NATE AC Service key terms

The number of degrees a refrigerant vapor is above its saturation temperature at a given pressure. Measured as suction line temperature minus the saturation temperature corresponding to suction pressure.

The number of degrees a liquid refrigerant is below its saturation temperature at a given pressure. Measured as saturation temperature at the liquid line pressure minus the liquid line temperature.

The temperature at which a refrigerant changes phase at a given pressure. In a condenser it is the condensing temperature; in an evaporator it is the boiling temperature.

A reference table or graph listing the saturation pressure of a refrigerant at each temperature. Technicians use it to convert gauge readings into saturation temperatures for superheat and subcooling calculations.

A metering device that regulates refrigerant flow into the evaporator by maintaining a set superheat at the evaporator outlet, using a sensing bulb and modulating the valve opening.

The component that drops refrigerant pressure between the liquid line and the evaporator, controlling flow and creating the low-side pressure that drives boiling and heat absorption.

The indoor coil where low-pressure liquid refrigerant boils and absorbs heat from the airstream, cooling and dehumidifying the air delivered to the conditioned space.

The outdoor coil where high-pressure refrigerant vapor rejects heat to outdoor air and condenses into liquid, dropping from a hot gas to a subcooled liquid.

The pump that raises refrigerant pressure and temperature, moving vapor from the low side to the high side of the system and driving the entire refrigeration cycle.

The insulated line carrying low-pressure refrigerant vapor from the evaporator back to the compressor inlet. Its temperature and pressure are used to measure superheat.

The line carrying high-pressure liquid refrigerant from the condenser outlet to the metering device. Its temperature and pressure are used to measure subcooling.

The line carrying hot, high-pressure refrigerant vapor from the compressor outlet to the condenser inlet. It is the hottest line in the system.

The high-side pressure read at the compressor discharge or condenser. It reflects condensing conditions and rises with high outdoor temperature or restricted condenser airflow.

The low-side pressure read at the compressor inlet. It corresponds to the evaporator boiling temperature and indicates load, airflow, and charge conditions.

A service tool with high-side and low-side gauges plus hoses used to read system pressures, evacuate the system, and add refrigerant during service.

The amount of refrigerant contained in a sealed system. Correct charge is verified by superheat, subcooling, or weigh-in methods depending on the metering device type.

The process of removing refrigerant from a system and storing it in a recovery cylinder for reuse, recycling, or reclaim, as required by environmental regulations before opening the circuit.

The federal regulation governing handling, recovery, and disposal of ozone-depleting and substitute refrigerants. Certification is required to purchase refrigerant and service sealed systems.

A pump used to evacuate a refrigeration system, removing air and moisture so the circuit can be opened, repaired, and recharged without contaminants.

An electronic instrument that measures deep vacuum in microns of mercury. It verifies that a system has been evacuated to a level that removes moisture before charging.

The process of locating refrigerant leaks using electronic detectors, UV dye, soap bubbles, or pressure testing, so the system can be repaired before recharging.

The volume of air moved across the evaporator or condenser, usually stated in cubic feet per minute. Low airflow causes low superheat, icing, and high head pressure.

The resistance to airflow in a duct system, measured in inches of water column. High static pressure reduces airflow and can indicate undersized ducts or a dirty filter.

The indoor fan that moves conditioned air through the duct system. Its speed settings and cleanliness directly affect airflow across the evaporator coil.

A device that traps dust and particles from return air before it reaches the evaporator. A loaded filter raises static pressure and reduces airflow.

Water that condenses on the evaporator coil as air is cooled below its dew point. It must drain away through a trap and drain line to prevent water damage.

A pan under the evaporator coil that collects condensate and routes it to the drain line. A clogged pan or line can cause overflow and water damage.

A safety device installed in the drain pan or line that shuts off the unit when condensate rises to a set level, preventing overflow and water damage.

The amount of moisture in air expressed as a percentage of the maximum it can hold at that temperature. Cooling coils reduce it by condensing water out of the airstream.

The temperature at which air becomes saturated and moisture begins to condense. An evaporator coil below dew point removes moisture from the airstream.

Heat that changes the temperature of air or a substance without changing its phase. It is measured by dry-bulb temperature change across a coil.

Heat absorbed or released during a phase change without a temperature change, such as refrigerant boiling or water vapor condensing on a coil.

The temperature of air measured by an ordinary thermometer, unaffected by moisture content. It is used with wet-bulb to find enthalpy and coil performance.

The temperature of air measured with a wetted thermometer bulb, reflecting evaporative cooling and moisture content. It indicates how much latent heat the air carries.

The total heat content of air, combining sensible and latent energy per pound of dry air. Psychrometric charts use it to calculate coil capacity and airflow.

A graph relating dry-bulb, wet-bulb, humidity ratio, and enthalpy of air. Technicians use it to analyze cooling, heating, and dehumidification performance.

The difference between return-air and supply-air temperatures across the evaporator. It is a quick check of cooling performance, typically 15 to 22 degrees Fahrenheit in normal conditions.

An electrically operated switch that closes to energize the compressor and condenser fan on a call for cooling. Pitted contacts cause intermittent operation.

A component that stores electrical energy and helps start or run motors. A failed run capacitor causes the compressor or fan to hum and overheat without starting.

The electronic circuit board that sequences the blower, compressor, and safety circuits in response to thermostat signals and sensor inputs.

The temperature-sensing control that calls for cooling or heating by closing low-voltage circuits to the equipment. It also sets fan mode and setpoint.