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.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 whilst the cooling system is fighting to reduce temperatures. It is therefore useful to look at how each element of an HVAC system complements the other and fine tune each part to save energy and money.