Gas Furnace
The heat is supplied by the burning of natural gas. A mixture of gas and air flows into the burner and is ignited by the pilot. Combustion occurs, and warm air from the burner flame rises to fill a chamber known as a heat exchanger. The heat exchanger becomes hot. Indoor air passing around the heat exchanger absorbs that warmth, continues into the air ducts and the heat is distributed throughout the interior area. The by-products of combustion pass upward through a venting system and escape through a vent in the roof.
A-Coil
Evaporator coil that sits on top the Gas Furnace. Air that travels across the evaporator coil gives up heat (the colder coil absorbs it) and humidity (moisture condenses upon contact with the cold surface of the coil). The cooler, drier air that continues through the air ducts is vented throughout your home to maintain your desired comfort level.
The condensing unit
Outdoors, at the condensing unit, an air conditioner releases the heat that was captured indoors. The same refrigerant that absorbed the heat indoors at low pressure is now pressurized by the compressor and is circulated through another coil, the condensing coil. In the condensing coil, under high pressure, the refrigerant releases its heat very quickly, making the coil itself hot. A fan blows across the coil, cooling its temperature down and transferring the heat to the outside air.