What overpressure must a pressurised room or analyser house maintain?

At least 25 Pa above the adjacent spaces. IEC 60079-13:2017 requires the pressurisation system of a room to be able to hold 25 Pa with 50 % of all outlets other than doors open, and treats a pressure differential below 25 Pa as failure of the pressurisation. When a door without an airlock is opened, the pressure may fall below 25 Pa for that time only if clean air flows outwards through the door at 0.3 m/s or more.

Short answer

• Minimum: 25 Pa (about 2.5 mm water column, roughly 0.1 inch of water) relative to adjacent spaces, for both levels pb and pc.
• Test condition: the value must be reached with half of the outlets (vents, dampers, openings, not doors) open, at the minimum pressurisation flow.
• Failure criterion: below 25 Pa. A sensor in the room must detect it and be able to raise an alarm.
• Doors: an airlock, or at least 0.3 m/s outward through an open door with half of the other openable outlets open. The 0.3 m/s figure assumes low wind.
• Analyser houses: the same 25 Pa applies when they are pressurised. Internal sources are controlled separately, by ventilation that keeps the room below 25 % of the lower flammable limit, which is monitored by the air flow in the ducting, not by the room pressure.
• Not the same as Ex p equipment: IEC 60079-2 requires 50 Pa for pressurised enclosures with level pxb or pyb, and 25 Pa for pzc.

Where the 25 Pa requirement comes from and how it is tested

IEC 60079-13 deals with rooms that people enter, located in Zone 1, 2, 21 or 22. Its pressurisation clauses set linked requirements:

The continuous air flow only has to compensate for leakage and hold the pressure; the much larger purge flow (normally 10 room volumes before switch-on) is a separate requirement. The draft third edition of IEC 60079-13, at enquiry stage in 2026, keeps the 25 Pa minimum.

Why 25 Pa, and when to design for more

A pressure differential works because air always flows from high to low pressure through every crack. Twenty-five pascals is small, about the dynamic pressure of a wind of 6.5 m/s (from ½ × 1.2 kg/m³ × v²). That comparison explains the practical points:

• Wind. On the windward side of a building, wind pressure can locally exceed the room overpressure in strong wind. The standard notes that its 0.3 m/s door velocity is based on low wind and may need to be increased for local conditions; the same logic supports a higher set point at exposed sites, such as offshore modules.
• Stack effect. Warm air in a tall room or a multi-storey building changes the pressure difference between the top and the bottom. The 25 Pa has to hold where it is lowest, so check the differential at those openings.
• Extract equipment inside. A fume hood, compressor or other air-consuming device in the room must be supplied with its own air; otherwise it can pull the room towards zero.
• Doors. At 25 Pa a 0.9 m × 2.1 m door carries about 47 N of pressure load; at 50 Pa, about 95 N. Doors must still close and latch against the pressure, so very high set points create their own problems.

A practical design therefore controls to a set point above 25 Pa and alarms at 25 Pa, so that ordinary fluctuations do not trip the room; the margin is a design choice, not a figure from the standard.

Analyser houses: pressure and dilution together

An analyser house in a hazardous area typically has both problems: the outside atmosphere must be kept out, and sample lines inside can leak. IEC 60079-13 then applies both techniques. Pressurisation keeps 25 Pa against the outside. Artificial ventilation, sized with IEC 60079-10-1, keeps the concentration from internal releases below 25 % of the lower flammable limit; it is monitored by flow or pressure switches in the air ducting, not by the room pressure, and gas detectors confirm the dilution. Pipes bringing flammable substances in must be isolatable from outside the room, and flow restrictors should limit any leak, because no practical pressure differential helps against a large leak inside the room. The draft third edition clarifies that its scope covers analyser houses.

In North America, NFPA 496 (2027 edition, which replaced the 2024 edition) covers pressurised control rooms and analyser rooms; check its own values when that standard applies.

Common mistakes

• Setting the alarm at the operating pressure. Every door opening then gives a nuisance alarm, and operators start ignoring it.
• Measuring against the wrong reference. The differential that matters is to the adjacent space where the hazardous atmosphere can be, not to another pressurised room.
• Using the Ex p enclosure value (50 Pa) for rooms, or the room value for enclosures. They are different standards and different numbers.
• Relying on overpressure for internal leaks. Pressure keeps gas out; only ventilation dilutes gas released inside.

Related on excodex

• Pressurised rooms and analyser houses: making an area non-hazardous by ventilation
• Pressurisation (Ex p): px, py and pz, purging and protective gas
• Can a control room inside a hazardous area be made safe by pressurisation?
• What happens when pressurisation fails in a pressurised room?
• Standards spaces: IEC 60079-13, IEC 60079-2

Standards and further reading

• IEC 60079-13:2017 (edition 2), equipment protection by pressurized room "p" and artificially ventilated room "v", clauses on room pressure differential, airlocks, door velocity and verification tests: law.resource.org; IEC webstore
• IEC 60079-2:2014, equipment protection by pressurized enclosure "p" (50 Pa for pxb and pyb, 25 Pa for pzc): law.resource.org
• Draft third edition, prEN IEC 60079-13: standards.iteh.ai
• IEC 61285, industrial-process control, safety of analyser houses
• NFPA 496 (2027 edition), purged and pressurized enclosures for electrical equipment: nfpa.org

This article is general technical information prepared by the excodex editorial team. It does not replace the applicable standards, the equipment certificates and instructions, national regulations or the assessment of a competent person.