Car Park Ventilation Design: AS 1668 Requirements

Why Car Parks Need Ventilation

Cars produce carbon monoxide (CO) when they start and idle. In an enclosed car park, that CO has nowhere to go. It builds up. At high enough levels, it causes headaches, dizziness, and in extreme cases, death.

AS 1668.2 sets three CO exposure limits for enclosed car parks:

• 60 ppm maximum averaged over one hour. This is the primary design limit. It covers the worst-case peak traffic scenario - the morning rush or evening exit. (AS 1668.2:2012, Appendix N)
• 30 ppm time-weighted average over eight hours. This applies to staffed car parks where attendants, detailers, or security guards work full shifts. (AS 1668.2:2012, Appendix N)
• 100 ppm short-term peak. No area of the car park should exceed this at any time. (AS 1668.2:2012, Appendix N)

The NCC requires every enclosed car park storey (except open-deck car parks) to have ventilation. It must be either mechanical ventilation to AS 1668.2, or natural ventilation to AS 1668.4. Open-deck car parks with permanent openings on two or more sides do not need a system.

The Deemed-to-Satisfy (DTS) path under AS 1668.2:2012 is prescriptive. Follow the numbers and you comply. No modelling needed.

• Minimum exhaust rate of 2.5 L/s per m² of car park floor area for small car parks (AS 1668.2:2012, Table B1)
• Supply air must be at least 80% of the exhaust rate to maintain slight negative pressure and prevent CO migrating into adjacent spaces (AS 1668.2:2012)
• Exhaust grilles at high and low level. CO is lighter than air at operating temperatures. High-level extract catches the warm exhaust plume. Low-level extract catches heavier cold-start emissions near ground level (AS 1668.2:2012)
• Supply and exhaust points must be spaced to avoid dead spots. No part of the car park should be more than 25 m from an exhaust point (AS 1668.2:2012)
• Minimum 2.2 m clear headroom under ductwork, sprinklers, and services for standard bays. 2.5 m clear above accessible parking bays and their shared areas (AS 2890.1)
• Exhaust discharge must be at least 3 m from any opening into an occupied space, and 6 m from any fresh air intake (AS 1668.2:2012, Section 4)
• A time switch is required on any mechanical ventilation system with an airflow rate above 1,000 L/s. It must switch power on and off at pre-programmed times (NCC 2025, Part J6)

The DTS path is simple. Apply the prescribed exhaust rate, position the grilles, and you are done. Most small car parks (under 40 spaces) go this route. The downside: fans run at full speed all the time, even when the car park is empty at 2 AM.

A performance solution uses CO sensors to control fan speed. When CO levels are low, fans slow down or stop. When CO rises, fans ramp up. This approach can cut ventilation energy use by 40–60% compared to a fixed-speed DTS system.

Performance solutions require a fire engineer and mechanical engineer to work together. The design must demonstrate that CO stays below the 60 ppm one-hour limit and the 30 ppm eight-hour TWA under all traffic scenarios. CFD (computational fluid dynamics) modelling is common for large or complex basements. The certifier reviews the performance solution report before issuing the Construction Certificate.

Most car parks over 100 spaces use a performance solution because the energy savings pay back the extra design cost within two to three years.