A forced venting unit that extracts gas from beneath a building needs its own gas monitoring and air extraction in the venting unit room. Gas can leak slowly or suddenly, and it can sit high or low in the room. The answer is paired sensors at ceiling and floor level, a panel that works on its own, and a fan that clears the room in about 10 minutes. This is how that was designed for a car park building on a business park in Ireland.
The challenge: a safe room for a venting unit
The venting unit was to go in the shaft of a new spiral car-access ramp, a spot with little other use that hides the unit from sight and sound. A standby generator would sit alongside so the unit could keep running during a power cut.
The unit mostly extracts air with only trace amounts of other gases. In an enclosed space, though, methane, carbon dioxide and hydrogen sulphide can still reach levels that are explosive or harmful. Two failure modes had to be covered:
- A gradual build-up from a small leak at the unit or from the surrounding ground.
- A sudden release, for example from a major leak or a component being knocked off the pipework.
The rest of the park used distributed ambient air monitoring, which suits a slow build-up. The sudden case needs continuous monitoring in the room itself, so that a rapid change is not missed.
The approach: sensors, panel and extract fan
An ambient air monitoring station
Methane is lighter than air and carbon dioxide is heavier. The behaviour of the mixture depends on how it enters the room. So each gas gets two sensors, one at ceiling level and one at floor level. The proposal covered:
- Carbon dioxide, methane, hydrogen sulphide and oxygen sensors, each as a high and low pair.
- An IP65 control panel receiving all eight signals.
- Two flashing lights on the panel. Yellow means a panel or equipment fault, or a soft alarm for gas levels. Red means hazardous levels.
- A cable link to the central computer, so the status is also seen remotely.
The panel works as a stand-alone unit and reacts straight away. It does not depend on the central computer to raise an alarm.
An ambient air extract system
The extract system runs with the venting unit and starts automatically on an alarm. It also runs for set periods each day. That gives the room basic ventilation and proves the fan still works when it is needed.
It was sized to clear the room in 10 minutes. For a room of about 100 m³ that means an air blower moving 600 m³/h at 20 mbar head, with inlet and discharge silencers and a duct to open air. The sums are simple: 100 m³ at 600 m³/h is 10 minutes of fan time for one room volume.
Safeguards for people
The fan can be started remotely, so there are interlocks to protect anyone in the room:
- A key-lock switch allows or disallows remote start.
- A sounder and flashing light operate for 30 seconds before the blower starts.
Why it works
Each part answers one of the risks. Paired sensors cover both lighter and heavier gases. The stand-alone panel covers the sudden leak. The scheduled fan run covers the slow build-up and tests the fan at the same time. The interlocks make sure the fan cannot start with someone inside.
Sensor maintenance matters as well. On the same site, a related note covered a sensor in a stairwell. Access needed safe standing for a ladder, so the sensor should sit near the wall rather than the stairs, plus a removable vent cover of at least 200 mm by 100 mm, ideally hinged and lockable. Planning access at the design stage is much cheaper than fixing it afterwards.
What to consider for your own project
- Put sensors high and low. Do not assume which way the gas will move.
- Keep alarms independent. The room should alarm even if the network is down.
- Size the extract fan to a target time. Ten minutes for one room volume is a clear, checkable figure.
- Interlock remote starts. Add a lock, a warning delay and a sounder.
- Design in access. Every sensor needs a safe way to be reached for calibration.
Equipment in a room like this falls under hazardous area rules. The HSE explains the duties in its guidance on dangerous substances and explosive atmospheres.
FAQ
Why use sensors at two heights?
Methane tends to rise and carbon dioxide tends to sink, but a leak can behave differently. Sensors high and low catch both.
Why measure oxygen?
A low oxygen reading shows that the air has been displaced by other gases, which is an asphyxiation risk even when no gas is toxic or explosive.
How long should an extract fan take to clear the room?
That is a design decision. Here the target was 10 minutes for one change of the room’s air.
Can the fan be started remotely?
Yes, but only with a key-lock switch to prevent it and a 30-second warning before it starts.
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