Linked from
The 11 pages that link to Heating, ventilation, and air conditioning, each with the reason it gives.
HumidityRelated: HVAC systems humidify or dehumidify air to meet comfort and building requirements.
British thermal unitRelated: HVAC systems commonly state heating output and cooling capacity in Btu.
Building designRelated: Their equipment and distribution compete for space with structure and rooms.
Thermal engineeringRelated: HVAC applies thermal design to occupant comfort and building energy use.