Air Conditioning glossary

Air conditioning glossary

The AC-specific parts and measurements, defined in plain English. For cross-system terms like SEER2, refrigerant, and ton, see the master glossary.

Capacitor

A small cylindrical part that gives the compressor and fan motors the jolt they need to start and keep running. A failed capacitor is a common, inexpensive cause of an AC that hums but won't start.

Compressor

The pump in the outdoor unit that pressurizes the refrigerant and drives the whole cooling cycle — the heart, and the most expensive part, of an AC or heat pump.

Condensate drain

The line that carries away the water the evaporator coil pulls from your air. When it clogs, the water backs up and a float switch shuts the system off — a frequent cause of an AC that stops on a humid day.

EEREnergy Efficiency Ratio

Cooling efficiency measured at a single peak condition, rather than across a season like SEER2. It's useful for comparing performance on the hottest days.

Metering device

The valve (often a TXV, thermostatic expansion valve) that meters refrigerant into the evaporator coil at the right rate. It's what lets the system match its cooling to the load.

Short cycling

When a system switches on and off too frequently instead of running steady cycles. It wears parts and wastes energy, and points to an oversized unit, low refrigerant, or an airflow problem.

Superheat & subcooling

The temperature measurements a technician uses to confirm the refrigerant charge is correct. They're why charging a system takes gauges and training, not a guess.