real fire than just the fire alarm system.

Not that they like responding to false alarms, they're used to fire alarm systems going into false alarm. But unlike smoke detectors in the fire alarm system, the building sprinkler system activates with heat, not just cooking smoke.

If heat has activated the sprinklers, and water is flowing in the building's sprinkler fire suppression system, they assume the building is actually on fire; the firefighters come into the building prepared for battle.

Waterflow Switch Description

The waterflow switch is a mechanical/electrical assembly.

As it only detects the water flowing in the pipes, the waterflow switch doesn't turn on or off the water. It just sets off the alarms by activating electrical switches.

To detect the water's movement, the assembly has a paddle across the inside of the pipe. When water starts moving in the pipe, it pushes on the paddle. The paddle is on a lever that, when the water has pushed aside the paddle, allows a pair of switches to activate, after a time delay.

There are two switches inside to set off two alarm systems:
  1. The local fire alarm system -- It evacuates the building and calls the fire department
  2. An outside horn/strobe -- It tells the fire department on their arrival that the sprinkler system has activated

Delayed Signal from the Waterflow Switch

The sprinkler system is a long water tank; inside it water naturally moves a little. The pipe itself can slightly expand and contract with changes in water pressure, and air bubbles inside the pipe will compress more or less with changes in water pressure.

As the water pressure momentarily increases, water can move the paddle of the waterflow switch for a moment or so, and this movement could set off false alarms. This particularly happens as the city water pressure goes up and down.

To prevent these false alarms, the waterflow switch assembly has a retarder. The retarder is an air damper that prevents the lever from the paddle from activating the switches, until after a specified delay. This time delay is adjustable from 0 to 90 seconds.

Pressure type waterflow switch assemblies, used in dry sprinkler systems, deluge, preaction, or chemical fire suppression systems, activate as soon as the pressure of the suppression system floods the pipes. They don't have a delay.

Wired as a Conventional Fire Alarm Input Device

One switch inside the waterflow switch assembly is for the fire alarm system. It's wired Class B or Class A, and like a pull station, sends an alarm to the panel when it shorts out the IDC (Initiating Device Circuit). When no water is flowing, the switch contacts are open, presenting a normal IDC to the panel; when water flows, the switch contacts close, sending an alarm to the panel.

Often from the fire alarm installer's point of view, the labels on the flow switch are not correct; so use an ohmmeter to determine which contacts are open when no water is flowing in the sprinkler system.

The other switch in the waterflow switch assembly turns on and off the outside horn/strobe. It's wired almost like a light switch on the wall. When water isn't flowing, the switch is off and the outside horn/strobe is off; when water flows, the switch turns on and the outside horn/strobe turns on.

Waterflow Switch

The waterflow switch is an alarm device. It sets off the building fire alarm system, and the fire department reacts to the alarm as if there is a real fire.

Wired conventionally, it's a delayed action, mechanical/electrical device.

For the fire alarm system, a waterflow switch is conventional. Learn more about the conventional fire alarm system in the book Make It Work - Conventional Fire Alarms. This book covers the basics of the Conventional Fire Alarm System, and shows how Life Safety and internal supervision affects the fire alarm system.

Douglas Krantz -- Fire Alarm Engineering Technician, Electronic Designer, Electronic Technician, Writer

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