Displacement Ventilation Systems

What are they?

Displacement ventilation systems use the principles of natural convection to maintain a comfortable internal air temperature within the occupied space while allowing hot air to rise to the ceiling. Air is diffused into the occupied space at floor level (normally through floor grille diffusers or columns) and is extracted at high level (normally at ceiling height). As the air rises through the space, it collects heat from people and equipment.  The occupied space is therefore a “pool” of cooler air that it not mixed with the heat gains, smells and occupant CO2.

As with other types of cooling system, displacement ventilation systems are served by central air handling units (AHUs) and chillers.

Advantages / Disadvantages

  • Good energy efficiency potential
  • Air quality is generally better compared to fan coil units (FCUs) or variable air volume (VAV) systems due to the lack of mixing of air within the space
  • Larger floor to ceiling height required.
  • For open plan offices, raised floors required.
  • Significant duct riser space requirements.
  • May not provide sufficient cooling in a building with a high cooling load, in which case a supplementary cooling system will be required.

Energy efficiency

This system can provide good efficiency provided that the fans are well controlled and maximum use is made of free cooling.  Enhanced results can generally be achieved where the system operates n a variable volume basis rather than constant volume.

Running costs

Running costs should be lower than conventional FCU or refrigerant based systems. Displacement ventilation is often supplemented by localised cooling systems for areas with higher than normal cooling requirements, such as IT server rooms. Typically, these areas will form a small proportion of the total floor space. Therefore the overall benefits will not be negated.

A building management system (BMS) is essential to provide adequate control for successful operation of the system

Retrofit / improvement opportunities

The major areas for improvement for displacement ventilation systems are:

  • Control and commissioning of airflows within occupied spaces to ensure that these are as low as permissible
  • Optimisation of dehumidification controls
  • Upgrade of controls for boilers, chillers and associated pumps
  • Upgrade of chiller plant.  Chillers have 15-20 year lifespan, although they generally have been superseded in terms of efficiency well before the end of their operational life.

Control improvements can be implemented with the tenants in-situ

The potential for retrofitting displacement ventilation into an existing building is constrained by the need for duct riser spaces and adequate floor to ceiling heights.

Applicable buildings

Best suited to building / space type with high ceilings, such as atria and auditoria and low density office space with 3m+ floor to ceiling heights.

Floor plate implications

These systems require a high ceiling (ideally 3m+) and preferably a floor void /  plenum (with a depth of 0.3–0.45m) to operate.  Alternatively, wall or column diffusers can be used but these take up floor space.

Temperature control / Occupant comfort

These systems can provide a comfortable working environment as they typically provide a higher level of outside air, and with less mixing with return air, than conventional overhead supply systems.

In some instances, however, they can be perceived as creating uncomfortable temperature gradients or draughts. Floor grilles are sometimes covered over by occupants (deliberately or accidentally); this adversely impacts on the system’s performance.

Maintenance implications

Comparable maintenance levels to other AHU systems.  May have fewer terminal units, which will reduce maintenance but may negatively affect efficiency


Supply grilles are typically be located at floor level (requiring a raised floor) and are fairly distinctive.  Wall mounted diffusers and column diffusers are identifiable as large structures typically with perforated metal faces, from which air is supplied at very low velocity.

Extract grilles are located at high level.

Questions to ask

  • Is the air supply variable volume or fixed volume?
  • Is there a supplementary cooling system in any space or capacity for one to be fitted?
  • Is there a building management system (BMS)?
  • Is the system regularly maintained (at least annually) and seasonally commissioned?
  • Is the system variable volume or constant volume?

More information

Displacement ventilation (* references to go in here)