Heating, Ventilating, and Air Conditioning, and HVAC systems are, effectively, everything from your air conditioner at home to the large systems used in industrial complexes and apartment blocks. A good HVAC system aims to provide thermal control and indoor comfort, and one that is designed using the principles of thermodynamics, fluid mechanics, and heat transfer.
The big air conditioner boxes that you might see on top of apartment blocks or offices are examples of (the visible part of) HVAC systems. They’re typically deployed in large industrial buildings, skyscrapers, apartment blocks, and large interior environments. They’re also an essential component of environments where there are health regulations requiring that temperature and humidity be kept at certain levels, using air taken from outside.