HVAC systems work by circulating air throughout a building or space. The system is composed of several components, including an air handler, a furnace, an evaporator coil, a condenser coil, and a thermostat. The air handler draws in air from the outside and circulates it through the evaporator coil, which cools the air. The cooled air is then circulated throughout the building or space. The furnace then heats the air, and the condenser coil cools it before it is circulated back outside. The thermostat monitors the temperature and adjusts the system accordingly.