Air left inside a wet pipe sprinkler system is the number one cause of the corrosion that shortens pipe life, and it makes water hammer worse on every fill. This article brings the NFPA 13 (2025) venting requirements, the NFPA 25 in-service inspections and the FM Global DS 2-1 corrosion provisions together in one place, gives a step-by-step site procedure for filling and venting, and ends with a ready-to-use specification clause.
1. Purpose and Technical Rationale
In wet pipe sprinkler systems, air trapped inside the pipe is the primary cause of internal corrosion. Corrosion starts at the air/water interface where metal, water and oxygen coexist, and it concentrates around the air pockets at the system's high points. The pipe wall thins along that air-pocket line, and that is where the first leaks appear.
FM Global, contrary to the earlier view, treats oxygen rather than microbiologically influenced corrosion (MIC) as the primary cause; MIC accounts for only 10–20% of sprinkler leaks (FM Data Sheet 2-1 update).
The second harm of trapped air is mechanical: during filling and pump start the compressed air pocket compresses and expands abruptly and intensifies water hammer. Filling must therefore be slow and the air released progressively.
2. NFPA 13 (2025) — Design and Installation Requirements
2.1 Mandatory clauses
- §8.1.5 Air Venting: "A single air vent with a connection conforming to 16.7 shall be provided on each wet pipe system utilizing metallic pipe." (See A.16.7)
- §8.1.5.1: "Venting from multiple points on each system shall not be required." There is no upper limit; additional vents are at the designer's discretion.
- §16.7 Air Venting: The vent is located near a high point in the system, and air is released by one of the following: (1) a manual valve of at least ½ in. (15 mm), (2) an automatic air vent, (3) a remote inspector's test valve, (4) other approved means.
2.2 A.16.7 — Annex explanatory material
- Intent: to remove as much trapped air as practical from a single point each time the system is filled. Removing all of the air is neither intended nor practical.
- The vent connection is taken from the top of a horizontal pipe, or alternatively from the side of a riser or riser nipple.
- A manual air release valve must be accessible and preferably located no more than 2.1 m (7 ft) above the floor.
- Automatic vents are not subject to that accessibility requirement; however they are recommended in an open-ceiling area, above a lay-in ceiling or above an access panel, and the location is identified with a placard.
- Branch lines need not be interconnected for venting; the vent does not have to be connected to a drain line.
Numbering history: these provisions were 7.1.5 and 8.16.6 in NFPA 13-2016; they were renumbered 8.1.5 and 16.7 in the 2019 edition and keep those numbers in the 2025 edition.
2.3 Filling, flushing and pressure test clauses (NFPA 13-2025)
| Clause | Content |
|---|---|
| 6.11.2.1.3.2(A) | Flushing is carried out before the hydrostatic test. |
| 6.11.2.2.1 | Underground supply piping: 200 psi (14 bar) or working pressure + 50 psi (3.4 bar), whichever is greater, ±5 psi tolerance for 2 hours. [NFPA 24: 10.10.2.2.1] |
| 6.11.2.2.2 | Acceptance criterion: less than 5 psi (0.34 bar) pressure drop and no visible leakage. Trapped air corrupts this measurement. |
| 29.2.1.1 | Aboveground system: 200 psi (14 bar) for 2 hours without loss. |
| 29.2.1.3 | Portions operating above 150 psi (10 bar): working pressure + 50 psi (3.4 bar). |
| 29.2.1.4 (2025 change) | In pumped systems the test pressure is determined from the pump churn (shutoff) pressure plus the elevation-corrected maximum static supply pressure. |
| 29.2.1.5 | An interim air test (§29.2.2) may be used in cold weather; it does not remove the hydrostatic test requirement. |
| 29.2.1.6 | The test pressure is read at the gauge at the lowest elevation of the portion being tested. |
| 30.7.1 | Deluge systems are isolated and tested per §29.2.1. |
3. NFPA 25 — In-Service Inspection, Testing and Maintenance
Air venting devices entered NFPA 25 as a separate item with the 2023 edition (Chapter 13 — Valves, Valve Components, and Trim). The current edition of the standard is 2026.
| NFPA 25 clause | Frequency | Requirement |
|---|---|---|
| 13.11 Automatic Air Vents | Annually | Automatic air vents are inspected visually from floor level; verify there is no physical damage and no sign of leakage. |
| 13.11.1 | Every 5 years | Strainers, filters and screens are removed and examined for damaged or corroded parts. |
| 13.11.2 | During the examination | Removed strainers, filters and screens are flushed until clean, or replaced. |
| Table 5.1.1.2 (summary table) | Annually | Automatic air vents are listed as an inspection item with a reference to Chapter 13. |
| Chapter 5 — pipe and fittings | Annually | Pipe, fittings and sprinklers are inspected from floor level for external corrosion, damage and missing hangers/seismic bracing. |
| Chapter 14 — internal condition assessment | Every 5 years | The condition of the pipe interior (corrosion, tuberculation, obstruction) is assessed. Corrosion caused by trapped air is found here. |
| Chapter 13 — main drain test | Annually | The main drain test is carried out. Because the system is refilled after every drain-down, venting is repeated. |
4. FM Global Data Sheet 2-1 — Corrosion in Automatic Sprinkler Systems
| Clause | Requirement |
|---|---|
| 2.2.1.9 | Remove trapped air from wet pipe systems: install a minimum ½ in. (13 mm) FM Approved automatic air release valve or FM Approved manual valve at the system high points. Vent the air every time the system is drained and refilled. |
| 2.2.1.13 A | Where nitrogen is used on wet systems, pressurise with an FM Approved nitrogen generator (see §3.3.1). |
| 2.2.1.13 C | Install a minimum ½ in. (13 mm) FM Approved automatic air release valve or manual valve at the system high points. |
| 2.2.1.13 D | Vent trapped air every time the system is drained and refilled. |
| 3.3 / 3.4 | Common causes of corrosion: trapped air, corrosive water chemistry and periodically feeding oxygenated water into the system during maintenance. §3.4 illustrates corrosion and leakage around an air pocket. |
| 3.3.1 | Nitrogen fill/purge procedure for wet systems (below). |
4.1 FM DS 2-1 §3.3.1 — Nitrogen fill/purge procedure (manual method)
- Drain the wet pipe system completely.
- Charge the system with compressed nitrogen to about 30 psi (2 bar).
- Measure the nitrogen concentration at the vent at the remote end of the system.
- Purge the system down to 0 psi (0 bar).
- Repeat steps 2–4 until the concentration is at least 98% (typically 3–4 cycles depending on system geometry).
- Once 98% is reached, fill the system with water.
The automatic method can be applied with a pre-installed auto-purge device while the system is in service.
5. NFPA 24 and EN 12845
- NFPA 24 (current edition 2025): acceptance and test provisions are §10.10.2.1 (flushing) and §10.10.2.2 (hydrostatic); NFPA 13-2025 §6.11 refers directly to these clauses. Annex C.4 recommends flushing the line, removing trapped air and leaving the line under pressure one day before the test to minimise pressure loss.
- EN 12845: contains no explicit NFPA 13-style "one vent required" clause for wet systems; charging with air and water is regulated mainly for dry and pre-action systems under §9.5.5 "Charging with air and water". Air venting on European and Turkish projects must therefore be imposed through the specification; typical specification language covers automatic air vents at high points, drains at low points, a lockshield isolating valve on each vent and a discharge routed to a visible location.
6. Site Procedure — Filling and Air Venting
1 Identify the vent points
At least one vent per wet system (NFPA 13 §8.1.5), at the highest point farthest from the riser. Riser tops, roof-main peaks, inverted-U crossovers and intermediate high points are assessed separately; additional vents do not violate the code (§8.1.5.1).
2 Verify pipe pitch
At a horizontal high point the pipe must be level or pitched back towards the riser; otherwise the air never reaches the vent.
3 Fill the system slowly
Open the main control valve (OS&Y or butterfly) gradually; charge at a controlled low flow from the city main, the pump bypass or the fill connection. Sudden opening produces water hammer.
4 Vent air progressively while filling
Keep the high-point vents, end caps and inspector's test valves open; air escapes until water arrives. On an automatic vent the ball valve stays closed before filling and is opened slowly once the system is full; when the air is gone the float closes and prevents water discharge.
5 Then pressurise
No jockey pump pressurisation until the air has been fully released.
6 Finish flushing and venting before the hydrostatic test
NFPA 13 §6.11.2.1.3.2(A), NFPA 24 Annex C.4. Trapped air falsely trips the 5 psi loss criterion of §6.11.2.2.2. Run the test: §29.2.1.1 at 200 psi (14 bar) for 2 hours; portions above 150 psi (10 bar) at working pressure + 50 psi (3.4 bar) (§29.2.1.3).
7 Repeat after every drain-down, repair or alteration
NFPA 13 A.16.7, FM DS 2-1 §2.2.1.9 / 2.2.1.13 D. In service: inspect automatic vents visually every year (NFPA 25 §13.11); remove and flush/replace strainers every 5 years (§13.11.1–13.11.2).
Where water chemistry is poor or MIC is suspected, take oxygen out of the equation altogether: consider nitrogen inerting of the wet system (98% target, FM DS 2-1 §3.3.1) or an auto-purge device.
7. Device Selection Note
Rather than settling for a single vent, prefer an FM Approved automatic vent at the system's real high points. A remote inspector's test valve is accepted under NFPA 13 §16.7(3) but it is manual and needs personnel intervention on every fill; an automatic vent releases the air that accumulates over the life of the system by itself. Look for: a strainer-equipped body, dielectric isolation (to prevent galvanic corrosion), float shut-off, at least a DN15 inlet and a discharge routed to a visible, drained point.
8. Suggested Specification Clause
Frequently Asked Questions
Why does trapped air matter so much in a wet pipe sprinkler system?
Trapped air is the primary cause of internal corrosion: corrosion starts at the air/water interface where metal, water and oxygen coexist, and concentrates around air pockets at high points. FM Global DS 2-1 also treats oxygen as the primary cause; microbiologically influenced corrosion (MIC) accounts for only 10–20% of sprinkler leaks. Trapped air also intensifies water hammer during filling and pump start.
How many vents does NFPA 13 (2025) require on a wet system?
§8.1.5: every wet pipe system using metallic pipe shall be provided with a single air vent connected per §16.7. §8.1.5.1 does not require venting at more than one point; there is no upper limit and additional vents are at the designer's discretion. Per §16.7 the vent is located near a high point and air is released through a manual valve of at least ½ in. (15 mm), an automatic air vent, a remote inspector's test valve or another approved method.
How often are automatic air vents inspected under NFPA 25?
NFPA 25 §13.11: automatic air vents are inspected visually from floor level annually for physical damage and signs of leakage. §13.11.1: strainers, filters and screens are removed every 5 years and examined for damage or corrosion; §13.11.2: removed parts are flushed until clean or replaced. Air venting devices entered NFPA 25 as a separate section in the 2023 edition.
How is the FM Global DS 2-1 nitrogen fill procedure carried out?
§3.3.1 manual method: drain the wet system completely; charge with compressed nitrogen to about 30 psi (2 bar); measure the nitrogen concentration at the vent at the remote end; purge the system to 0 bar; repeat until the concentration is at least 98% (typically 3–4 cycles); once 98% is reached, fill with water. The automatic method uses a pre-installed auto-purge device while the system is in service.
How is air venting handled on EN 12845 projects?
EN 12845 has no explicit "one vent required" clause for wet systems like NFPA 13; charging with air and water is mainly regulated for dry and pre-action systems under §9.5.5. Air venting on European and Turkish projects must therefore be imposed through the specification: automatic air vents at high points, drains at low points, a lockshield isolating valve on every vent and a discharge routed to a visible location.
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Download SprinkCalc on the App StoreNFPA 13, Standard for the Installation of Sprinkler Systems, 2025 edition — §8.1.5, §8.1.5.1, §16.7 and A.16.7 (air venting), §6.11 and §29.2.1 (flushing, hydrostatic testing), §30.7.1. NFPA 25, Standard for the Inspection, Testing, and Maintenance of Water-Based Fire Protection Systems — §13.11, §13.11.1, §13.11.2 (added in the 2023 edition; current edition 2026), Table 5.1.1.2, Chapters 5 and 14. NFPA 24 (2025) — §10.10.2.1, §10.10.2.2, Annex C.4. FM Global Property Loss Prevention Data Sheet 2-1, Corrosion in Automatic Sprinkler Systems — §2.2.1.9, §2.2.1.13, §3.3, §3.3.1, §3.4; FM Global Research Technical Report P14180. NFPA, "Corrosion in Sprinkler Systems". EN 12845:2015 — §9.5.5. Manufacturer bulletins: United Fire Systems AR-1 installation instructions, Reliable Model AAV, AGF (interpretation of NFPA 13 air vent requirements), Potter corrosion control catalogue; MeyerFire (water hammer), McKinney Fire Department technical bulletin (A.16.7 excerpt).