Automatic Sprinkler and Fire Alarm Systems

Don’t believe everything you see in the movies; here is a layman’s guide to fire sprinkler and fire alarm basics.

The myths and misconceptions surrounding fire alarm and fire sprinkler systems in movies and television is pretty staggering. As a design professional, I roll my eyes every time I see a character activate a manual pull station resulting in every fire sprinkler going off. Of course you might say, “It’s a movie; lighten up!”. On one hand; you’re right. It is only fiction. On the other hand, the consistency in which fire alarm and fire sprinkler systems are incorrectly shown actually leads to complacency and misunderstanding. In the not so distant past, I actually explained how a sprinkler works to a coworker, and they were floored that what they’d see in the movies was not correct. Entertainment is fine; I’m a big movie person myself. When no one actually explains how the systems really work to the average person, and the movies are all they know, it could actually lead to a delay in evacuation or other events that could put a person’s life in danger. I’m not suggesting changing the movies; let’s just talk about how the systems actually work. I’ll try and do so in the least technical way possible. To start, here’s some examples (from two movies I actually like) of movie imagination with regards to fire alarm and sprinkler systems. After the show, I’ll follow with some basic concepts and history for you:

Concepts and History:

Automatic sprinkler and fire alarm systems are two crucial components of a building’s life safety systems. They are what’s known as active protective systems. Passive protective systems such as fire-resistive construction, protected stairways, smoke barriers and fire walls are intended to be the main line of defense in the protection of occupants against a fire. Fire alarms are there to actively notify occupants of a fire so they can evacuate, and automatic sprinkler systems are intended to help suppress the fire long enough for the fire department to arrive and extinguish the blaze. While typical light-hazard sprinklers may often extinguish a small fire, their purpose is to keep it from growing; not put it out. More specialized systems are actually designed to fully put out a fire, but those are normally reserved for high hazard occupancies such as large storage buildings, hazardous material handling and very large mercantile buildings such as big box stores like Home Depot, Costco or Walmart.

Automatic sprinkler systems have been in existence since the late 1800’s, beginning with Henry S. Parmelee’s invention of the first sprinkler head in 1874. While improvements have been made in sprinkler design and the various system components, such as valves and piping; the basic concept of automatic sprinkler systems has not changed that much in almost 150 years.

The earliest fire alarm systems involved nothing more than a person keeping watch in the streets who would then alert the fire brigade. With the invention of the telegraph in the early 1840’s by Samuel F. B. Morse, cities such as New York began to construct municipal alarm systems of communication between fire stations and government buildings. This reduced the time needed to alert the fire brigade and improved response time through better directions. By 1880, manual fire alarm signal boxes had been patented by John Gamewell and his brother-in-law James M. Gardiner. At that time, the Gamewell Company held a 95% share of the United States market for fire alarm systems. William B. Watkins designed the first electric fire sensor / heat detector in the early 1870’s and in 1873 formed the first private fire alarm company, Boston AFA; known today as AFA Protective Systems.

In the early 1900’s, leading fire alarm and fire sprinkler companies such as ADT (American District Telegraph), Holmes Protective, AFA, Grinnell and Automatic Fire Protection (AFP), established contracts with each other to supply detection equipment, sprinkler systems, sprinkler system supervisory equipment and central station monitoring services.

In 1939, Swiss physicist Ernst Meili invented an ionization chamber which could detect combustible gases in mines and a cold cathode tube that could amplify the small signal generated to a level sufficient to activate an alarm. By 1951, the first ionization smoke detectors were sold in the US. The photoelectric (optical) smoke detector was then invented by Donald Steel and Robert Emmark of Electro Signal Lab in 1972. The technologies and devices used in fire alarm systems have continued to advance, but the basic concepts remain the same.

Fire Alarm Systems: The Basics

Fire alarm systems are required to be installed in accordance with NFPA 72, the National Fire Alarm Code. (Disclaimer: The following explanations are in layman’s terms and intentionally omit and generalize some technical details, devices and system components for clarity).

Fire Alarm Systems are made up of three basic categories of devices:

  1. Initiating
  2. Notification
  3. Control / Transmission

Initiating Devices:

These components do exactly what you think they would; they initiate an alarm. They include both automatic and manual means of activating the alarm system. The most common manual device is the “Manual Pull Station”. This would be the red device on the wall labeled “Pull in case of fire” or something similar. When the handle of this pull station is moved it literally flips a switch inside the box that tells the system to go into “Alarm”.

The three most common automatic initiating devices are smoke detectors, heat detectors and sprinkler system flow switches. The first two are self-explanatory. When a smoke detector sees (or senses) smoke, it tells the system to go into alarm mode. If the heat detector senses a rise in temperature above a set point, it also tells the system to go into alarm mode. The third item is a component that few people outside the construction, design, or fire protection industries even know exists; the flow switch. A flow switch is a device installed into the piping of the fire sprinkler system just after the water pipes enter the building. The typical flow switch has a round “paddle” or disc that is directly inside the pipe and connects to a lever and electronic sensor. When the sprinkler system activates, water begins to flow through the pipes. When the water moves, this ‘paddle’ also moves; raises the lever and sets off the sensor. This tells the fire alarm system that a sprinkler has activated; also resulting in an alarm.

In a typical light-hazard occupancy sprinkler system ,which are the most common, this is the ONLY interaction between the sprinkler system and the fire alarm system. When water flows; the alarm goes off. It does NOT work the other way. Setting off the fire alarm DOES NOT activate any or all of the fire sprinklers. Unless the system being discussed is for very unusual situations like hazardous materials, locations with jet fuel, etc.; the fire alarm system and sprinklers are not set off electronically in any way. We’ll discuss how a sprinkler system works next.

Notification Devices:

These include two main appliances, commonly referred to as a horn and a strobe. The horn is what it sounds like; it’s that really loud and annoying noise maker that lets you know there is a fire and to GET OUT. The strobe is the light emitting device that flashes when the alarm activates. Strobes are there in case occupants are hard of hearing or ambient noise levels are high enough to prevent people from hearing the horn. The horn can also be a recorded announcement in some occupancies. These two devices are there to let you know a fire or another emergency is present and you need to act accordingly. Very commonly both devices are combined into a single appliance called… you guessed it: a horn-strobe. We look for humor in this subject matter wherever we can find it.

Control / Transmission Devices:

This is my own general category for the main Fire Alarm Control Panel (FACP), Fire Alarm Annunciator Panels (FAAP), and automatic dialing devices. All of the initiating and notification devices are connected to the FACP. When an initiating device is tripped, the FACP activates all of the notification devices to let people know there is a problem. If any systems are required to shut off when an alarm is activated (mechanical units, electronic door locks, magnetic door hold-opens, etc.), the FACP will send signals to that equipment accordingly. Most modern FACP’s also include the auto-dialer device. This is literally a phone / modem that calls a monitoring service and says, “We have an alarm. The following initiating devices were set off….” The monitoring company will then notify the fire department. As all of this is done electronically, there is very little delay between when a device is set off and when the fire department is notified. The FAAP (as shown below) is basically a twin to the control panel from the FACP that can be located at strategic locations without having to install the larger control box in public areas.

Fire Alarm Systems

Those are the fire alarm basics. Although the wiring, programming and code requirements surrounding what devices are required, where they go, and how they must work are more complex, the basic concepts of initiation, notification and transmission are pretty simple.

Fire Sprinkler Systems: The Basics

Automatic Fire Sprinkler Systems must be installed in accordance with NFPA 13, NFPA 13R or NFPA 13D. 13R is generally for multifamily and hospitality occupancies like apartments, condos, hotels, motels and dormitories. 13D systems are for single family homes or duplexes. The NFPA 13 compliant system is what you would normally see in any commercial, institutional, educational or other public type building. The 13R and 13D systems use the same types of sprinkler heads; they just have less restrictive requirements in where sprinklers are required, and what kinds of equipment and other devices are installed.

The central component to a sprinkler system is… no big surprise: the sprinkler.

Fire Sprinkler
Typical ‘pendant’ type sprinkler head

The most typical sprinkler installed has 4 main parts:

  1. The cap: a disc that covers the hole the water shoots out of;
  2. The thermal linkage: the colored glass tube or metal piece that bursts or melts at a certain temperature. The linkage holds the cap in place;
  3. The deflector: The metal disc or plate that the spreads the water out, and
  4. The frame: the “arms” of the sprinkler head that hold the deflector, cap and linkage all together.

So how does a sprinkler go off? Its rather simple. The most common thermal linkage you will see today is a glass bulb with a colored liquid inside. That liquid is a mixture of water and alcohol or glycol with a colored tint. The color indicates the temperature at which the bulb is designed to burst. Per NFPA 13, bulbs that are red in color activate at 155°F. Other colors such as green, yellow, or blue indicate higher temperature ratings.

When a fire gets large enough to raise the air temperature in a room (at the head itself) to a level equal to or above the head’s rating; the glass bulb bursts. With nothing to hold it in place; the water pushes the cap out of the way; water rushes through the opening (called the orifice), and then spreads out in a pre-defined pattern as it hits the deflector. That’s how a sprinkler gets activated. There are no electronics, no manual levers or special keyed switches. The bulb gets hot; it breaks; water comes out. The only heads that will go off in a fire are those that have been heated to that specific temperature.

If EVERY head went off in a building at once (as the movies like to show), the amount of water necessary would be as much as fifty to a hundred times greater than any normal building has available. Sprinkler systems are designed to have no more than 4-6 heads go off in a fire (that is a very general statement I know; the exact number of heads is a code requirement and up to the system designer and is dependent on a number of different factors). What you see in the movies is just not physically possible in any typical building. There are specialized systems in which a large number of heads are activated at the same time, but those are not used in any building the general public normally occupies.

There is of course more to a sprinkler system than just the sprinkler heads. Every head is connected to pipes made of steel or CPVC (specially designed plastic pipe). The pipes with heads on them are commonly referred to as branch lines, while the larger pipes that feed the branch lines are called mains. The single big pipe that supplies the mains is called the riser. The riser is located inside the building right after the main water pipe enters the building. The riser typically includes shut-off valves, pressure gauges, test valves, the flow switch (connected to the fire alarm system as mentioned above), backflow preventers, and other devices needed to maintain and test the system. A typical NFPA 13 system will also have a Fire Department Connection (FDC) at the riser (or a remote one that connects to the riser) which allows the fire department to connect a pump truck and supply the system with a higher flow and pressure than the normal water system may be capable of.

Main riser with valves, main drain and pressure gauges

The kind of system described above is known as a “wet” system because the pipes are always full of pressurized water. Water freezes however, so another common system known as a “dry pipe” system is often used in attics or other areas where no heat is available during winter months. In this system, the heads are actually the same, but the branch lines and mains are filled with compressed air. The water from the utility provider is kept at bay in the riser, by a special valve and the compressed air in the system itself. When a fire causes the sprinkler head bulb to burst, the process is the same, except the system has to first purge itself of the compressed air, and then the water comes rushing in behind. Same concept; it just slightly delays the water actually hitting the fire.

The basic concepts and components of a fire alarm and fire sprinkler system are not difficult to understand. In fact, they are simple enough that everyone should know how and when each system would go off, and how it generally works. Knowing these basics makes using them that much easier and safer.

Would you like know more?

This is a VERY broad overview of fire alarm and fire sprinkler systems. What components or requirements of these systems would you like to know more about?

Sources, Disclaimers and Copyrights:

  • Fire Alarm System Research – Where it’s been and where it’s going, Wayne D. Moore, P.E.

  • Sprinkler image: Courtesy of
  • Fire Alarm device images are the property of Potter Roemer Fire Pro., Copyright 1937-2016
  • Fire alarm control and annunciator panels images are the property of Silent Knight, by Honeywell, Copyright 2017.
  • Excerpt from Lethal Weapon 4, Copyright Warner Bros. Pictures, 1998
  • Excerpt from Casino Royale, Copyright Columbia Pictures, 2006
  • Fire Truck Image is the property of the City of Leander Texas Fire Department (actually one block from my office; thank you gentlemen).
  • Fire riser image is by DFD Architects, Inc., Copyright 2017.

All images and videos included or linked to are for reference and educational purposes only. All copyrights are the property of their respective owners.

Disclaimer: I am not a licensed fire alarm or fire sprinkler RME (Responsible Managing Employee) or installer. All fire alarm and fire sprinkler systems shall be designed and installed by appropriately licensed professionals in accordance with applicable state and local laws. The above commentary is from my personal experience and code knowledge, but should not be substituted for advice and direction from trained and licensed professionals in fire alarm and fire sprinkler design and installation.