Natural Ventilation and Mixed Mode Systems

What are they?

Natural ventilation systems use external air to cool the building without using mechanical cooling. Air supply and extract is provided through opened windows and / or air intake louvres, either manually controlled  or automatically linked to a building management system (BMS). More  complex systems make use of the ‘stack effect’, by which the natural  buoyancy of warm air moves air up to high level exhausts (e.g. atria).  Pressure differences caused by external wind can also be used to enhance air movement and cooling. For example, ‘windcatchers’ can  be placed on roofs to provide downward ventilation into a space. Natural ventilation is typically supplemented with a heating system incorporating  a boiler and radiators.  High summer temperatures will generally lead to poor comfort conditions but this may be acceptable in some environments such as warehouses.

Office buildings can be designed to be ‘mixed mode’, a system which uses natural ventilation when external temperatures are moderate and air conditioning when higher temperatures are experienced. A system can be included which informs tenants when windows should be opened to optimise internal comfort and when they should be closed and the  mechanical system relied upon.  This approach is particularly applicable to cell offices, where an individual can make decisions about their own window without affecting others; in open plan configurations, the natural ventilation component of mixed mode operation needs to be automatically controlled and requires relatively subtle operation to be successful.

Advantages / Disadvantages

  • No energy is required to provide ventilation or cooling where the building operates a natural ventilation strategy only.
  • For the Sydney climate, mixed mode is more practical than natural ventilation except in spaces that have limited comfort requirements.
  • Opening windows have higher infiltration than sealed windows, which can increase heating and cooling loads in extreme conditions.
  • Management of opening windows in open plan areas is difficult due to wind effects and different perceptions of comfort for different occupants.
  • Open windows and louvres can permit external noise and pollutants into the space, especially in an urban setting.
  • Natural ventilation is difficult to provide to deep plan buildings

Energy efficiency

A very low energy solution when no active cooling is provided.

A mixed mode strategy will also be a low energy solution compared to conventional systems, although actual savings will depend on  how frequently the mechanical cooling system is used instead of  natural ventilation.

Running costs

The cooling effect from opening a window is ‘free’. If windows are open and the mechanical cooling or heating system is also running then this will waste energy. Effective building management will help reduce running costs in this situation e.g. if the system is automatically controlled by a building management system (BMS).

Retrofit / improvement opportunities

The major areas for improvement for natural ventilation and mixed mode systems are:

  • Measures to eliminate windows being open when the building is actively heating or cooling
  • Optimisation of the control changeover between natural ventilation and active conditioning
  • Review and improvement of airflows during natural ventilation mode
  • Upgrade of controls for boilers, chillers and associated pumps

Control improvements can be implemented with the tenants in-situ

Replacing non-openable windows with openable windows to provide natural ventilation is not a cost effective solution to saving energy.  If, however, a building has been provided with openable windows in addition to an air conditioning system, that system may be turned off at some points  during the year to save energy / cost and become a mixed mode building.

Applicable buildings

Natural ventilation is possible for light industrial spaces, but most other spaces will use mixed mode as active cooling is required on hot summer days.

Natural ventilation and mixed mode operation is typically not suited to buildings with surrounding pollution issues e.g. city centres.

Floor plate implications

Opened windows on a single elevation will only ventilate to a depth of  circa 7m, which increases to 15m (including allowance for 1m wide  central walkway across floor plate) if windows are on opposing elevations.  Spaces deeper than 15m need a more complex ventilation strategy which typically requires an open central atrium to achieve the required stack effect.

Temperature control / Occupant comfort

Natural ventilation is generally unsuitable for buildings with a high cooling demand or in city centres where noise and pollution make opening windows undesirable. Windows provide limited control of how much air enters a space, which will differ depending on external conditions.  For example, air flow will fluctuate depending on whether it is a still or windy day.  Natural ventilation is difficult to implement in open plan floor plates owing to the conflicting perceptions and preferences of occupants, and may cause occupant dissatisfaction as a result.

Maintenance implications

Manual windows will require occasional adjustment to ensure they are in a fully operable condition. Automatic window / louvre opening devices will require more maintenance.


There will be no or very few air grilles and no mechanical plant where the building operates a natural ventilation strategy. The majority of windows will be openable. There may be a horizontal transfer grille for the air to leave a room or floor area and enter an atrium.  A building operating a mixed mode strategy will feature an air conditioning system as well as openings such as windows and louvres.

Questions to ask

  • Is the building wholly or partly naturally ventilated?
  • Do the windows open automatically?
  • If windows open automatically, how are they controlled?
  • Are the windows regularly maintained?

More information

More refs to be provided.