Accumulator - A storage tank located in the suction line of a compressor, It allows small amounts of liquid refrigerant to boil away before entering the compressor. Sometimes used to store excess refrigerant in heat pump systems during the winter cycle.

Air Conditioner - Equipment that conditions air by cleaning, cooling, heating, humidifying or dehumidifying it. A term often applied to comfort cooling equipment.

Air Handler - The device that moves the air across the heat exchanger in a forced air system-normally considered to be the fan and its housing.

Air Sensor - A device that registers changes in air conditions such as pressure, velocity, temperature, or moisture.

Air Vent - A fitting used to vent air manually or   automatically from a system.

Alternating Current - An electric current that reverses its direction at regular intervals.

Ambient Temperature - The surrounding air temperature.

Amperage - Amount of electron or current flow in an electric circuit.

Automatic Combination Gas Valve - A gas valve for gas furnaces that incorporates a manual control, gas supply for the pilot, adjustment and safety features for the pilot, pressure regulator, the controls for and the main gas valve.