Factors to determine the capacity of HVAC system

What factors determine the capacity of HVAC system required :

You should know first the full forms of HVAC. 

• Heating: The amount of thermal energy that needs to be added to the space.
• Cooling: Thermal energy will sometimes need to be removed from a particular space in order to bring down the temperature.
• Humidifying : This process says that increasing the relative humidity through the addition of water vapor into the system.
• Dehumidifying: Process of removing water vapor from the given space.
• Cleaning: Purifying the air by the removal of dust, smoke, pollen and other pollutants.
• Air Movement: By this factor ensuring that there is ample movement of air to keep the inhabitants comfortable.