Further information on ventilation in buildings

Air Changes

The replacement of a quantity of air in a space within a given period of time, typically expressed air changes per hour. If a building has one air change per hour, this is equivalent to all of the air in the building being replaced in a one-hour period. Under current regulations (Part F) an air change rate of 8 litres per second must be achieved for new build offices.

Chilled Beams

A chilled beam is a building cooling device that circulates air using the principles of natural heat convection. The major advantage of a chilled beam over more common forced air systems is that it circulates building air without the noise and expense of ductwork and air handlers. Typically mounted overhead near or within a ceiling, the beam is a type of radiator, chilled by an external source such as Recirculated water. It cools the space below it by acting as a heat sink for the naturally rising warm air of the space. Once cooled, the air naturally drops back to the floor where the cycle begins again.


A condenser is a heat exchanger in which the refrigerant, compressed to a hot gas, is condensed to liquid by rejecting heat to achieve a cooled space. The condenser in an air conditioning unit is very similar to that used in a common refrigerator.

Constant Air Volume (CAV)

Constant Air Volume (CAV) is a type of heating, ventilating, and air-conditioning (HVAC) system. In a simple CAV system, the supply air flow rate is constant, but the supply air temperature is varied to meet the thermal loads of a space. Most CAV systems are small, and serve a single thermal zone. However, variations such as CAV with reheat, CAV Multizone, and CAV primary-secondary systems can serve multiple zones and larger buildings.

Convection Heating

In convection heating, air is heated when it comes into contact with hot surfaces in the heater. People feel warmer because of the higher air temperature. Some convection heaters use a fan to draw the cool air in.

Fan Coil System

A Fan coil system is an air conditioning system used in buildings. A fan unit is placed at each place which needs to be heated or cooled. A central plant delivers hot or cold water to fan units. The fan draws air from the room, blows it over the water coil and returns it to the room. Dehumidified air from a central plant or fresh air from outside may also be used by a fan coil system.

Internal Environment

In the context of mechanical building services the internal environment refers to the strategy employed to heat, cool and distribute air around a building. The Internal environment can be heated and/or cooled, whilst air distribution could be through natural or mechanical methods, or a mixture of the both for a mixed-made strategy. If comfort cooling is provided throughout the internal environment would be fully air conditioned.

Mixed Mode

A mixed mode system combines the best aspects of both natural ventilation and mechanical ventilation/air conditioning. The simplest example of a mixed mode system is the opening of windows to enable natural ventilation with air conditioning available when windows can not be opened.

Natural Ventilation

Ventilation systems are considered natural if the air is supplied and removed from the indoor space by non mechanical means. The use of natural ventilation reduces the need for mechanical energy consuming plant and is therefore more efficient. The requirement for natural ventilation to be utilised effectively in a building calls for early implementation in the design of a building.

Radiant Heating

Radiant heating heats a building through radiant heat. The heat energy is emitted from a warm element (floor, wall, overhead panel) and warms people and other objects in rooms rather than directly heating the air. The internal air temperature for radiant heated buildings may be lower than for a conventionally heated building to achieve the same level of body comfort (when adjusted so the perceived temperature is actually the same).

Terminal Unit

A terminal unit is the final device in an air conditioning system located in the space being heated or cooled. The terminal unit can be utilised to determine the flow and direction of air whilst re-heating/re-cooling to achieve the desired local temperature.

Variable Air Volume (VAV)

Variable air volume (VAV) is a technique for controlling the capacity of a heating, ventilating, and/or air-conditioning (HVAC) system. The simplest VAV system incorporates one supply duct that, when in cooling mode, distributes approximately 55 degree F supply air. Because the supply air temperature, in this simplest of VAV systems, is constant, the air flow rate must vary to meet the rising and falling heat gains or losses within the thermal zone served.

Variable Speed Drive (VSD)

An electronic device designed to be used with a motor to provide a variable flow output, thus reducing the energy required to turn the motor.

Further reading