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 air reservoirs throughout the system. This article sets out, in field terms, which is required where.
1. Relief Valve Requirements on Wet Pipe Systems (Section 8.1.2)
NFPA 13 sets the following rules to protect a wet system against pressure build-up:
- Basic requirement (8.1.2.1): 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 mm).
- Setting (8.1.2.1): 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 (8.1.2.3): a relief valve complying with 8.1.2.1 is also required downstream of the check valves that the standard requires by reference to 16.16.2.2(1), so that pressure rise in water trapped behind the check valve can be safely relieved. The rule applies to those check valves, not to every check valve.
- Opening and closing range (Annex A.8.1.2.1): according to the annex, standard relief valves open within 90% to 105% of their rating and close at a pressure above 80% of that rating, so the rating should be chosen so that normal system pressure stays out of this range.
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 (8.1.2.2). 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 (definition 3.3.3).
Expansion in antifreeze systems
The expansion of antifreeze solution as it warms produces a significant pressure rise. There are two routes:
- Expansion chamber (8.6.3.3.1–8.6.3.3.2): 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 (8.6.3.4): 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 (16.9.7.3)
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. Where operation would discharge water onto interior walking or working surfaces, Annex A.16.9.7.3 advises piping the discharge to a drain connection or another safe location.
4. Marine Wet Systems (Chapter 31)
Vessels are exposed to greater vibration, temperature swing and impact, so the rules are stricter:
- Mandatory protection (31.3.1): relief valves are required on all wet pipe systems on vessels covered by Chapter 31.
- Pressure tank safety (31.7.2.3.1): 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 mm) | 175 psi (12 bar) or maximum working pressure plus 10 psi |
| Antifreeze systems (volume not exceeding 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 | No size given in Chapter 31; at least 1/2 in (15 mm) under 8.1.2.1 | Relief valve on every wet system; relief valves on pressure tanks |
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 clause 8.1.2 therefore requires either a 1/2 inch (15 mm) 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 standard relief valve opens within 90% to 105% of its rating and closes at a pressure above 80% of that rating (NFPA 13 8.1.2.1 and Annex A.8.1.2.1).
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 (NFPA 13 8.1.2.2). 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 (15 mm) relief valve is permitted instead of a costly and space-consuming expansion chamber (8.6.3.4). On larger systems an expansion chamber sized with the pre-charge air temperature and pressure taken into account is required (8.6.3.3.1–8.6.3.3.2).
Is a relief valve mandatory at a pressure-reducing valve outlet?
Yes. Under clause 16.9.7.3 a listed relief valve of at least 1/2 inch (15 mm) is installed on the discharge side of the PRV, set so that the rated pressure of the system components is not exceeded. Where operation would discharge water onto interior walking or working surfaces, Annex A.16.9.7.3 advises piping the discharge to a drain connection or another safe location.
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.
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Download MEP Calc on the App StoreNFPA 13, Standard for the Installation of Sprinkler Systems, 2025 Edition — 8.1.2 (relief valves and auxiliary air reservoirs), 8.6.3.3–8.6.3.4 (antifreeze expansion), 16.9.7.3 (pressure-reducing valve outlet relief), 31.3.1 and 31.7.2.3.1 (marine systems), 3.3.3 (air reservoir definition), Annex A.8.1.2.1 and A.16.9.7.3. Values are for information; the full text of the standard governs any actual design.