HVAC Excellence AC key terms

The number of degrees a refrigerant vapor is above its saturation temperature at a given pressure. Measured at the evaporator outlet, it confirms the metering device is feeding enough refrigerant and that liquid has fully boiled off.

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

The temperature at which a refrigerant changes phase at a given pressure. On a pressure-temperature chart, each pressure corresponds to one saturation temperature, which is the basis for reading superheat and subcooling.

A reference table or graph listing the saturation temperature of a specific refrigerant at various pressures. Technicians use it to convert gauge readings into temperatures when calculating superheat and subcooling.

A thermostatic expansion valve that meters refrigerant into the evaporator by balancing refrigerant pressure, spring force, and a remote bulb signal. It maintains a set superheat at the evaporator outlet as load changes.

Any component that controls refrigerant flow into the evaporator and creates the pressure drop between high and low sides. Common types include thermostatic expansion valves, fixed orifices, pistons, and capillary tubes.

A metering device with a non-adjustable opening, often a piston or capillary tube, that feeds refrigerant at a rate depending mainly on pressure difference. Systems using it are usually charged by superheat.

The indoor coil where low-pressure liquid refrigerant absorbs heat from the airstream and boils into vapor. Its performance depends on airflow, coil cleanliness, and correct refrigerant feed.

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

The pump that raises refrigerant pressure and moves vapor through the system. It separates the low side from the high side and is the component most sensitive to liquid slugging and overheating.

The process of removing refrigerant from a system into an approved container without venting it to the atmosphere. Recovery is required by regulation before opening a system and must follow EPA handling rules.

A DOT-approved, properly rated tank used to store recovered refrigerant. It must not be filled beyond the marked capacity and must be labeled with the refrigerant type it contains.

A pressure below atmospheric, measured in microns, used to remove air and moisture from a system before charging. A deep, stable vacuum confirms the system is dry and leak-free.

An instrument that measures very low absolute pressure during evacuation. It is placed at the system, not at the pump, to show the true vacuum level and whether moisture is still boiling off.

Flowing dry nitrogen through a system during brazing to displace oxygen and prevent internal oxidation, or using nitrogen to pressure-test for leaks. It protects the compressor and metering device from debris.

A joining method that melts filler metal at high temperature to connect copper refrigerant lines. Proper heat control and nitrogen flow are needed to avoid leaks and internal scale.

Forming a cone-shaped end on copper tubing so a flare nut can seal it against a fitting. Correct flare depth and torque prevent leaks on connections that cannot be brazed.

The process of finding refrigerant leaks using electronic detectors, soap bubbles, pressure tests, or dye. Finding and repairing leaks is required before recharging a system.

The volume of air moved through a system, usually stated in cubic feet per minute. Correct airflow is essential for proper heat transfer, superheat, subcooling, and temperature rise across the furnace.

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

The difference between return and supply air temperatures across a furnace or heat pump. It confirms that airflow and heating output are within the manufacturer's listed range.

A safety procedure that de-energizes equipment and prevents accidental startup during service. The technician applies a lock and tag to the disconnect before working on electrical components.

A switch or breaker near the equipment that removes power from the unit for service. It must be opened and verified before electrical work begins.

A component that stores electrical energy and helps start or run motors. A failed run capacitor often causes hard starting, overheating, or low motor torque.

An electrically controlled switch that connects power to the compressor and fan motors. Pitted contacts or a burned coil can prevent the unit from starting.

The control that senses space temperature and signals the equipment to heat, cool, or fan. Wiring and configuration errors at the thermostat often cause no-cool or no-heat calls.

A system that moves heat in either direction using a reversing valve, providing both heating and cooling. In heating mode the outdoor coil becomes the evaporator and the indoor coil the condenser.

A four-way valve in a heat pump that changes refrigerant flow direction to switch between heating and cooling. A stuck valve can cause the system to heat in cooling mode or fail to defrost.

A timed or demand-initiated mode in which a heat pump temporarily reverses to melt frost from the outdoor coil. It is controlled by sensors and a board and ends when the coil reaches a set temperature.

Measuring flue gas oxygen, carbon monoxide, and temperature to verify safe and efficient fuel burning. It is a core skill for gas furnace and boiler service and is part of specialty certification testing.

A colorless, odorless gas produced by incomplete combustion that can be deadly. Technicians test for it in flue gas and ambient air and must correct causes such as poor draft or dirty burners.

The metal component in a furnace that separates combustion gases from the supply air. Cracks or holes allow carbon monoxide into the airstream and require replacement or shutdown.

The components that light a burner, such as a hot surface igniter, spark igniter, or standing pilot. Failure to ignite or prove flame causes lockout and no-heat complaints.

A rod or rectification device that proves flame is present by sensing current through the flame. A dirty or oxidized sensor commonly causes short cycling and ignition lockout.

The study of air and moisture properties, including dry-bulb, wet-bulb, humidity ratio, and enthalpy. It is used to evaluate comfort, coil performance, and load calculations.

The temperature of air measured with a wet thermometer bulb, reflecting evaporative cooling. It indicates moisture content and is used to set airflow and charge on cooling systems.

The temperature at which air becomes saturated and moisture condenses. In HVAC it helps predict condensation on ducts and coils and supports humidity control design.

A method such as Manual J that estimates heating and cooling needs from building size, insulation, windows, and climate. Correct sizing prevents short cycling and poor humidity control.

Planning duct size, layout, and fittings to deliver required airflow with acceptable pressure loss. Poor design raises static pressure and reduces system capacity.

The process of verifying that a new system is installed, adjusted, and operating as designed. It includes airflow, charge, control, and safety checks documented for the owner.

A structured method of gathering symptoms, testing components, and confirming the repair. Following a logical sequence prevents guesswork and repeat callbacks in professional service work.

Converting a system from one refrigerant to another, often from an ozone-depleting type to a newer blend. It requires compatible oil, seals, and metering changes and careful labeling.

A fluid used in a cooling system that absorbs and releases heat as it changes pressure and phase. Common types include R-410A, R-32, and R-454B, each with specific handling rules.

A mildly flammable refrigerant classification, such as R-32 or R-454B, with lower global warming potential than many older refrigerants. Handling requires ignition-source awareness and updated safety practices.

The process by which an educator demonstrates technical knowledge and teaching competence to be recognized as a qualified HVAC instructor. It often involves an exam and documented field experience.

A structured evaluation of whether a person can perform required tasks to a defined standard. In education it verifies hands-on skill as well as written knowledge.

Statements describing what students should be able to do after completing a course or unit. Educators use them to align lessons, labs, and assessments with industry needs.