Constant Volume Systems

Constant volume cooling is a simple system often used to provide tempered fresh air in multi-zone buildings containing partially centralised air/water systems (such as a fan coil installation). Constant volume systems can also be used to cool single zones, such as clean rooms, and operating theatres in hospitals.

As the name suggests, constant volume systems provide a fixed volume of air at a temperature and humidity determined by the conditions of the space being served. They are therefore not generally suitable as the primary cooling system for buildings with multiple zones, as each zone will have varying cooling requirements. The exception to this is a constant volume system that incorporates re-heaters in each zone: if the supply air is too cold for the conditions in a zone, the re-heater will raise its temperature slightly, thereby providing local control. However, reheaters are no longer widely used since this approach is generally wasteful of energy.

A major application for constant volume systems is to provide fresh air in partially centralised air/water systems, such as fan coil or chilled beam installations. For these applications, a constant volume system would provide tempered ventilation air, and may meet a small proportion of the heating or cooling load. The primary heating and cooling requirement would be met by the room units, such as fan coils or chilled beams.


  • Suited to certain types of public spaces and foyers; commonly used to provide fresh air in air/water systems such as fan coil and chilled beam installations
  • Can be used for single zone applications, where only one set of internal conditions need to be satisfied
  • Simple and relatively easy to maintain


  • Not generally suitable for multiple-zone applications (such as offices), as each zone will have varying cooling requirements.