Carpark ventilation

Carparks that are not adequately naturally ventilated must be mechanically ventilated.

Ventilation systems usually consist of exhaust and supply fans with large power ratings.

Many carpark ventilation systems are either running at full speed; or are turned off; or operating on timers during perceived “peak periods”. The first approach imposes a significant energy cost and the latter two can compromise occupant health and safety.

Relevant Standards

The National Construction Code (NCC) requires that every storey of a carpark, except an open-deck carpark, must have:

  • a system of mechanical ventilation complying with AS 1668.2: The use of ventilation and airconditioning in buildings - Mechanical ventilation in buildings; or
  • a system of natural ventilation complying with Section 4 of AS 1668.4: The use of ventilation and airconditioning in buildings – Natural ventilation of buildings

The primary requirements are:

Ventilation systems must operate at all times, unless they are automatically controlled by a carbon monoxide (CO) monitoring system

The system must also provide at least 1 air change in every 24 h period

  • The levels of CO that will trigger operation of the system vary depending on whether the carpark is occupied or not. For example, if a building manager’s office is located in the carpark, the system will trigger at a lower CO concentration level
  • If the system has automated controls, ventilation rates can vary depending on the concentration of CO detected (see Variable Speed Operation below)
  • No point in the car park can be more than 25m away from a sensor
  • Additional energy saving measures are permitted for small car parks - 40 or less car spaces (see Small car parks section below)
  • The systems must be serviced twice-yearly

There are some variations in the standards that are best interpreted and assessed by a qualified service provider.

Variable Speed Operation

Since car park fans are very power hungry, variable speed drives can make a significant difference to the power consumed. The standards provide for the fans to operate at different speeds, depending on the level of CO detected by the sensors.

Ventilation rates vary according to the concentration of CO detected in relation to the system’s configured CO exposure limit (EL) - the concentration of CO that triggers fan operation:

  • At 75% of the EL and above, the system runs at the full ventilation rate
  • Between 50% and 75% of the EL, the system operates at a variable ventilation rate as defined in the standard
  • Below 50% of the EL, the system can operate at 25% of full ventilation rate
  • Below 25% of EL, intermittent ventilation is allowed
  • The system can turn off below 15% of EL and restart again at 25% of full ventilation rate when 25% of EL is reached

NOTE: There is a cubic relationship between fan speed and power consumption i.e. if the fan speed doubles, power consumption increases 8-fold, and vice-versa.


Graph of relationship between fan speed & power consumption

Graph of cubic relationship between fan speed & power consumption

Small car parks

For car parks with 40 or less car spaces, energy saving measures additional to variable speed operation are allowed.

The airflow rate can be halved if all vehicles remain parked and engines don’t operate for more than 2 hours. If the car park is also unoccupied at the same time, the system can be shutdown. The system can be reactivated by motion detectors in doorways and lift doors and whenever vehicle entrance doors are opened.

Potential Cost Savings

  • Decreased operational costs through reduced energy consumption – any reduction in speed results in significant energy savings
  • Peak Demand cost savings - installing VSDs reduces peak demand by removing the high surge currents of power associated with starting fans at full speed
  • Maintenance cost savings – a softer start at lower speeds reduces wear and tear on the equipment


Example configuration of fans, CO sensors and VSDs

Example configuration of fans, CO sensors and VSDs

Last Updated: 
Tue 13/10/2015

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