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  • Writer's pictureKianWee Chen

What is HVAC system?

Updated: Jun 29, 2023

HVAC stands for Heating Ventilation and Air-Conditioning. In some regions of the world that do not require heating, it is also referred to as ACMV, which stands for Air-Conditioning and Mechanical Ventilation. HVAC systems perform seven main processes:

  1. Heating - supplying heat to the occupants to maintain thermal comfort.

  2. Cooling - removing heat from the occupants to maintain thermal comfort.

  3. Humidifying - adding moisture to the air to maintain comfort.

  4. Dehumidifying - removing moisture from the air to maintain comfort.

  5. Cleaning - removing particulates (dust, etc.) and biological contaminants (pollen, etc.) from the air to maintain good air quality.

  6. Ventilating - exchanging air between indoors and outdoors to remove gaseous contaminants (CO2) to maintain good air quality.

  7. Air movement - circulating air in interiors to achieve proper ventilation and thermal heat transfer.

How do you design a HVAC system is dependent on the building type (commercial, residential etc.) and the climate. For example, a residential home in New Jersey (hot and humid summer and cold and dry winter) will require heating, cooling, dehumidifying, cleaning and air movement, while a home in Singapore (hot and humid year round) only requires cooling, dehumidifying, cleaning and air movement. It is common not to provide active ventilation in homes as you can open windows for ventilation. For most commercial buildings, occupants do not have the option to open windows. Most countries have regulations that mandate commercial buildings to have minimal ventilation rate to ensure acceptable indoor air quality for the occupants' well-being. At its most basic, a HVAC system need to at least heat and cool a building's occupants.

The principles of heating and cooling

Heating is a more straightforward process as compared to cooling. The general principle of heating is using a heater to generate heat and then through a medium (air/water) deliver it to the occupants (Figure below). One of the most primitive heating system is a fireplace. The fireplace is the heater, through combustion of wood (its fuel), the fireplace heats up the air to keep the occupants warm. As illustrated below.



In cooling, you are removing heat from the occupants and rejecting it to the outside. Before the invention of modern air-conditioning system by Willis Carrier in the 20th century, cooling systems are passive. Buildings are designed to passively catch wind to cool down the occupants indoor. The heated wind is then exhausted out of the building.

With the invention of air-conditioning, we are able to actively reject heat from our buildings to the outside with a chiller powered by electricity. In a typical window air-conditioning (AC) unit, the chiller is usually a vapor compression machine powered by electricity. The unit remove heat from the interior by cooling the air and rejecting the heat to the outside. As illustrated below.


  1. Occupants are cooled by the chilled air, the air now warmed by the occupants is recirculated and cooled by the chiller.

  2. Outside air removes the heat from the AC unit and dissipates it to the environment

Vapor compression cycle

To better understand how air-conditioning removes heat from a space and cool the occupants, we need to explain the vapor compression cycle used in most chillers for cooling. The illustration below is a blow up diagram of the components inside the air-conditioning window unit. The vapor compression cycle contains four main components, the evaporator, compressor, condenser and expansion valve. The diagram below briefly explains the process.

  • Evaporator - the low pressure cold refrigerant moves to the evaporator absorbs the heat from the incoming warm air/water and changes from liquid to gas. The warm air/water loses its heat and get cooled and exit the evaporator.

  • Compressor - the low pressure refrigerant in its gaseous state moves to the compressor, gets compressed, and exit the compressor as a high pressure hot gas.

  • Condenser - the high pressure hot gaseous refrigerant loses its heat as it exchanges heat with the incoming cool air/water and changes state to liquid. The cool air/water is heated and leaves the condenser.

  • Expansion Valve - high pressure cold liquid refrigerant moves through the expansion valve and gets depressurized. It leaves the expansion valve as a mixture of liquid and vapor. The refrigerant flows to the evaporator and absorbs heat from the warm air/water. The cycle repeats.

Electricity is used to power the refrigeration cycle. The vapor compression cycle is used in refrigerators for keeping our fruits and vegetables fresh. The cycle can be reversed to provide heating in a heat pump. As we decarbonize our buildings, using combustion for heating (fireplace, gas furnace etc.) will become unacceptable, as combustion will generate CO2. We will move towards electrifying our heating system through the use of heat pumps. Thus the vapor compression cycle will play a key role in the mitigation of climate change.

The vapor compression machine in the form of chillers/ heat pump is a primary component in a HVAC system to move heat around to maintain thermal comfort for the occupants. It can be used directly to cool the air in the interior space as illustrated by the window air-conditioning illustration above. In more complex HVAC setup in a bigger commercial building, it can be used to produce either chilled or hot water for heating, cooling and dehumidification in a building.

Now that we have a conceptual understanding of the principles of heating and cooling in HVAC systems. We can move on to talk about the two main categories of HVAC systems 1) Convective Systems and 2) Radiant Systems in our next post.


Further Readings

  1. McDowall, R., (2007a). Chapter 1 - Introduction to HVAC, in: McDowall, R. (Ed.), Fundamentals of HVAC Systems. Elsevier, Oxford, 1–10.

  2. McDowall, R., (2007b). Chapter 6 - Single zone air handlers and unitary equipment, in: McDowall, R. (Ed.), Fundamentals of HVAC Systems. Elsevier, Oxford, 71–91.

  3. Szokolay, S.V., (2014). Introduction to architectural science : the basis of sustainable design, 3rd ed. Routledge, London.

  4. Hall, F., (2017). Building services handbook, 9th ed. ed. Routledge, Abingdon, Oxon.



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