NCCER HVACR key terms

The study of the properties of moist air, including dry-bulb temperature, wet-bulb temperature, humidity ratio, and enthalpy. Technicians use it to predict how air behaves as it is heated, cooled, humidified, or dehumidified.

The temperature of air measured by an ordinary thermometer with a dry sensing element. It reflects only sensible heat and does not account for moisture content or evaporative cooling effects.

The lowest temperature air can reach through evaporative cooling, measured with a thermometer whose bulb is kept wet and moving. It combines dry-bulb temperature and humidity and is used to check refrigerant charge.

Heat that changes the temperature of a substance without changing its state. In air conditioning it is the portion of the load that raises or lowers room temperature, measured in Btu per hour.

Heat absorbed or released when a substance changes state at constant temperature, such as refrigerant boiling or water vapor condensing. In comfort cooling it is the energy removed when moisture condenses on a coil.

The total heat content of a substance per unit mass, combining sensible and latent energy. On a psychrometric chart it is read along slanted lines and used to calculate total coil load.

A cooling capacity equal to 12,000 Btu per hour, roughly the heat needed to melt one ton of ice in 24 hours. Equipment capacity and load estimates are commonly stated in tons.

The amount of heat required to raise one pound of water one degree Fahrenheit. It is the basic unit for expressing heating and cooling quantities in North American HVACR work.

A room-by-room estimate of heat gain and heat loss used to size equipment and ducts. It accounts for insulation, windows, occupancy, appliances, infiltration, and local design temperatures.

A widely recognized residential load calculation method published by the Air Conditioning Contractors of America. It produces room-by-room heating and cooling loads used to select equipment and design duct systems.

A residential duct design method that sizes ducts from the equipment airflow and the pressure available after subtracting component losses. It helps ensure correct room airflow and acceptable static pressure.

A required safety procedure in which an energy-isolating device is locked and tagged so equipment cannot be energized while someone services it. Only the person who applied the lock removes it.

Wearable gear that reduces injury risk, such as safety glasses, gloves, hearing protection, and arc-rated clothing. Selection depends on the hazard present at the specific task and site.

A sudden release of energy caused by an electrical fault through air, producing intense heat, light, and pressure. It can occur without contact and requires shock and flash hazard assessment before work.

A test instrument that measures voltage, resistance, and often current and continuity. Choosing the correct function and range before touching probes prevents meter damage and personal injury.

A meter that measures current by surrounding a single conductor with a split jaw, avoiding a series connection. It is useful for checking compressor and fan motor amp draw against nameplate ratings.

A tool with two gauges and valves that connects to service ports to read system pressures and to add or remove refrigerant. Hoses and gauges must be rated for the refrigerant in use.

A pump used to evacuate air, nitrogen, and moisture from a sealed system before charging. A deep vacuum verified with a micron gauge confirms the system is dry and leak-free.

An electronic instrument that measures very low absolute pressure during evacuation. Readings in microns show whether moisture has been removed and whether the vacuum holds.

Equipment that removes refrigerant from a system and stores it in a recovery cylinder for reuse, recycling, or reclamation. Venting refrigerant to the atmosphere is prohibited by federal regulation.

The repeating process in which refrigerant absorbs heat at low pressure and rejects it at high pressure. Its four stages are compression, condensation, expansion, and evaporation.

The component that raises refrigerant pressure and temperature by squeezing vapor and moving it through the circuit. Common types include reciprocating, scroll, and rotary, each with different service considerations.

The coil where high-pressure refrigerant vapor rejects heat to outdoor air or water and condenses into liquid. Dirty coils or poor airflow raise head pressure and reduce capacity.

The indoor coil where low-pressure liquid refrigerant boils and absorbs heat from the airstream. Its temperature must stay above freezing to avoid icing under normal operating conditions.

A restriction such as a thermostatic expansion valve, fixed orifice, or capillary tube that drops refrigerant pressure between the condenser and evaporator. It also regulates the flow of liquid refrigerant.

A modulating metering device that controls refrigerant flow to maintain a set superheat at the evaporator outlet. It responds to bulb temperature and evaporator pressure through a diaphragm.

The temperature of refrigerant vapor above its saturation temperature at a given pressure. Measured at the evaporator outlet, it confirms the refrigerant is fully boiled before reaching the compressor.

The temperature of liquid refrigerant below its saturation temperature at a given pressure. Measured at the condenser outlet, it indicates how much liquid is stacked in the condenser and helps verify charge.

The temperature at which a refrigerant boils or condenses at a specific pressure. Technicians compare it with line temperature to find superheat and subcooling values.

The high-side pressure read at the compressor discharge or condenser. It rises with high outdoor temperature, dirty coils, or non-condensable gas and affects compressor load and efficiency.

Air or nitrogen trapped in a refrigerant circuit that does not condense at normal condenser temperatures. It raises head pressure, causes high subcooling readings, and must be removed by evacuation.

The federal Clean Air Act rule governing refrigerant handling, recovery, leak repair, and technician certification. It sets requirements for ozone-depleting and substitute refrigerants in stationary equipment.

A refillable, DOT-approved vessel used to hold refrigerant removed from equipment. It must not be filled beyond the marked capacity and must be identified with the refrigerant it contains.

The air supplied to a fuel-burning appliance for complete burning of gas or oil. Inadequate supply causes carbon monoxide production and is a common cause of unsafe operation.

A colorless, odorless gas produced by incomplete combustion that can cause illness or death. Technicians test for it in flue gas and ambient air and check for proper venting.

The movement of flue gases out of an appliance and up a vent or chimney. It may be produced by natural buoyancy or by a fan, and it must be verified so gases do not spill into the space.

A pair of dissimilar metal wires that generates a small voltage when heated, used to prove a pilot flame is present. A weak signal typically prevents the gas valve from opening.

A group of thermocouples connected in series that produces enough millivolts to power a gas valve. It is common on standing-pilot heating equipment and is tested under load.

The ordered steps a control board follows to start a burner, such as proving draft, energizing the igniter, opening the gas valve, and sensing flame. Understanding it guides diagnosis of no-heat calls.

A rod that uses flame rectification to tell the control board a burner is lit. A dirty or corroded sensor causes short cycling or lockout because the board cannot confirm flame.

A reversible system that moves heat into a space for heating and out of it for cooling by changing refrigerant flow direction. Its heating capacity drops as outdoor temperature falls.

A four-way valve that switches refrigerant flow direction in a heat pump, swapping the roles of the indoor and outdoor coils. A failure can leave the system stuck in one mode.

A timed or demand-initiated mode in which a heat pump temporarily reverses to melt frost from the outdoor coil. Controls terminate defrost based on temperature or pressure to avoid overheating.

The resistance to airflow measured across the blower and components of an air handler, expressed in inches of water column. High readings indicate restrictive filters, coils, or undersized ducts.

The volume of air moved through a system per unit time, usually stated in cubic feet per minute. Proper airflow is required for capacity, efficiency, and safe equipment operation.

The amount of refrigerant contained in a system. It is verified by weight, superheat, subcooling, or sight glass depending on the metering device and system type.

The current a motor draws when its rotor cannot turn, typically several times running current. Comparing measured amps with nameplate values helps diagnose compressor and fan failures.

A test that confirms an electrical path is complete with very low resistance. It is used to check switches, fuses, and wiring while the circuit is de-energized.