HVAC system basics

What is HVAC system?

HVAC stands for Heating, Ventilation and Air conditioning system, which is used to maintain a controlled environment within a building for the occupants or for the process being conducted. To maintain the environment the HVAC system controls four factors:

  • Temperature
  • Humidity
  • Air distribution
  • Air quality

Classification of HVAC Systems:

HVAC systems are classified based on the medium which is used to transfer heat within the system. The basic types are:

  • All air: An all air system supplies only conditioned air to space. No chilled or hot water crosses the boundary between the location of the central equipment and the conditioned space

  • Air-water: Usually, heated or chilled water is supplied to the conditioned space and used in fan coil units and induction units.

  • All water: Air and water are treated in a central plant and are distributed to terminal units in the conditioned space


  • Heat is released by burning fossil fuels or by passing an electric current through a wire. This heat is used to warm the occupants by radiant.

  • Heating systems can be categorised into one of two main types these are; indirect heating systems and direct heating systems.

  • Direct heating systems use individual stand alone heaters in each room where heating is required. The most common form of direct heating is the use of gas, coal or electric heaters in a domestic property.

  • Indirect heating systems are known as central heating systems in houses because they generate heat at a central location, the boiler. The heat must then be removed from the boiler and delivered to each room. It is carried there by a heat transfer medium, which can be water, steam or air.

Temperature Control:

  • Vary the temperature of the supply air to space while keeping the air flow rate constant. This is the constant volume, variable temperature approach.

  • Vary the air flow rate while keeping the supply air temperature constant. This is the variable volume, constant temperature approach

Humidity Control:

  • HVAC systems typically over-cool the air to remove water vapour, and they may have to heat the air back up – this is called reheat and requires additional energy.

  • Humidification – the air is too dry and water vapour must be added for comfort.

  • Dehumidification – The air is too wet and water vapor must be removed for comfort.