Car Park Ventilation Design

What You Need to Know

Enclosed car parks need mechanical exhaust systems to remove carbon monoxide (CO) from vehicle engines. AS 1668.2 sets the ventilation rates. The NCC caps CO at 60 ppm over one hour and 30 ppm over eight hours. Get the system wrong, and the car park fails its Essential Safety Measure inspection every year.

• CO must stay below 60 ppm (1-hour average) and 30 ppm (8-hour TWA) between 750 mm and 1800 mm above the floor (NCC 2025, Table F6V2)
• The peak CO reading must never exceed 100 ppm at any point (NCC 2025, Table F6V2)
• A 15-minute rolling average must not exceed 90 ppm (NCC 2025, Table F6V2)
• Every enclosed car park storey needs mechanical ventilation complying with AS 1668.2, unless it qualifies as an open-deck car park with natural ventilation under AS 1668.4 (NCC 2025, F6D11)
• Car park exhaust systems must have CO monitoring complying with AS 1668.2 clauses 4.11.2 or 4.11.3 (NCC 2025, J6D4(3))
• The exhaust airflow rate is the greatest of three calculation methods: contaminant generation, staff exposure, or 2.5 L/s per m² of floor area (AS 1668.2-2012, Section 4)
• Supply air must equal 75–90% of the exhaust rate (AS 1668.2-2012, Section 4)
• Exhaust discharge must be at least 3 m above ground and 6 m from walkways or adjacent properties (AS 1668.2-2012, Section 4)

Take a 2,000 m² basement car park with 60 spaces. Using the area-based method alone, you need at least 5,000 L/s of exhaust air (2,000 m² × 2.5 L/s/m²). That is about 5 m³/s, which typically requires two or three large axial fans.

But the area-based method is just the floor. AS 1668.2 also requires a contaminant generation calculation based on the number of spaces, average driving distances, and how many engines run at once. In a busy retail car park, this calculation often produces a higher number than the area-based rate. You must use whichever method gives the largest exhaust airflow.

The 2024 edition of AS 1668.2 halved the base contaminant generation rate for a single engine. Modern vehicles produce less CO than the older fleet the 2012 edition assumed. Where the 2024 edition applies, you may be able to reduce fan sizes and duct dimensions compared to a 2012-based design. Check which edition your certifier requires.

CO sensors drive the system. Space them no more than 25 m apart, mounted between 750 mm and 1800 mm above the floor. Wire them to a variable speed drive (VSD) on the exhaust fans. When CO rises, the fans speed up. When the car park is empty, they slow down, cutting energy use by 30–50%.