Sprinkler head (sprinkler)Opens alone when heat reaches it; the others stay shut.
Riser (system riser)The vertical supply pipe of a sprinkler system.
Fire department connection (FDC)Lets a fire engine pump extra water into the system.
Control valve (OS&Y valve)Normally open; closing it shuts off water to the sprinklers.
Water main (water supply)The public pipe under the street that supplies the system.
Branch lineThe pipe that the sprinkler heads are connected to.
DeflectorBreaks the stream of water into a spray pattern.
Glass bulb (heat-sensitive element)Its liquid expands with heat until the glass shatters.
Street to ceiling 10
Pressure gaugeOne sits below the alarm check valve and one above it.
Backflow preventerKeeps sprinkler water from flowing back into the drinking supply.
Alarm check valve (alarm valve)Its clapper lifts when a sprinkler opens, sending water to the alarms.
Water motor gong (water motor alarm)A bell driven by flowing water; it needs no electricity.
Main drainNormally shut; opened to empty the system, discharging outside.
Pipe hangerA rod and ring that hold the pipe to the building.
Water flow switch (flow switch)Its paddle inside the pipe moves with flow and signals an alarm.
Tamper switch (supervisory switch)Signals a warning if the control valve starts to close.
Cross mainCarries water from the riser to the branch lines.
Retard chamber (retarding chamber)Absorbs brief water surges so they do not ring the alarm.
Sprinkler head up close 5
FrameKeeps every other part of the head in place.
Threads (threaded inlet)Screw the head into a fitting on the pipe above.
Cap (plug)Seals the orifice until the bulb breaks and releases it.
OrificeThe opening water flows through once the cap is released.
Escutcheon (trim ring)A trim ring that covers the hole cut in the ceiling.
How this list was compiled: Part names and every detail line were checked, for facts only and with nothing copied, against pages read on 2026-09-20. The National Fire Protection Association's own articles, under https://www.nfpa.org/news-blogs-and-articles/blogs/: 2023/11/13/sprinkler-piping-arrangements (cross mains and branch lines in tree, looped and gridded systems), 2022/02/18/types-of-sprinklers (a heat-activated element, either a glass bulb or a fusible metal link; a plug is released, water moves through an orifice and collides with the deflector; pendent, upright and sidewall; escutcheons), 2022/04/18/hangers-and-support-of-sprinkler-system-piping (risers as the vertical supply pipes; a hanger as a clamp on the building, a threaded rod and an adjustable swivel ring round the pipe), 2022/05/20/fire-department-use-of-sprinkler-systems (a fire department pumper supplying water through the fire department connection), 2022/10/28/backflow-preventer-types (protecting the potable water supply), 2021/06/22/the-basics-of-sprinkler-thermal-characteristics, 2024/03/27/floor-control-valve-assemblies (the control valve, normally open and supervised; the check valve; the pressure gauge; the main drain valve, normally shut; the flow switch), 2021/03/26/sprinkler-system-basics-types-of-sprinkler-systems, 2021/06/07/types-of-water-supplies (a connection to a water main at the street level) and 2022/06/03/fire-alarm-notification-delay-from-sprinkler-waterflow (retard). The United States Occupational Safety and Health Administration, Fire Service Features of Buildings and Fire Protection Systems: https://www.osha.gov/sites/default/files/publications/OSHA3256.pdf (chapter 8: control valves, the valve tamper switch, water flow indicators, risers and feed mains; chapter 10: fire department connections, their inlets, plugs and caps, and two-inlet connections called siamese). A fire protection supplier's articles, under https://blog.qrfs.com/: 257-fire-risers-part-1-essential-fire-sprinkler-riser-components (the riser itself, main drain, control valve, pressure gauges, flow or pressure switch), 266-fire-sprinkler-system-risers-part-2-wet-pipe-components-and-assemblies (the alarm check valve, gauges on the supply side and the system side, the backflow preventer between two OS&Y valves), 10-how-a-fire-sprinkler-works-thermal-sensitivity (the pip cap, also known as a plug; frame arms), 131-which-fire-sprinkler-do-i-have-and-how-do-i-replace-it (plug, heat-sensitive element, deflector, and a frame to keep everything in place), 53-the-role-and-components-of-fire-department-connections, 86-draining-a-fire-sprinkler-system-main-drain-auxiliary-drain-and-more (the main drain discharges outside), 360-how-does-a-water-motor-gong-work-in-fire-sprinkler-systems (a bell that needs no electricity; the retard chamber absorbs water surges), 12-what-is-a-flow-switch-or-water-flow-detector-in-a-fire-sprinkler-system (a vane or paddle in the pipe), 39-what-is-a-tamper-switch-for-fire-protection-systems (signals if the valve partially or fully closes), 38-fire-sprinkler-covers-vs-escutcheons-the-definitive-guide (a trim ring that covers the cut-out in the ceiling), 320-nfpa-13-seismic-bracing-requirements (system risers distribute water to cross mains) and 296-fire-sprinkler-pipe-how-to-perform-an-obstruction-investigation-part-1 (valves, risers, cross mains and branch lines). A sprinkler maker's technical data: https://www.vikinggroupinc.com/sites/default/files/2026-09/062293_03.pdf (an alarm check valve: the clapper, the alarm port, the optional retard chamber, the water motor alarm, inlet and outlet pressure gauges, the system main drain) and https://www.vikinggroupinc.com/sites/default/files/migrated/081714.pdf (a pendent sprinkler: frame casting, deflector, glass bulb, pip cap and sealing spring, thread size, orifice, escutcheon; the liquid in the bulb expands and the glass shatters). A second maker's bulletins: https://www.reliablesprinkler.com/files/bulletins/612.pdf (the mechanical sprinkler alarm, also called a water motor and gong, usually mounted on an outside building wall) and https://www.reliablesprinkler.com/files/bulletins/074.pdf (drawings that label deflector, glass bulb, frame, cap and seal assembly, threads). Wikipedia: https://en.wikipedia.org/wiki/Fire_sprinkler (the plug pushed out of the orifice, the deflector that forms the spray pattern, each sprinkler activating independently), https://en.wikipedia.org/wiki/Fire_sprinkler_system (OS&Y, outside screw and yoke, valves) and https://en.wikipedia.org/wiki/Backflow_prevention_device. The United States Fire Administration's page https://www.usfa.fema.gov/prevention/home-fires/prepare-for-fire/home-fire-sprinklers/ was read as the plan asks; it says that only the sprinkler above the fire sprays, and names no parts. Licence: own work
The drawing: Drawn for this site on 2026-09-28; a generic wet-pipe system, not to scale and not any maker’s product; nothing traced or copied. Licence: own work
How it works
The parts of a fire sprinkler system run from the water main under the street, through a riser hung with valves, gauges and alarms, to the heads under the ceiling. This drawing cuts a small building open and draws one pendent head large.
Read the full guide
From the street to the ceiling, in flow order
In a wet-pipe system the piping is always full of water. It comes from the water main, passes the backflow preventer and the control valve, and climbs the riser to the cross main and the branch lines. When a head opens, flow lifts the clapper in the alarm check valve, and a little water rings the water motor gong outside.
Parts of a fire sprinkler head
The threads screw into the pipe fitting. The frame holds a glass bulb against a cap that seals the orifice. Heat expands the liquid in the bulb until the glass shatters; the cap is released, and water strikes the deflector, which forms the spray.
Pick the group Sprinkler head up close for 7 parts alone, from threads to deflector. The drawing zooms in.
Start here: 8 main parts runs from the water main to the glass bulb, the names a building owner hears first.
Questions and answers
What are the main components of a fire sprinkler system?
A water supply, a control valve, an alarm check valve, a riser, cross mains and branch lines, and the sprinkler heads. On the outside wall are a fire department connection and often a water motor gong. This drawing numbers 23 parts.
Do all fire sprinklers go off at once?
Not in a wet-pipe system like this one. Each sprinkler head opens on its own, when the heat around it breaks its glass bulb or fusible link. The heads away from the fire stay shut.
What is an FDC on a building?
A fire department connection. Firefighters run hoses from a fire engine to its inlets and pump extra water into the sprinkler system. One with two inlets is often called a siamese connection.
What does a tamper switch do?
It watches a control valve, which is normally open. If the valve is partly or fully closed, the switch signals a warning at the fire alarm panel, because sprinklers behind a closed valve get no water.