Fire Sprinkler Systems: How They Work, Types, and What Firefighters Need to Know

Dry Pipe Systems

Dry pipe systems fill the piping with pressurized air or nitrogen instead of water. A dry pipe valve holds back the water supply until a sprinkler head activates. When a head opens, the air pressure drops, the dry pipe valve trips, water fills the pipes, and flows through the open head.

Key characteristics for firefighters

• Delayed water application. After a head activates, there is a delay of 30 seconds to over a minute while water fills the pipes. This is the system's primary operational disadvantage compared to wet pipe.
• Used where wet pipe would freeze. Any unheated space where wet pipe water would freeze uses a dry pipe system: loading docks, parking structures, attic spaces in cold climates, refrigerated warehouses.
• Larger pipe sizes required. To compensate for the delay, NFPA 13 limits dry pipe systems to 500 gallons of volume and requires accelerators or exhausters in larger systems to reduce trip time.
• Trip notification. The dry pipe valve tripping sounds the alarm. This is an indicator that a head has opened — but water has not yet reached the fire.

Deluge and Pre-Action Systems

Deluge systems

Deluge systems have open sprinkler heads (no thermal element) and an empty pipe. A separate detection system (heat, flame, or smoke detectors) opens the deluge valve when a fire is detected, flooding the entire piping system and flowing water through all heads simultaneously. Used in very high hazard occupancies where rapid flooding of an entire area is required: aircraft hangars, ammunition storage, chemical processing areas, and certain industrial applications.

Pre-action systems

Pre-action systems require two conditions to be met before water flows: the detection system must activate AND a sprinkler head must open. This double-interlock design prevents accidental water discharge, which makes it the system of choice for water-sensitive occupancies: data centers, libraries, art storage, museums. The operational tradeoff is increased complexity and slower activation than wet pipe.

Fire Company Response in Sprinklered Buildings

When an engine company arrives at a fire in a building with a sprinkler system, operations change significantly from a non-sprinklered building response:

Do not shut off the system

The most critical rule: do not shut off a functioning sprinkler system without a compelling reason. An operating sprinkler is providing suppression, cooling the structure, and protecting both victims and firefighters. Shutting it off removes the one system that has been actively fighting the fire since it activated. Only shut down the system after the fire is confirmed fully extinguished and the IC makes a deliberate decision to do so.

Support the system first

Before stretching attack lines, connect to the FDC to boost system pressure. A sprinkler system operating from municipal supply alone may not maintain adequate pressure for simultaneous supply to multiple heads or for water delivery to upper floors. Engine company water supply to the FDC is the highest-priority task in a sprinklered building fire.

Find and confirm the operating head

The waterflow alarm and flow indicators (or simply following the wet ceiling and floor) locate the operating head. The fire is in the area directly below or near the operating head. If the fire is already controlled by the sprinkler, a small attack line to extinguish the residual fire may be all that is needed.

Attack with the system, not against it

Direct the attack line at the fire in coordination with the operating sprinkler. Avoid directing hose streams at the operating sprinkler head in a way that cools it below activation temperature — this will cause the head to close before extinguishment is complete.

Fire Department Connection (FDC)

The FDC is the Siamese connection on the exterior of the building that allows engine companies to pump water into the sprinkler system to boost pressure and supplement supply.

• Connect early, before stretching attack lines. FDC connection is the highest-priority task in a sprinklered building fire. In a large commercial fire, the system may need 250–500 GPM or more sustained.
• Use correct pressure. The FDC connection pressure should be sufficient to maintain system pressure at the flowing heads. A typical target is 150 PSI at the FDC connection. Consult your pre-incident plan or local fire sprinkler coordinator for system-specific requirements.
• Locate the FDC before you need it. Pre-incident planning identifies FDC location, required pressure, and the system it serves. In a large complex building, multiple FDC connections may serve different system zones.
• Verify the FDC check valve is functional. The FDC has a check valve that prevents building supply water from back-flowing out through the FDC. Occasionally these fail, resulting in water spraying from the FDC instead of entering the system.

Impaired Sprinkler Systems

An impaired sprinkler system is one that is not capable of functioning normally due to a closed control valve, a depleted water supply, a damaged component, or any other condition. Impaired systems require specific management:

• NFPA 25 requires notification of the authority having jurisdiction (AHJ) and the building owner when a system is impaired for more than 10 hours.
• Hot work permits may require a fire watch when the system is impaired during construction or maintenance. See the Fire Watch guide for NFPA 25 requirements.
• Closed control valves are the most common impairment. A PIV or OS&Y valve that has been partially or fully closed (intentionally or not) eliminates or reduces system function. Check valve position status during pre-incident planning and on arrival at a building fire.
• Treat an impaired building like an unsprinklered building. Adjust your tactical expectations. The life safety and property protection benefits of sprinklers are absent.

Frequently Asked Questions

Do all sprinklers go off at once when one activates?

No. Each sprinkler head is an independent heat-detection device. Only heads that reach their individual activation temperature will open. In the typical residential or commercial fire, only 1–2 sprinkler heads activate, which is sufficient to control or suppress the fire. The simultaneous activation of all heads shown in movies is fictional and only occurs in deluge systems, which are specialized high-hazard systems.

How much water does a fire sprinkler use?

A residential sprinkler head (NFPA 13D) typically flows 8–24 gallons per minute. A standard commercial sprinkler head (NFPA 13) flows 13–25 GPM or more depending on hazard classification. Because only 1–2 heads typically activate, the total water used by a sprinkler-controlled fire is dramatically less than the water applied by fire department hose streams, significantly reducing water damage.

What is the fire department connection (FDC) on a building?

The FDC is a Siamese connection on the building exterior that allows engine companies to pump water into the sprinkler or standpipe system to boost pressure and supplement the building water supply. Connecting to the FDC is typically the first action engine companies take at a fire in a sprinklered building, before stretching attack lines.

What should firefighters do when a sprinkler system is operating at a fire?

Do not shut off the system. Connect an engine to the FDC immediately to boost system pressure. Locate the operating head to find the fire. Attack the fire in coordination with the operating sprinkler. Shut down the system only after the fire is confirmed fully extinguished and the IC makes a deliberate decision to do so.