Types of Fire Sprinkler Systems: Wet, Dry, Pre-Action, Deluge and More

Sprinkler systems all deliver the same thing — water onto a fire — but they get there through different arrangements of pipes, valves and detection.

That arrangement is the fitter's work: it decides whether the piping holds water or air, whether a detection system has to act first, and which acceptance tests the finished system faces.

System type.	In the piping.	How water flows.	What fitters add to the job.
Wet pipe.	Water, held in the piping.	Each sprinkler opens individually at the heads the fire reaches.	The core craft: piping, hangers, heads, alarms, the hydrostatic test.
Dry pipe.	Air or nitrogen, with water held back at the dry-pipe valve.	A sprinkler opens, air escapes, water follows into the piping.	Air supplies and compressors, the dry-pipe valve, air tests and trip tests.
Pre-action (single or double interlock).	Air in the piping; water held behind an interlocking valve.	Detection — and on double interlock, sprinkler operation too — admits water first.	Detection interfaces, the pre-action valve, heavier testing.
Deluge.	Dry piping behind a deluge valve, feeding open sprinklers.	A detection system opens the valve and every sprinkler discharges at once.	The deluge valve, its detection circuit and the open-head piping.
Antifreeze.	An antifreeze solution in runs of wet piping.	Sprinklers open individually, as on a wet system.	Solution-filled runs of piping tied into wet systems.
ESFR and storage sprinklers.	Water, under designs built around the storage below.	Heads selected and positioned for the racked or piled storage.	Storage-specific heads, with clearances checked above the storage.
Water mist.	Water, through nozzles and piping built for mist.	A separate technology with its own design basis and tests.	A specialty track beside wet and dry installation.

NFPA 13 — NFPA's industry benchmark for the design and installation of automatic fire sprinkler systems — is in its 2025 edition as of October 2026.

Water mist has its own standard, NFPA 750, covered in the water mist section below.

Companion standards cover the water supply around these systems: NFPA 14 (2024 edition) for standpipes and hose systems, NFPA 20 (2025) for stationary fire pumps and NFPA 24 (2025) for private fire service mains.

NFPA 25 (2026) covers the inspection, testing and maintenance of water-based systems once they are in service — the work sprinkler service techs and ITM inspectors live in.

NFSA notes that regular fire sprinkler inspection, testing and maintenance is required by the International Fire Code and by NFPA 1, 101 and 5000.

For how NFPA 13 itself is organized, and the requirements fitters meet on every one of these systems, see NFPA 13 for sprinkler fitters.

Still mapping the role itself?

Start with the sprinkler fitter career guide.

A wet pipe system keeps water in the piping at all times.

Every sprinkler sits closed until the heat at that individual head opens it, and water discharges only from the heads that have opened.

NFPA's fire data counts sprinkler systems of all types together, not wet systems alone, but it shows how few heads usually open.

In 77% of US structure fires where sprinklers operated between 2017 and 2021, only one sprinkler operated, and in 96% of them five or fewer did — NFPA's U.S. Experience with Sprinklers report, published in 2024.

Wet installation is the core craft the other system types modify: piping, hangers and bracing, heads, the fire department connection and the waterflow alarm, all installed from approved drawings.

Acceptance runs a hydrostatic test of the piping.

On the Idaho State Fire Marshal's NFPA 13 acceptance checklist — written on the 2018 IFC and the 2019 edition of NFPA 13 — that test is 200 psi held for 2 hours, including the fire department connection piping.

The same checklist wants the waterflow alarm to initiate within 5 minutes.

A dry pipe system holds air or nitrogen in the piping, with water held back at the dry-pipe valve.

When a sprinkler opens, the air escapes, pressure falls, and water follows through the piping and out the open heads.

Dry systems exist for the exposure wet piping cannot take: runs through spaces that are not heated.

The Idaho State Fire Marshal's checklist even requires the riser room on these systems to be heated.

Testing is heavier than wet work.

The piping still gets the 200 psi, 2-hour hydrostatic test, and dry systems add a 40 psi air test held for 24 hours with less than 1.5 psi of loss.

Before handover, the checklist requires the dry-pipe valve to be trip-tested, the quick-opening device tested on systems of 500 or more gallons, and systems of 750 or more gallons to deliver water within the time Table 7.2.3.6.1 of the 2019 edition of NFPA 13 sets.

The air supply is its own work: compressors must restore normal system air within 30 minutes, and system air pressure is set at least 20 psi above the trip pressure.

A pre-action system is a dry system with an additional guard: an automated valve holds water out of the piping until a signal arrives.

On a single-interlock system, a detection event admits water into the piping, and a sprinkler that then opens discharges it.

On a double-interlock system, both must happen — detection and a sprinkler operating — before water enters the pipe.

The design serves spaces where water from an accidental discharge would be costly in its own right, so the piping stays dry until the evidence of fire is real.

That adds testing.

The Idaho checklist puts double-interlock pre-action systems under the same 200 psi, 2-hour hydrostatic test plus the 40 psi, 24-hour air test that dry systems carry.

It also has pre-action systems over 20 sprinklers supervised — under constant pressure monitoring — at a minimum of 7 psi.

A deluge system turns the individual-sprinkler idea inside out.

The sprinklers are open, with no heat-operated closing element, and the piping sits dry behind a deluge valve.

When a separate detection system activates, the valve opens and water discharges from every sprinkler across the protected area at once.

That is the layout for hazards that need water across the whole area simultaneously rather than head by head.

The monitoring hardware differs too: the Idaho checklist rules out paddle-type waterflow switches on dry, pre-action and deluge systems alike.

Deluge also borders the wider fixed-extinguishing field.

Where a system is installed to meet an OSHA standard, 29 CFR 1910.160 covers fixed extinguishing systems — except automatic sprinkler systems, which fall under 29 CFR 1910.159.

The sprinkler rule, 29 CFR 1910.159, lists the acceptance tests it calls proper: flushing underground connections, hydrostatic tests of piping, air tests in dry-pipe systems, dry-pipe valve operation and tests of drainage facilities.

An antifreeze system is a wet system with a twist at the cold end: runs of piping are filled with an antifreeze solution instead of plain water, so piping exposed to cold can stay protected off a wet system.

The sprinklers behave as they do on any wet system, opening individually.

The solution is what stands between that piping and freezing.

Washington's State Fire Marshal rules (WAC 212-80) define an antifreeze sprinkler system as a wet pipe system containing a liquid solution to prevent freezing.

On sprinkler operation it discharges the solution, followed immediately by water.

The rules for the solution itself have changed under NFPA 25.

According to NFSA (2022), NFPA 25 required existing antifreeze sprinkler systems to be converted to a listed antifreeze by September 30, 2022.

The same NFSA post says new antifreeze systems were not allowed after September 12, 2012 unless a listed antifreeze was available, and that the first listed antifreeze (tested to UL 2901) appeared in 2019.

It also reports a later NFPA 25 committee vote on letting legacy antifreeze remain within concentration limits.

What governs a specific system is set by the edition the AHJ adopted — confirm the requirements there before you install, fill or service one.

ESFR — early suppression fast response — sprinklers are storage sprinklers: heads selected and positioned around the commodity and rack layout beneath them.

Storage work makes the installation details unforgiving, because clearances above the storage are part of how the design performs.

The Idaho State Fire Marshal's checklist makes the point with numbers: ESFR sprinklers must sit at least 1 ft horizontally from the bottom edge of a bar joist or open truss, and at least 36 in. above the top of storage.

The checklist's general sprinkler rules also apply: heads must match the plans in type and temperature rating, must not be painted or covered, and must sit at least 4 in. from walls.

Water mist systems discharge water as a fine mist through nozzles.

The working medium is water, but the equipment, design basis and test procedures are a separate technology from the sprinkler types above.

For a sprinkler fitter's career, mist is a specialty track — nearer the suppression technician's special-hazards lane than to standard wet and dry installation.

NFPA's standard for these systems is NFPA 750, Standard on Water Mist Fire Protection Systems, in its 2027 edition as of October 2026.

It standardizes design, installation, maintenance and testing requirements for water mist systems.

The 2024 International Fire Code (Section 904.11) requires automatic water mist systems to be designed and installed in accordance with NFPA 750 and the manufacturer's instructions.

Career information, not licensing, legal, code, inspection, engineering or safety advice. The requirements that govern any system you work on are set by the edition the authority having jurisdiction adopted — confirm them there.