NATE Core key terms

Heat energy that changes the temperature of a substance without changing its state. Measured with dry-bulb temperature and expressed in BTU per hour for load calculations.

Heat energy absorbed or released when a substance changes state, such as water evaporating or refrigerant boiling, with no change in temperature during the phase change.

The temperature of air measured by an ordinary thermometer, unaffected by moisture content. Used to calculate sensible heat gains and equipment temperature rise.

The lowest temperature air can reach through evaporative cooling, measured with a wetted wick and moving air. Indicates moisture content and evaporative capacity.

The ratio of actual water vapor in air to the maximum vapor the air could hold at that temperature, expressed as a percentage. Changes as temperature changes even when moisture stays constant.

The temperature at which air becomes saturated and water vapor begins condensing into liquid. Air cooled below this point forms condensation on coils, ducts, or surfaces.

The temperature of refrigerant vapor above its saturation temperature at a given pressure. Measured at the evaporator outlet to verify the expansion device is feeding the coil correctly.

The amount a liquid refrigerant is cooled below its saturation temperature at a given pressure. Measured at the condenser outlet and used to confirm adequate liquid seal and charge.

The temperature at which a refrigerant boils or condenses at a specific pressure. On a pressure-temperature chart, each pressure corresponds to one saturation temperature for a given refrigerant.

A reference table or graph listing the saturation pressure and temperature relationship for a specific refrigerant. Technicians use it to convert gauge readings into saturation temperatures.

The repeating sequence in which refrigerant evaporates, is compressed, condenses, and is expanded to move heat from a cooler space to a warmer one.

The component that raises refrigerant vapor pressure and temperature, creating the pressure difference that drives circulation through the system. Common types include reciprocating, scroll, and rotary.

The indoor heat exchanger where low-pressure liquid refrigerant absorbs heat and boils into vapor, cooling the air or product passing across its surfaces.

The heat exchanger where high-pressure refrigerant vapor rejects heat to outdoor air, water, or another medium and condenses into liquid for return to the metering device.

A valve or restrictor that drops refrigerant pressure between the condenser and evaporator and regulates how much liquid enters the coil, controlling superheat.

Thermostatic expansion valve; a modulating metering device that senses evaporator outlet temperature and pressure to maintain a set superheat across changing loads.

British thermal unit; the heat needed to raise one pound of water one degree Fahrenheit. Cooling capacity is commonly rated in BTU per hour, with 12,000 BTU per hour equaling one ton.

A cooling capacity equal to 12,000 BTU per hour, roughly the heat absorbed by melting one ton of ice in 24 hours. Used to size equipment and compare loads.

A structured estimate of heat gain and loss for a building based on envelope, orientation, occupancy, and internal sources. Determines required equipment capacity and airflow.

Cubic feet per minute; the volumetric airflow rate through ducts, coils, or equipment. Proper CFM is required for correct heat transfer and humidity control.

The outward force per unit area exerted by air against duct walls, measured in inches of water column. Used to check blower performance and duct restrictions.

An instrument that measures low air or gas pressures, typically in inches of water column, to verify duct static pressure, gas supply pressure, or filter loading.

A fluid that absorbs and releases heat by changing state within a refrigeration system. Selection depends on pressure range, capacity, oil compatibility, and environmental regulations.

A charging technique for fixed-orifice systems that sets refrigerant charge by measuring evaporator outlet superheat at specified indoor and outdoor conditions.

A charging technique for TXV systems that sets charge by measuring liquid-line subcooling at the condenser outlet under steady operating conditions.

Removing refrigerant from a system into a container for reuse, recycling, or disposal. Required by regulation before opening a circuit to the atmosphere.

A pressure below atmospheric, measured in microns, used to remove moisture and non-condensables from a system before charging. Deep vacuum confirms a leak-free, dry circuit.

A unit equal to one-thousandth of a millimeter of mercury, used to measure deep vacuum. A standing vacuum test shows whether moisture or leaks remain.

Testing flue gas for oxygen, carbon dioxide, carbon monoxide, and temperature to verify safe, efficient fuel burning and to set airflow and gas pressure.

The flow of combustion gases through a vent or flue, created by natural buoyancy or a mechanical inducer. Measured to confirm safe venting and prevent spillage.

The relationship stating voltage equals current multiplied by resistance. Used to calculate expected readings when troubleshooting motors, controls, and heating elements.

A circuit with a single path so the same current flows through every component. A break anywhere stops current; voltage divides across the loads.

A circuit with multiple current paths where voltage is the same across each branch and total current is the sum of branch currents. Common in HVAC control wiring.

A control device that senses space temperature and switches heating or cooling equipment to hold a setpoint. May be low-voltage, line-voltage, or communicating.

The target value a control system maintains, such as desired room temperature or supply-air temperature. Compared against sensor input to start or stop equipment.

A temperature sensor made of two joined dissimilar metals that generates a small voltage when heated. Commonly used to prove a pilot flame is present.

A printed circuit assembly that sequences blower, ignition, and safety functions using relay logic or microprocessors. Fault codes and LED indicators aid diagnosis.