A pressure relief vent is designed to release excess pressure from a room during the discharge of a gas fire suppression system. Without it, the sudden release of gas can create a rapid pressure build-up. This can damage walls, ceilings, or doors and cause system failure due to the extinguishing agent leaking out of the room.

For pressure relief venting to work properly, the vent needs to be the right size. If it’s too small, pressure may rise too high. Correct sizing with pressure relief vent calculations, helps protect both people and property. It also ensures the system meets safety standards like ISO 14520 and NFPA 2001.

In this article, we’ll explain what affects vent size, which standards apply, and how to calculate the right venting area for your space.

Pressure relief and room integrity

When a gaseous fire suppression system activates, it releases gas into the protected space within seconds. This sudden release creates a sharp increase, or decrease in pressure, depending on the extinguishing agent. Without a way for that pressure to escape, the force can push against walls, ceilings, doors, or windows. This may cause serious damage.

Pressure relief vents are there to prevent that. They allow the excess pressure to escape in a controlled way, helping to maintain the structural integrity of the room. This becomes even more critical in tightly sealed rooms where the suppression agent needs to remain effective for extended periods of 15 minutes or longer.

If the room is damaged during discharge, the gas may leak out too quickly. That can reduce the effectiveness of the system and risk the fire not being properly suppressed. In some cases, it can also pose a danger to people nearby.

By allowing just the right amount of pressure to escape, a properly sized vent keeps the room intact, protects equipment, and helps ensure the suppression system does what it’s meant to do.

Factors influencing vent size

Sizing a pressure relief vent isn’t guesswork. It’s based on a range of factors that affect how quickly pressure builds up in a room during discharge. Below are the key elements that determine the vent size you’ll need. 

By taking all these factors into account, it’s possible to size a pressure relief vent that protects both the suppression system and the space.

Volume of the protected room (m³)

Room volume is an essential input factor to any gas calculation. The more space there is to protect, the more gas needs to be released during discharge. This increase in gas displaces more air and hence needs larger vents to accommodate this. To relieve that extra volume and prevent structural damage, a larger venting area is needed in larger rooms.

Peak discharge pressure or blast overpressure (bar/kPa)

With gas suppression, the peak pressure refers to the maximum pressure the room structure can withstand. The standard values used depend on the type of construction:

• Light construction: standard peak pressure vale is 250 pascals (5 psf).
• Heavy construction: standard peak pressure value is 500 pascals (10 psf).

Different suppression systems and gases create varying pressure effects. Some produce high overpressure, others create negative pressure. To size a vent correctly, it’s important to know the required peak pressure during discharge and how it relates to the structural limits of the room.

If the vent is too small, it won’t be able to relieve pressure quickly enough, which can cause the room to exceed its peak pressure threshold and sustain damage. A properly sized vent acts like a safety valve, allowing the right amount of pressure to escape to prevent structural damage to the room.

Type of gas (inert or chemical)

The type of gas your gas fire suppression system has a major impact on how pressure behaves during and after discharge. This directly affects what kind of pressure relief vent is needed to safely manage those pressure changes and prevent structural damage.

Inert agents

Inert gases, such as nitrogen or argon, cause a rapid increase in the room’s volume. The greatest pressure impact happens within the first one or two seconds of discharge. This sudden expansion results in a sharp pressure spike. 

For inert gases, it is important the vent must respond instantly and provide sufficient surface area to release the pressure as quickly as it builds. Even a brief delay or an undersized vent can cause structural damage.

Chemical agents

Chemical gases, such as FM-200 or NOVEC 1230, increase the room volume by about 15 percent, which is less than inert gases. However, they introduce different challenges during discharge.

When Novec evaporates, it cools the surrounding air rapidly. Cooler air takes up less space than warm air, causing the air volume in the room to contract quickly. This contraction leads to a sudden drop in pressure, known as negative pressure. Negative pressure can pull on walls and windows, putting stress on the room’s structure by pushing inward.

After the discharge, as the air gradually warms up again, the pressure rises once more, creating a positive pressure effect. Because of these fluctuations, chemical gas systems require pressure relief vents that can manage both negative and positive pressure changes. This ensures the room remains protected throughout the entire pressure cycle.

Discharge time

For both chemical and inert gases, discharge time is a necessary input for gas calculations, as it directly affects how quickly pressure builds up in the room. Faster discharges create a more abrupt pressure rise, requiring larger vents to relieve it safely, while slower discharges give the pressure more time to dissipate and typically require less venting.

For inert gasses the discharge time is generally either 60 or 120 seconds. A 120 second discharge produces a lower initial pressure spike and so requires smaller pressure vents. A regulated valve releases the gas in a more controlled manner, reducing the initial pressure spike. 

Common discharge times:

• Regulated valves: 120 seconds
• Non-regulated valves: 60 seconds

Inert gas fire suppression systems with a regulated valve require less venting than inert gas fire suppression systems with non-regulated valves. Regulated valves are often used when pressure relief is a concern, as they minimize the initial pressure spike.

You can typically find out what type your system has by checking the valve specifications provided by the system manufacturer or installer. 

Formulas and pressure relief vent calculations

Vent sizing is not just based on best guesses or rough estimates. Industry standards provide detailed guidance to ensure vents perform as intended during a gas discharge. The most widely used standards are:

• NFPA 2001 (used in the US and many international projects).
• ISO 14520 (global standard, often referenced in data centres and critical infrastructure).
• EN 15004 (European equivalent of ISO 14520).

All three give recommendations on maximum allowable pressure rise, typical discharge times, and how to calculate the free vent area (the actual open space required for air and gas to escape).

Example formula

A common starting point is: A = (V × ΔP) / (K × √T)

• A = Free vent area (m²).
• V = Volume of the protected room (m³).
• ΔP = Maximum allowable pressure rise (Pa).
• K = Discharge coefficient (based on gas type and vent performance).
• T = Discharge time (s).

This is a simplified example. In practice, the formula varies depending on the gas used, the discharge rate, and whether the vent is gravity-operated or pressure-actuated.

• Gravity-operated: The vent opens automatically when the internal pressure rises enough to lift a weighted flap or panel, relying solely on gravity to close it once pressure normalizes.
• Pressure-actuated: The vent uses a spring or other mechanism calibrated to open at a specific pressure threshold, providing more precise control over when the vent opens and closes.

Use of safety factors

To account for uncertainties, like variations in room construction, gas flow, or slight design changes, it’s common to add a safety factor. This might be 10% to 20% more vent area than the minimum calculated, just to ensure there’s a margin for error. The exact factor depends on the project risk level and engineering judgment.

The importance of certified testing

Formulas and standards provide a solid foundation for vent sizing, but real-world conditions often differ from theoretical predictions. That’s why certified testing is essential to ensure accuracy.

Standard HVAC testing and certification methods focus on measuring free vent area with vent blades fixed in the open position. These tests do not simulate the sudden pressure increases that occur during gas suppression system discharges and can therefore be misleading for this application. 

Certified testing provides reliable data on the actual free vent area, which can be quite different from the manufacturer’s nominal or claimed figures. To address this, AFP developed a certification protocol in partnership with BRE based on the 2008 live gas discharge test. All AFP vents are tested under real-world conditions using the AFP blast simulator.

The test starts with the blades of the vent in the closed position. When real gas is released, the test simulates the actual pressure spike found in real-world conditions. This allows measurement of the pressure at which the blades open, how far they open, and the true free vent area during discharge.

Only AFP vents are certified according to the BRE 2008 live discharge tests, providing reliable data on their performance in practical scenarios.

When to get expert advice

In short: always.

Venting is a small part of the overall fire suppression system, but it can make or break the performance. Getting it wrong can lead to structural damage, failed discharges, or non-compliance with fire safety regulations. That’s why it’s important to involve venting experts early in the design process.

An experienced advisor can look at the full picture. They will look at the room construction, system type, layout, and local standards and then suggests the right venting solution. They can also assist with calculations, recommend tested products, and advise on placement and installation.

Even if the suppression system is already designed, it’s worth getting an expert to review the venting strategy. It’s a small step that can prevent big problems down the line.

Get expert advice from AFP

At AFP, we specialise in pressure relief vents, and only vents. That focus allows us to go beyond theory and offer real, tested solutions you can rely on.

Our vents are independently tested using live gas discharges to measure the actual free vent area under blast conditions. This ensures they perform as expected when pressure builds rapidly during a gas discharge. We also test for repeatability, seal strength, and air tightness. These are all crucial to maintaining room integrity before and after activation.

Whether you’re working with an inert gas system or a chemical agent, we help you choose the right vent for your space, based on both industry standards and real-world performance data. We can also assist with sizing calculations and product selection, making sure you meet the requirements of ISO 14520, EN 15004, and/or NFPA 2001.

If you’re unsure about vent sizing, have a complex room layout, or just want a second opinion, get in touch with us. Our team is here to help from the early design stage through to installation and testing.