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Starter question — a frequently asked question posted by the MEP Center team to open this forum. Share your own experience below.
We are designing the ventilation for an enclosed underground car park of about 4000 m² with a clear height of 3.0 m. The architect prefers jet fans instead of ductwork, and the system must also serve smoke clearance in a fire.
- What airflow rates are used in European and US practice for normal (pollution) mode and fire mode?
- How should CO and NO₂ detection control the fans, and where are the sensors placed?
- What are the layout rules for jet fans and the main exhaust fans?
Size the main exhaust for the fire or smoke-clearance case required by your code, run it at reduced speed under CO/NO₂ control in normal operation, and use jet fans only to move air towards the exhaust points. Normal-mode design is a dilution problem; fire mode is usually verified by CFD.
1. Airflow rates
- UK and many European practices: 6 air changes per hour for normal ventilation and 10 ach for smoke clearance (Approved Document B; BS 7346-7 for jet fan smoke control). Many national codes, including Turkey’s, follow similar figures; check yours.
- IMC 404 (US): 0.75 cfm/ft² (3.8 l/s·m²), with reduced operation permitted under CO and NO₂ detection control.
- ASHRAE Applications Handbook gives a contaminant-based method for CO dilution when the code allows it.
2. Worked example
Volume: 4000 m² × 3.0 m = 12,000 m³.
- Normal mode at 6 ach: 12,000 × 6 / 3600 = 20 m³/s (72,000 m³/h).
- Smoke clearance at 10 ach: 12,000 × 10 / 3600 ≈ 33.3 m³/s.
- IMC check: 3.8 l/s·m² × 4000 m² ≈ 15.2 m³/s.
Use two or more exhaust fans, for example 2 × 16.7 m³/s, so that one fan failure leaves half the capacity, with a separate standby where the authority requires it. In normal mode both fans run at about 60 % speed (flow is proportional to speed), which needs only about 0.6³ ≈ 22 % of full-speed power. With CO control they run lower or stop most of the day.
Make-up air of roughly 75 to 80 % of exhaust enters through ramps or dedicated shafts, located at the far end from the exhaust so the air sweeps the whole floor.
3. CO and NO₂ control
- Typical setpoints: low speed above about 25 to 30 ppm CO, high speed above about 50 ppm. As a reference, the EU occupational limit for CO is 20 ppm over 8 hours and 100 ppm for 15 minutes.
- Add NO₂ sensors where diesel vehicles park (typical stage around 1 to 3 ppm).
- Mount CO sensors at breathing height (about 1.5 to 1.8 m), at ramps, near exits and in dead corners, at the spacing given by the sensor’s coverage data.
- In fire mode the fire alarm overrides gas control.
4. Jet fan layout
- Place jet fans in series along the flow path towards the exhaust, clear of beams and services directly in front of the outlet, with their throw overlapping the next fan.
- Avoid blowing against the incoming make-up air or into dead-end bays without an exhaust point.
- Fire-mode fans and their cables must be rated for smoke temperature (EN 12101-3 classes such as F300 or F400, as the authority specifies) with secondary power supply.
- Verify smoke mode by CFD: visibility and temperature along escape routes and firefighting access, reversing the flow for different fire locations if the scheme requires it.
Common mistakes
- Sizing exhaust fans only for 6 ach while the fire strategy needs 10 ach.
- Make-up air entering right next to the exhaust.
- Too few CO sensors, or sensors near the ceiling or at the ramp mouth only.
- Ramps and bays with no air movement because jet fans ignore beam layout.
Related: Car Park Ventilation with Jet Fans: Quick Design. For deeper study: HVAC Systems Design Course.
Check the edition of the code that applies to your project.
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