Temperature in a building is controlled by a small appliance called a thermostat. Thermostats, with built-in thermometers, regulate the heat output of a furnace through a system of relays, valves, and switches. The thermostat generates electrical signals when the temperature rises above or falls below the desired level. It usually controls the flow of fuel to the heating unit. Thermostats are also used to regulate central air-conditioning units.
Air is drawn through a filter to remove dust, smoke, and pollen.
It then passes over the cooling coils, causing some condensation, thereby dehumidifying the air as well as cooling it. The condenser fan directs excess moisture onto the condensing coils, where it either evaporates or is channeled into a drain outside the house. Heat from the condenser is dissipated outside the house, while cool air is blown by a fan through vents into a room. Humidifiers and Dehumidifiers Air quality is also modified by smaller appliances such as fans, humidifiers, dehumidifiers, and air purifiers. Most of these are motor-driven, electrically powered appliances.