Climate control system for a pressurized control room in a hazardous area (the air handling that keeps flammable gas out as well as cooling the room)
Air conditioning for a control room inside or beside a hazardous area · clean air intake, at least 25 Pa above adjacent spaces, 0.3 m/s outward through an open door, gas detection at a Zone 2 intake, loss-of-airflow and loss-of-pressure alarms, purge before re-energizing
A control room in or next to a hazardous area can hold general-purpose electrical equipment when a purging and pressurizing system keeps flammable gas out. The air handling unit that cools and ventilates the room then also does the protecting: it draws air from a clean source, holds the room above the pressure outside, and its failure has to be detected and acted on. The climate control specification therefore carries safety requirements as well as comfort and equipment cooling.
Air handling and pressurization for control rooms and other occupied rooms protected by pressurization in hazardous areas. General air handling units are covered on the Air handling unit page; fan modules on the Fan assembly for air handling units: the fan, motor and drive module page; portable units for shelters on the Environmental control unit (portable cooling and heating unit for tents, shelters and field facilities) page; pharmaceutical cleanroom air handling on the Cleanroom conditioning system (pharmaceutical HVAC air-handling unit) page; HVAC controllers on the Building automation controller (DDC controller) page.
CNFX editorial research · Sources checked October 13, 2026 · 2 sources
IEC 60079-13:2017 lets the clean air supply include heating, ventilation and air conditioning equipment, and says "Any outdoor air used by heating, ventilation and air conditioning equipment shall be provided from the clean air supply." For pressurization "pb" the clean air comes from a non-hazardous area; for "pc" the intake may be in a Zone 2 area. [1]
The pressurization system "shall be capable of maintaining a pressure of at least 25 Pa in the room with respect to adjacent spaces, with 50 % of all outlets (not doors) open". Without an airlock, the outward velocity through an open door has to be at least 0.3 m/s, and pressure may drop below 25 Pa while that velocity is held. An airlock needs a continuous clean-air flow of at least five airlock volumes per hour, or gas detectors alarming at 25% of the limiting value. [1]
With the intake in a Zone 2 area, the standard requires at least one flammable gas detector with alarm in the intake and one in the room, and intake detection "arranged to shut down the intake of air containing flammable vapour or gas into the room on detection of 25 % of the limiting value". It also notes that it does not cover air quality for personnel with regard to toxicity and temperature. [1]
An EC&M article by James Stallcup and William G. Lawrence (2000), citing NFPA 496-1998, gives at least 0.1 in. of water column (25 Pa) "with all openings closed", and 60 ft/min (0.3 m/s) through open doors; the IEC figure is measured with half the outlets open, so check which standard the project follows. [2]
Requirement
IEC 60079-13:2017
NFPA 496-1998 as reported by EC&M
Room overpressure
≥25 Pa, 50% of outlets (not doors) open
≥0.1 in. w.c. (25 Pa), all openings closed
Open door without airlock
≥0.3 m/s outward
60 ft/min (0.3 m/s)
Airflow failure detection
At the fan discharge, with alarm
At the fan discharge, alarm at an attended position
Before re-energizing
Purge ≥10 room volumes at ≥5 air changes/h, unless verified otherwise
Gas detectors, or a purge timer for at least four air changes
Name the standard the room is designed to, the hazardous zone around it and the intake location, and give the HVAC supplier the room volume, leakage paths and door arrangement.
02
Airflow loss, pressure loss and what happens to the power
Under IEC 60079-13, "Loss of air flow shall be detected at the discharge end of the fan and shall activate an alarm"; loss of pressure is sensed in the room, and the alarm may be delayed when an open door is the cause and inflow is sufficient. The pressurization system is energized independently of the room's power. [1]
For pressurization "pb", failure must cut power automatically to equipment without an acceptable EPL, or operation continues under administrative controls such as gas detectors confirming the room is below 25% of the limiting value; for "pc", power may be kept for a limited duration. [1]
EC&M's summary of NFPA 496 says a fan-running interlock is not reliable for detecting failure "because of the possibility of a broken belt or other equipment failure", and that the air system's power is taken ahead of any service disconnects feeding the control room. [2]
Ask the supplier for the airflow and differential pressure sensors, alarm and trip logic, door sensors, and the purge sequence before power is restored, and how these are tested at commissioning.
03
Pressurized control room HVAC details
Paste it into your enquiry below or into an email to a maker, then replace every bracket.
Standard: [IEC 60079-13 pb/pc; NFPA 496 type]
Area: [zone around room; intake location]
Room: [volume; doors; airlock; leakage]
Pressure: [setpoint; sensor; alarm delay]
Door flow: [velocity; door sensors]
Gas detection: [intake; room; set points]
Failure action: [trip; administrative control]
Cooling: [equipment heat; occupants; design climate]
Explore company information and manufacturing capabilities, then discuss the scope of your project with a relevant supplier.
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Compiled and source-reviewed by the CNFX editorial team.
Each manufacturer above links to the page on that company's own website where the product is listed, with the date we checked it. How we source and review
· Page last updated 13 October 2026
Buyer questions
What pressure must a pressurized control room hold?
IEC 60079-13:2017 requires at least 25 Pa above adjacent spaces with half the outlets (not doors) open. [1]
Can the air intake be in a hazardous area?
For pressurization pc it may be in Zone 2, with gas detectors in the intake and the room and intake shutdown at 25% of the limiting value. [1]
Is a fan-running signal enough to detect failure?
No; airflow loss is detected at the fan discharge. EC&M notes a belt can break while the motor runs. [1][2]
How much purging before power returns?
IEC 60079-13 sets at least 10 room volumes at five air changes per hour unless a reduced volume is verified. [1]
Sources and what still needs a direct check
Sources checked 2026-10-13. IEC 60079-13:2017 is read from the public copy hosted by Public.Resource.Org; EC&M published Stallcup and Lawrence's article on NFPA 496 purging and pressurizing (2000).
