Car Park and Jet Fan Ventilation Systems

What problem it solves

Ventilation in an enclosed car park has two separate jobs, and they work in different scenarios.

In normal operation the job is to prevent carbon monoxide and other gases from vehicle exhaust from building up. Carbon monoxide is colourless and odourless; users do not notice it accumulating. That is why the system is monitored continuously by sensors, and the fans start when the concentration exceeds a set threshold.

In a fire scenario the job changes completely: the aim is no longer to dilute gas but to clear smoke in a controlled way, keeping visibility along escape routes and making it easier for firefighters to reach the fire. In this scenario the fans run in a different pattern, often at higher capacity, and must be fire-rated.

Because the same system has to do two different jobs, the design is more than a mechanical capacity calculation. The scenarios are set up together with the controls and the fire detection system.

What a jet fan system is

In the traditional solution, ducts are run to every point of the car park and air is collected through them. A jet fan system largely eliminates the ducts: small axial fans mounted on the ceiling push the air in a set direction. Air entering through the fresh air shaft at one end of the car park is pushed step by step by the jet fans to the exhaust shaft at the other end.

In practice this means ceiling height is preserved, duct costs fall, installation is faster and the risk of dead zones is reduced. On the other hand, the jet fan layout must be calculated correctly; wrongly placed fans push air against each other and in one corner of the car park the air does not move at all.

Applications

• Shopping centre and office building car parks
• Enclosed car parks in residential complexes
• Hospital and hotel car parks
• Vehicle parking areas at factories and logistics facilities
• Enclosed loading and unloading ramps

System components

Jet fans

Unidirectional or reversible. Because reversible fans can reverse the flow direction in the smoke clearance scenario, they allow the clearance direction to be changed according to where the fire starts. Fans that operate in the fire scenario must be certified to run at high temperature for a set period.

Fresh air and exhaust shafts

The entry and exit points of the system. Shaft sections must be large enough to pass the total airflow pushed by the jet fans. If a shaft is undersized, the air cannot leave the car park, however powerful the jet fans are.

CO and LPG sensors

Distributed according to the volume and geometry of the car park. Sensors are placed where exhaust gas accumulates, usually at vehicle level. In car parks with LPG-fuelled vehicles, LPG sensors are also provided; since LPG is heavier than air, the sensor is placed close to the floor.

Control panel

Runs the fans in stages according to sensor values. In normal operation, one group of fans starts at a low threshold and all of them at a high threshold. On a signal from the fire detection system, the controls switch to the smoke clearance scenario.

What drives the design

Commissioning

Unlike other systems, commissioning of car park ventilation includes scenario testing. After each fan is confirmed to run, the tests check that the controls respond correctly to sensor thresholds and that, when a fire signal arrives, the right group of fans runs in the right direction.

A smoke test can be carried out to confirm smoke movement visually. The results are reported; this report is a reference document both for building inspection and during operation.

How the jet fan layout is determined

The most critical decision in a jet fan system is where the fans go and which direction they face. The layout is derived from three inputs: the positions of the fresh air and exhaust shafts, the column and beam layout, and the route of ramps and driving aisles.

The basic principle is that air moves in one direction and without interruption from shaft to shaft across the whole car park. Fans are spaced to feed this flow step by step. In a layout where two fans push air against each other, the air stops in that zone and gas builds up.

Behind columns, under ramps and in the corners of the car park are the points that carry a risk of dead zones. These may need an additional fan or a small change in a fan's angle. In large car parks with complex geometry, a flow simulation is valuable so the layout is not left to guesswork.

Fan height matters too: the jet fan must be high enough for its airflow to travel above vehicle level, but must not reduce headroom.

Operating costs and controls

Car park ventilation uses unnecessarily high energy if it runs at full capacity all the time. In reality, traffic varies a lot through the day; it peaks in the morning and evening and almost stops for most of the night.

Staged control with CO sensors makes use of this variation. When concentration is low, the system stays off or runs with a very small group of fans; as concentration rises, fans come in stage by stage. Combined with variable speed fans, the savings increase further.

The second benefit of the controls is evidence: when the system ran and at what concentration can be recorded. This record is useful both for the operator's own monitoring and for inspections.

Maintenance

Jet fans sit on the ceiling, at points that are relatively hard to reach. That is why the maintenance plan must be made from the start. Periodic checks cover dust build-up on fan blades, bearing noise, tightness of mountings and vibration.

CO sensors need calibration; a sensor past its calibration either runs the system unnecessarily or fails to run it when needed. The second is a serious risk. The fire scenario also needs testing at set intervals; a scenario that is not run at least once a year cannot be assumed to work.

Existing car park systems

A common situation in older car parks is a ducted system that has become inadequate over time or was never commissioned. In these facilities the move to a jet fan system can be done in phases, without removing the ducts or closing the car park.

The first step is measuring the existing shafts: are the sections large enough, are the exhaust and fresh air directions correct? If the shafts are suitable, the jet fan layout is drawn, the controls and sensor infrastructure are installed and the fans are mounted section by section. The car park remains partly usable at every stage.