Note that this is not only for aircon servicing technicians but also for the end consumers to understand.Here we describe the most common systems used to air condition and heat houses. These may have two units, one inside and one outside; or be all in one single unit, which may be on the roof or on the ground next to the house. In a smaller version, the heating and cooling may be all in one unit which fits in or under a window, and works only for one room. The essential parts for the system are the air conditioning and/or heating unit(s), supply and return ductwork, grilles and registers, and controls. Cold or hot air is produced by the unit, in accordance with the controls settings, and is blown through ductwork to registers or grilles to comfort condition the rooms. Air is returned from the rooms back to the unit to be recooled or re-heated and used again.
The Heating Cycle
Heating may be from gas, electric, or a special form of electric called a heat pump (see cooling cycle below). The heat is transferred to the air by a heat exchanger or coil which is positioned in the airflow of the unit. A gas furnace burns gas on one side of a metal box or cell, called a heat exchanger, and the air passes over the other side to get hot. This way, the air is not exposed to the burnt gas fumes, which go out of the house through a vent. Electric heat is commonly a set of wires, or coil, which become hot when the electricity is on the wires are high resistance and the electrical current passing through the wires causes them to heat up. Again the air passes over them to heat up, and in this case there are no harmful fumes.
The Cooling Cycle
This is more complex, but not difficult to understand. It works like a refrigerator, to produce cooling out of electric power. The essential components of the refrigeration system are the condenser, expansion device, evaporator and the compressor. These items are described in more detail in the component section of the book, and the basic operating principles in producing cooling are detailed here.
The closed circuit connecting these components (usually constructed of copper tubing) contains a refrigerant. The refrigerant is a chemical compound, designed to have the required pressure and temperature characteristics, relative to whether it is a gas or a liquid, for the refrigeration cycle being used. The condenser, usually air-cooled to the outside, is a pipe coil that condenses hot refrigerant from gas to liquid, giving up heat. The restrictor, on smaller units often just a small opening that allows the refrigerant through slowly, reduces the refrigerant pressure, which in turn reduces the temperature. The cool, low pressure liquid refrigerant evaporates to a gas in the evaporator, a coil in the air that goes to the house, and in doing so drops in temperature to cool the air. To enable the cycle to repeat, the now warmer gas has to become a hot liquid again. This is done by the compressor, which compresses the gas, making it hotter than the outside air and at high pressure. When this is fed to the condenser (see above), the hot gas is cooled and becomes liquid again. This cycle repeats continuously while cooling is required. The special case of the heat pump is all electric, and cools as described above. However, for heating it reverses the actions above to make heat inside the house, with cooling outside. This is done by a “reversing valve” which literally just reverses the flows of the refrigerant in the system.
Information courtesy from Mike Newberry from the Aircon Repair