Heating, Ventilation and Air Conditioning systems (HVAC) controls the indoor climate by adding or extracting heat and adding or removing mass (e.g. water vapour and dust). To combat summer heat and winter cold, heating, ventilation and air conditioning systems (HVAC) provide solutions for poor indoor environment quality to meet the comfort levels required by its occupants.

Physiological comfort factors include:

• Dry bulb temperature (DBT, tdb): Temperature of air as registered by ordinary thermometer
• Mean radiant temperature (MRT): Temperature registered by a thermometer whose bulb is covered by a wetted wick and exposed to a current of rapidly moving air.
• Relative humidity (RH): Ratio of the actual water vapour pressure of the air to the saturated water vapour pressure of the air at the same temperature
• Humidity Ratio (aka Moisture Content/ Mixing Ratio): Ratio of the mass of water vapour to the mass of dry air
• Air movement
• Odour concentration
• Airborne dust