This document provides an overview of heating, ventilation, and air conditioning (HVAC) systems. It defines HVAC as the system that controls indoor air quality, temperature, and humidity. HVAC is needed to improve air quality, regulate moisture, maintain consistent temperature, and conserve energy. The document describes the basic components and functions of HVAC, including heating via conduction, convection and radiation; ventilation; air conditioning units; and compressor, condenser, evaporator functions. It also discusses types of HVAC installations and their advantages and disadvantages, as well as applications.