Jet fans turn a car park into a duct. Instead of distributing air through ceiling ductwork, a line of small high-velocity fans pushes it along the space toward extract points.
The result saves ceiling height, cost and a great deal of steelwork, which is why impulse systems dominate new car park design.
Jet fans induce airflow by momentum rather than by ducting it. They are spaced along the flow path, typically 20 to 35 metres apart, and push air toward extract shafts. Design covers two duties: everyday CO and pollutant control, and smoke clearance during a fire, which requires a high-temperature rating.
How impulse ventilation works
Each fan produces a high-velocity jet that drags surrounding air along with it.
That entrainment moves far more air than the fan itself handles, so a modest fan can shift a large volume of the space. Meanwhile a line of fans passes the flow from one to the next, building a coherent movement toward the extract.
Consequently the car park structure itself becomes the duct, and the ceiling stays clear.
Two duties, one system
| Duty | Requirement | Typical operation |
|---|---|---|
| Pollutant control | Dilute CO and NO2 | Low speed, demand controlled |
| Smoke clearance | Clear smoke after a fire | Full speed, rated fans |
| Smoke control | Maintain clear access routes | Modelled, high temperature |
| Purge | Full air change after an incident | Full speed |
Fans used for smoke duty need a temperature rating, commonly 300 °C for two hours, and certification covering the whole assembly.
Thrust and spacing
Jet fans are specified by thrust in newtons rather than by airflow, which surprises people.
- Thrust determines how much surrounding air each fan entrains.
- Typical units produce 20 to 100 newtons depending on size.
- Spacing along the flow path is commonly 20 to 35 metres.
- Beams and ramps disrupt the jet, so spacing tightens around them.
- Fans must be positioned to avoid blowing directly at obstructions.
- Finally, reversible units allow flow direction to change during a fire.
Spacing that works on a drawing often fails in a real structure, which is why modelling matters. Our tunnel ventilation guide covers the same thrust logic over longer distances.
Why modelling is not optional
A car park is not an empty box. Beams, ramps, columns and parked vehicles all disrupt airflow.
Computational fluid dynamics modelling shows where dead zones form and whether smoke would actually clear along the intended route. Meanwhile most authorities now expect that modelling as part of the fire strategy approval.
Typical findings include stagnant corners behind ramps, short-circuiting between supply and extract shafts, and jets deflected by downstand beams. All are cheap to fix on a model and expensive to fix in concrete.
Control strategy
- CO and NO2 sensors distributed through the space drive normal operation.
- Fans run at low speed or intermittently under normal conditions.
- Occupancy or traffic sensing can supplement gas detection.
- Fire detection switches the system to its smoke mode.
- Smoke mode runs fans and extract at full duty in a defined direction.
- Finally, a manual override is provided for the fire service.
Demand control matters commercially, since a car park ventilated continuously at full rate wastes a great deal of energy.
Practical points
Several details cause problems late in a project.
Fan noise matters, since car parks are reverberant and often sit below apartments. Mounting has to allow for the thrust reaction, which is a real force on the fixing. Meanwhile access for maintenance needs planning, because fans at high level in an occupied car park are awkward to reach.
Our industrial axial fan range covers the underlying fan technology, our hazardous area guide covers classified spaces, and ASHRAE guidance covers enclosed parking ventilation rates.
Car park ventilation FAQ
How far apart should jet fans be?
Commonly 20 to 35 metres along the flow path, though structure and geometry change that considerably. Modelling determines the final layout rather than a rule of thumb.
Do jet fans replace extract fans?
No. They move air toward extract shafts, which still need main extract fans. The jet fans replace the distribution ductwork rather than the extract plant.
What temperature rating is needed?
For smoke duty, commonly 300 °C for two hours, though the fire strategy determines the requirement. Certification must cover the complete fan assembly.
Can one system handle both pollution and smoke?
Yes, and that is the normal arrangement. The fans run at low duty for pollutant control and switch to full duty in a defined direction during a fire.
Jet fans replace ductwork with momentum, which saves height and cost. Model the layout properly, and specify the fire rating before anything else.
