Introduction

Heating, ventilation and air conditioning (HVAC) systems control the temperature, humidity and quality of air in buildings to a set of chosen conditions. To achieve this, the systems need to transfer heat and moisture into and out of the air as well as control the level of air pollutants, either by directly removing them or by diluting them to acceptable levels.

Heating systems increase the temperature in a space to compensate for heat losses between the internal space and outside. Ventilation systems supply air to the space and extract polluted air from it. Cooling is needed to bring the temperature down in spaces where heat gains have arisen from people, equipment or the sun and are causing discomfort. Heating, ventilation and air conditioning systems vary widely in terms of size and the functions they perform. Some systems are large and central to the building services – these were probably designed when the building was originally commissioned and use ventilation to deliver heating and cooling. Other systems may provide heating through boilers and radiators, with some limited ventilation to provide fresh air or cooling to certain parts of the building such as meeting rooms. In some cases, individual comfort cooling units have been added to a building to overcome a specific overheating problem that had not been thought of at the time of the original design.

So if heating, ventilation and air conditioning can be separate systems, why consider them holistically? The answer lies in the interaction of these services with each other and with the building. By considering HVAC systems as individual elements rather than as an interacting system, it would be easy to overlook a major area of energy wastage – that one component might impact on another. For example, it would be wasteful to increase heating inside a building while the cooling system is fighting to reduce temperatures. It is therefore useful to look at how the elements of an HVAC system interact with each other and fine tune each part to save energy and money.

HVAC system components

HVAC systems vary widely in terms of the individual components that make them up and how they are set up within a building. Most systems contain some common basic components:

Boilers produce hot water (or sometimes steam) for distribute to the working space. This is done either by heating coils which heat air as part of the ventilation system, or through hot water pipes to radiators

Cooling equipment chills water for pumping to cooling coils . Treated air is then blown over the chilled water coils into the space to be cooled through the ventilation system. As part of the refrigeration cycle in the chiller, heat must also be rejected from the system via a cooling tower or condenser

Pumps are used throughout the system to circulate the chilled and hot water to the required areas throughout the building

Stale air is extracted, usually using a fan, via separate ducts and expelled outside

Controls are used to make components work together efficiently. They turn equipment on or off and adjust chillers and boilers, air and water flow rates, temperatures and pressures. A controller incorporating one or more temperature sensors inside the workspace sends a signal to the heating or cooling coils to activate

If there is a demand for heating or cooling then the controls may also send a signal to the chiller and boiler to operate as required. There are often other control panels on the chiller or boiler too, allowing users to have greater control.


HVAC system types

There are many different approaches to using HVAC systems and buildings will typically use combinations and variations of a few standard approaches. HVAC systems are generally described according to how they use air, water or both to distribute heating and cooling energy to the space.