#217 – Flow Switches vs. Pressure Switches in Fire Protection: What’s the Difference?

Flow and pressure switches’ similar exteriors conceal important differences in function and code-compliance

One switch sounds the alarm when a fire sprinkler opens. Another does the same thing—yet it breaks in all but one type of sprinkler system. And still a third type of switch watches only for important, but not necessarily life-and-death, changes within system piping.

They’re called flow or pressure switches, and while all of these devices look nearly the same, the differences between them matter a great deal for fire protection systems. Read on to learn about:

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All flow switches are designed to detect fire sprinkler activation, but only some pressure switches play the same role

In fire protection, flow switches, or water flow detectors, are exactly that: devices that sense when water flows through a fire sprinkler system.

Usually, water within the pipes leading to sprinkler heads remains stagnant. When a sprinkler opens, a supply of fluid moves through pipes and sprays into a room. This change in how fast water moves activates the sensor. In turn, it sends a signal to a fire alarm control panel, a notification device (like a bell), or both.

Fire sprinkler pressure switches, on the other hand, watch for changes in pressure. Major manufacturers offer two main types:

  • Alarm pressure switches that monitor changes in water pressure only. These serve largely the same purpose as flow switches, sounding the alarm when a head or heads have apparently opened.
  • Supervisory switches that monitor the air pressure in dry and pre-action systems and may also watch for changes in water pressure in wet-pipe systems. They prevent damage to the system or unwanted activation.

Low-air supervisory switches watch for a loss of air or nitrogen in a dry-pipe system. These systems contain a supply of pressurized gas until sprinkler heads activate. An unwanted decrease in pressure could cause an important valve—the dry-pipe valve—to open, filling pipes with water. At low temperatures, this water expands within (and can damage) the system. By letting personnel know about a potential problem, these switches help to keep sprinkler systems intact.

Switches for high and low air pressure play the same role, but also detect increases in pressure. That added pressure could damage pressure-activated valves and other components in fire sprinkler systems. And if something catches fire while there’s excess pressure, valves controlling the supply of water to sprinklers may not open on time.

The basics of water flow switches

Vane or paddle-type flow switches work in wet-pipe systems only. As the name stipulates, they feature a vane or paddle that sits in the path of water and moves when it flows. But in dry systems where water quickly replaces compressed air when sprinklers activate, that paddle could be damaged or broken off—thus the restriction to only using them in wet-pipe systems.

For smaller pipe sizes (typically under two inches), these switches may take the form of a “t-tap.” They have a threaded inlet and a protruding paddle that connects to a threaded fitting (the tee). Others have a paddle and saddle with a horseshoe-shaped bolt. When tightened, this U-bolt holds the sensor in place over a hole cut into the pipe.

Tee-tap water flow switch
This t-tap switch threads directly into a pipe fitting.

Orientation and placement

Vane and paddle-type flow switches must face the flow of water. However, they may hang from a pipe that runs either vertically or horizontally. One standard location for these switches is slightly downstream from where the water enters the sprinkler system—on or after a check or isolation valve.

Activation and signaling

Water flow switches activate when the flow exceeds a predetermined rate. However, they may also use a mechanism to delay those signals, preventing false alarms due to pressure surges.

They typically feature two electrical switches: one that signals a fire alarm control panel, and another that sounds a bell or other audiovisual alarm. Flow switches connected to a control panel must have a non-silenceable circuit—meaning that the panel can’t deactivate the switch’s alarm signal.


T-tap flow switches typically protect residential fire sprinkler systems, which use small pipe sizes. They can also work on smaller lines (branch lines) in commercial systems. In contrast, installers typically use U-bolt-type switches in commercial systems, where they attach to pipes that carry the system’s supply of water.

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The basics of pressure switches

Some pressure switches, also known as pressure-type flow switches, activate when water flows through dry-pipe, preaction, and deluge systems. Others can help prevent damage from too much or too little pressure. These switches can also act as flow switches for wet-pipe systems with maintained excess pressure. Wet-pipe systems like these have added pressure that’s designed to keep the clapper—which prevents water from flowing back out of sprinkler systems—from opening unnecessarily.


One visible difference between pressure switches and paddle flow switches is the lack of a paddle in the latter. Pressure switches have no parts that reach the open pipe. It’s this distinction that makes these devices safe for dry-pipe, preaction, and deluge fire sprinkler systems that surge a lot of water pressure.

Alarm pressure switch
Alarm pressure switches like these trigger local alarms or a control panel.

Activation and signaling

Changes in pressure cause parts within the sensor to move. When that pressure rises or falls too far, the electrical contacts close, forming a circuit that sends a warning signal to a fire alarm control panel or sounds an alarm. Most pressure switches are adjustable, allowing installers to decide exactly what change in pressure is significant enough to warrant concern.

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NFPA 13 requires flow switches or alarm pressure switches for all systems

NFPA 13: Standard for the Installation of Sprinkler Systems tells contractors which fire sprinkler systems require monitoring of flow or pressure and how. In short, those requirements are as follows:

  • When included, all water flow and pressure switches—supervisory or not—must be listed (section A.7.1.1 in the 2022 edition)
  • Systems with more than 20 heads must have a local waterflow alarm that activates alarm bells or other audiovisual devices (
  • As the commentary in the NFPA 13 Handbook explains, NFPA 13 does not require waterflow detection devices to signal a fire alarm control panel
  • “Paddle-type waterflow devices” (what we and manufacturers commonly call “flow switches”) may only be used with wet-pipe fire sprinkler systems (
Diagram of paddle position in a flow switch
Paddles move out of the way of water flow—but installation in systems using compressed air can break the paddle off when water flows suddenly, causing a potentially deadly obstruction. Source: NFPA 13 Handbook

However, the requirements are stricter for high-rise buildings. More waterflow detectors are required, and they must connect to an alarm system.

From the 2022 edition of NFPA 13

16.11.10* Alarm Attachments — High-Rise Buildings. When a fire must be fought internally due to the height of a building, the following additional alarm apparatus shall be provided:

(1) Each sprinkler system on each floor shall be equipped with a separate waterflow device. The waterflow device shall be connected to an alarm system in such a manner that operation of one sprinkler will actuate the alarm system, and the location of the operated flow device shall be indicated on an annunciator and/or register. The annunciator or register shall be located at grade level at the normal point of fire department access, at a constantly attended building security control center, or at both locations.

(2) Where the location within the protected buildings where supervisory or alarm signals are received is not under constant supervision by qualified personnel in the employ of the owner, a connection shall be provided to transmit a signal to a remote central station.

(3) A distinct trouble signal shall be provided to indicate a condition that will impair the satisfactory operation of the sprinkler system.

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NFPA 13’s requirements address each type of switch in turn

Flow switches

Wet-pipe fire sprinkler systems require an alarm check valve—a device that prevents water from flowing in the wrong direction and features a flow switch—or another listed means of sounding an alarm ( Switches used to detect water flow must cause an audible alarm to sound within five minutes of when the rate of flow meets or exceeds the flow from the smallest sprinkler on the system, measured by orifice size, or K-factor (7.7). This signal continues until the flow stops.

Alarm pressure switches

It’s worth noting that while alarm pressure switches activate at a particular pressure, NFPA 13 doesn’t say what that pressure should be. But section 7.7 still applies: an audible alarm must sound within 5 minutes of the system’s smallest head opening. However, tests described in the NFPA 25 Handbook can verify that the switch works as intended:

  • Wet-pipe systems (no excess pressure): open the inspector’s test valve
  • Wet-pipe systems (with excess pressure) and dry-pipe systems: open the inspector’s test valve, water bypass test valve, or alarm test valve

Supervisory (pressure) switches

Nowhere in NFPA 13 are low- and high-pressure supervisory switches required. However, the NFPA 13 Handbook does note that it’s a good idea. While not mandated, supervision can reduce the number of inspections required after the system’s installation.

When should these switches activate? Those requirements are found in NFPA 72: National Fire Alarm and Signaling Code.

From the 2022 edition of NFPA 72* Dry Pipe Sprinkler System.

(A) A pressure supervisory signal-initiating device for a dry-pipe sprinkler system shall indicate both high- and low-pressure conditions.

(B) Unless otherwise permitted by the manufacturer’s published installation instructions, the off-normal signal shall be initiated when the pressure increases or decreases by 10 psi (70 kPa).

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While the NFPA relaxes some requirements for residential systems, many still require waterflow detection devices

Two other standards from the National Fire Protection Association may govern the installation of these sensors in residential settings:

  • NFPA 13D: Installation of Sprinkler Systems in One- and Two-Family Dwellings and Manufactured Homes
  • NFPA 13R: Installation of Sprinkler Systems in Residential Occupancies up to and Including Four Stories in Height


The home fire sprinkler systems covered by NFPA 13D must have a waterflow alarm if the house doesn’t have an NFPA 72-compliant series of smoke alarms or detectors (2022 edition, section 7.6). Unlike commercial fire sprinkler systems, home fire sprinkler systems do not need a listed waterflow detection device.

When a home fire sprinkler system has a waterflow alarm, that same system must also have a test connection that allows for testing of that alarm (7.2.4).


All NFPA 13R residential systems require a test connection (2016 edition, section 6.10.1). That requirement makes sense in light of the fact that all of these systems must have waterflow alarms (6.16.1).

Those alarms must connect to the building’s fire alarm system if it has one (6.16.2). For installation requirements, NFPA 13R has contractors turn to NFPA 13. Notably, these alarms need not provide signals zoned by every floor when an alarm system is present (6.16.4).

A low-rise motel
NFPA 13R motels and other commercial residential buildings at or under four stories high do need waterflow alarms—but they do not have to connect to a fire alarm system. Image source: Bernard Gagnon, CC BY-SA 3.0, via Wikimedia Commons

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Switch selection has a large impact on fire safety outcomes

Flow and pressure switches range from essential, code-required electronics to helpful—though not necessary—additions to a fire protection system. But taking the time to understand the differences between them can help installers enhance safety outcomes in buildings of all kinds.

QRFS offers flow, alarm pressure, and supervisory switches manufactured by Safe Signal. These UL and FM-approved switches come with a three-year warranty and connect to pipes in sizes from 1″ to 8″ in size, or to tees with 1/2″ or 1″ inlets.

8" flow switch
This eight-inch flow switch is our largest—but a range of sizes are available for a variety of wet-pipe fire sprinkler systems.

View our online catalog of fire protection switches.

Questions? Call us at +1 (888) 361-6662 or email support@qrfs.com.

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