Water trapped inside a wet pipe sprinkler system expands as the temperature rises. Add surge effects such as water hammer and the pressure can quietly climb past the rated limit of the equipment, typically 175 psi (12 bar). To prevent this, NFPA 13 (2025) requires small 1/2 inch pressure relief valves or auxiliary expansion reservoirs throughout the system. This article sets out, in field terms, which is required where.
1. Relief Valve Requirements on Wet Pipe Systems
NFPA 13 sets the following rules to protect a wet system against pressure build-up:
- Basic requirement: unless an auxiliary air reservoir is installed, every wet pipe system must have a listed pressure relief valve of at least 1/2 inch (15/16 mm).
- Setting: the relief valve is set to discharge at 175 psi (12 bar), or 10 psi (0.7 bar) above the maximum pressure the system can be subjected to, whichever is greater.
- Downstream of check valves: a relief valve must be provided on the downstream side of check valves, so that pressure rise in water trapped behind the check valve can be safely relieved.
- Opening and closing tolerance: listed relief valves are designed to open within 90% to 105% of their rated pressure and to close fully above 80% of that rating.
2. Auxiliary Air Reservoirs and Antifreeze Expansion Control
Auxiliary air reservoir
Where a suitable auxiliary air reservoir is installed to absorb pressure increases, the relief valve requirement is waived. The air reservoir is a sealed vessel that holds air at the same pressure as the water in the wet pipe system and acts as a flexible air cushion accommodating the expansion of the water.
Expansion in antifreeze systems
The expansion of antifreeze solution as it warms produces a significant pressure rise. There are two routes:
- Expansion chamber: sized with the pre-charge air temperature and pressure taken into account, and large enough to ensure that the maximum system pressure does not exceed the rated pressure of the components.
- 1/2 inch relief alternative: on small systems where the total antifreeze volume does not exceed 150 litres (40 gallons), a listed 1/2 inch (15 mm) relief valve may be used instead of a costly expansion chamber.
3. Relief at Pressure-Reducing Valve Outlets
On the discharge side of pressure-reducing valve stations, used in high-rise buildings and dedicated zones, a listed relief valve of at least 1/2 inch (15 mm) is installed, set so that the rated pressure of the system components is not exceeded. So that operation does not discharge water onto walkways or sensitive equipment, the outlet must be piped to a drain or to a safe external discharge point.
4. Marine Wet Systems
Vessels and marine structures are exposed to greater vibration, temperature swing and impact, so the rules are stricter:
- Mandatory protection: relief valves are required on all wet pipe systems used in marine applications.
- Pressure tank safety: safety valves are fitted on pressure tanks feeding the water supply, both to relieve over-pressurisation and to prevent expansion from causing false actuation of dry valve equipment.
Summary: What Is Required Where?
| Application | Minimum size | Setting / operating limit |
|---|---|---|
| Standard wet pipe sprinkler network | 1/2 in (15/16 mm) | 175 psi (12 bar) or maximum working pressure plus 10 psi |
| Antifreeze loops (volume below 150 L / 40 gal) | 1/2 in (15 mm) | Relief set to absorb expansion |
| Pressure-reducing valve outlets | 1/2 in (15 mm) | Set below the rated pressure of system components |
| Marine wet systems | 1/2 in (15 mm) | Continuous wet pipe protection and tank safety |
Frequently Asked Questions
Why does a wet pipe sprinkler system need a 1/2 inch relief valve?
Water trapped in the pipework expands as ambient temperature rises, and surges such as water hammer add to that. The resulting quiet pressure build-up can exceed the rated limit of the equipment, typically 175 psi (12 bar), and damage pipework and sprinkler heads. NFPA 13 therefore requires either a 1/2 inch relief valve or an auxiliary air reservoir on every wet system.
What pressure is the relief valve set to?
It is set to 175 psi (12 bar), or 10 psi (0.7 bar) above the maximum pressure the system can be subjected to, whichever is greater. A listed valve opens within 90% to 105% of its rated pressure and closes fully above 80% of that rating.
If I install an auxiliary air reservoir, is the relief valve still mandatory?
No. Where a suitable auxiliary air reservoir is installed to absorb pressure increases, the relief valve requirement is waived. The reservoir is a sealed vessel holding air at the same pressure as the system water, providing a flexible air cushion that accommodates thermal expansion.
For an antifreeze system, should I use an expansion chamber or a 1/2 inch relief valve?
On small systems where the total antifreeze volume does not exceed 150 litres (40 gallons), a listed 1/2 inch relief valve is permitted instead of a costly and space-consuming expansion chamber. On larger systems an expansion chamber sized with the pre-charge air temperature and pressure taken into account is required.
Is a relief valve mandatory at a pressure-reducing valve outlet?
Yes. A listed relief valve of at least 1/2 inch is installed on the discharge side of the PRV, set so that the rated pressure of the system components is not exceeded. The discharge must be piped to a drain or a safe external point so that operation does not flood walkways or sensitive equipment.
Is this the same as the large circulation relief valve at the fire pump?
No, they are entirely different. The large circulation relief valve at the fire pump discharge, required under NFPA 20, protects the pump from overheating at churn. The 1/2 inch auxiliary relief valves discussed here protect the sprinkler network, alarm valve trim and antifreeze loops from static and thermal pressure build-up.
SprinkCalc — Fire Sprinkler Design Across Three Standards
SprinkCalc covers hazard classification, design density and area, K-factor selection, water demand and hydraulic calculations for NFPA 13, FM Global and BS EN 12845 in a single iOS app, and exports a professional PDF report.
Download SprinkCalc on the App Store
MEP Calc — 86+ Engineering Calculators
MEP Calc bundles 86+ engineering modules in one iOS app: 21 fire calculations plus heating, cooling, HVAC, plumbing, steam and natural gas.
Download MEP Calc on the App StoreNFPA 13, Standard for the Installation of Sprinkler Systems, 2025 Edition — relief valves and auxiliary air reservoirs, antifreeze expansion provisions, pressure-reducing valve outlet safety, marine system requirements, and the associated Annex A explanatory material. Values are for information; the full text of the standard governs any actual design.