HVAC Excellence Master 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 coil is fully fed with liquid and protects the compressor from slugging.

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 indicates charge and metering balance.

The temperature at which a refrigerant changes phase at a given pressure. On a pressure-temperature chart it links gauge pressure to boiling and condensing points, and it shifts with pressure, not with load.

A table or graph giving the saturation temperature of a specific refrigerant at each pressure. Technicians use it to convert gauge readings into evaporating and condensing temperatures and to compute superheat and subcooling.

Heat absorbed or released when a substance changes phase without changing temperature, such as boiling liquid refrigerant in an evaporator. It is the dominant part of a system's cooling capacity compared with sensible heat.

Heat that changes the temperature of a substance without changing its phase. In air conditioning it is the portion of load that lowers room temperature, measured by dry-bulb change across the coil.

The total heat content of a refrigerant or air stream, combining sensible and latent energy per unit mass. On a pressure-enthalpy diagram, the difference in enthalpy across the evaporator defines refrigerating effect.

The heat a unit mass of refrigerant absorbs in the evaporator, equal to the enthalpy rise from expansion-valve outlet to compressor suction. Higher refrigerating effect means more capacity per pound of refrigerant circulated.

Absolute discharge pressure divided by absolute suction pressure. High ratios raise discharge temperature, reduce volumetric efficiency, and can require two-stage compression or unloading to stay within compressor limits.

The ratio of actual refrigerant volume pumped to the compressor's theoretical displacement. It falls as compression ratio rises because re-expansion of clearance gas and internal leakage take up cylinder volume.

The heat exchanger where low-pressure liquid refrigerant boils and absorbs heat from air or water. Its performance depends on airflow, coil cleanliness, and correct superheat at the outlet.

Thermostatic expansion valve, a modulating metering device that holds a set superheat by balancing bulb pressure, evaporator pressure, and spring force. It adjusts refrigerant flow as evaporator load changes.

The pump that raises refrigerant pressure and moves vapor through the circuit. Common types include reciprocating, scroll, screw, and rotary; each has specific suction superheat and oil-return requirements.

A vessel on the suction line of some systems that traps liquid refrigerant and oil and releases them gradually, preventing liquid slugging at the compressor. Common on heat pumps and systems with long line sets.

The relationship stating that voltage equals current times resistance. Technicians use it to calculate expected resistance of loads, verify current draw, and diagnose open, shorted, or grounded circuits.

Series circuits provide one current path so current is equal and voltage divides; parallel circuits provide multiple paths so voltage is equal and current divides. Control switches are wired in series, loads in parallel.

An electrically operated switch with a coil and heavy-duty main contacts that connects line power to a compressor or fan motor. Low-voltage thermostat signals energize the coil, which pulls the contacts closed.

A small electromagnetic switch that uses a low-power coil to control a higher-power circuit, such as a fan or valve. Relays isolate control voltage from line voltage and allow one signal to operate several loads.

A temperature-responsive control that opens or closes low-voltage circuits to call for heating, cooling, or fan operation. Modern units may be programmable or communicating and include staging and deadband settings.

The current a motor draws at startup when the rotor is stationary. Comparing measured startup current with nameplate LRA helps identify a seized compressor, failed capacitor, or low supply voltage.

A capacitor that stays in the circuit of a single-phase motor to shift current phase and improve running torque. A weak or bulged capacitor causes high current draw, slow starting, and overheating.

The temperature of air measured by an ordinary thermometer, unaffected by moisture content. It is the value used for room temperature setpoints and for sensible load calculations across a coil.

The lowest temperature air can reach by evaporative cooling, measured with a wetted thermometer in moving air. It indicates moisture content and sets the practical limit for evaporative cooling and condenser performance.

The percentage of moisture in air compared with the maximum it can hold at that temperature. It changes as air is heated or cooled even when the actual moisture content stays the same.

The temperature at which air reaches saturation and moisture begins to condense. Surfaces below dew point sweat, so duct insulation and chilled-water piping must be vapor-sealed to prevent condensation.

The resistance to airflow measured at the blower outlet and return inlet, excluding the air handler's internal components. It is compared with the fan performance table to set blower speed and verify duct sizing.

The volume of air moved per unit time, usually expressed in cubic feet per minute. Correct airflow is required for rated capacity, proper superheat and subcooling, and safe heat-exchanger temperatures.

A manufacturer's graph showing the relationship between airflow and static pressure for a specific blower at various speeds. It predicts delivered airflow when system resistance is known.

The pressure exerted by air against duct walls, measured along the distribution system. Excess static indicates undersized or restrictive ducts and causes noise, reduced airflow, and higher energy use.

A recognized residential duct design method that sizes ducts from room-by-room airflow and available static pressure. It accounts for fittings, friction rate, and terminal devices to balance the system.

Air escaping or entering through joints, seams, and penetrations in the duct system. Leakage in unconditioned space wastes capacity and can draw humid or contaminated air into the supply stream.

Closing duct joints with mastic, mesh tape, or approved sealants to reduce leakage. Sealing is most valuable on ducts in attics, crawlspaces, and garages where pressure differences drive losses.

The boundary separating conditioned space from outdoors, including walls, roof, windows, doors, and floor. Its airtightness and insulation level largely determine heating and cooling loads.

A low-permeance material that limits moisture diffusion through building assemblies. Placement depends on climate; in humid cooling climates it is typically on the exterior side of insulation.

A recognized method for calculating residential heating and cooling loads from building envelope, orientation, occupancy, and climate data. Accurate loads prevent oversized equipment and poor humidity control.

A refrigerant classification for lower-flammability fluids with mild burning velocity and heat of combustion. Common examples include R-32 and R-454B, which require charge limits, ignition-source rules, and leak detection.

A classification for highly flammable refrigerants such as propane and isobutane. A3 use is restricted by charge size and location, and service procedures demand strict ignition control and ventilation.

A measure of how much a refrigerant traps heat over a set horizon compared with carbon dioxide. Regulations increasingly favor low-GWP alternatives and require reporting of refrigerant emissions.

Finding refrigerant loss using electronic detectors, pressure tests, or ultrasonic and dye methods. Detection is critical for A2L and A3 systems because leaks can create flammable concentrations in enclosed spaces.

Removing refrigerant from a system into a suitable container for reuse, recycling, or destruction. Recovery is required before opening a circuit and is tracked to prevent venting to the atmosphere.

A programmed temperature adjustment during unoccupied hours that reduces heating or cooling energy. Recovery from setback must be sized so equipment can restore comfort without excessive auxiliary heat.

Cooling output in British thermal units per hour divided by electrical input in watts. Higher EER indicates more efficient operation; seasonal metrics such as SEER2 account for varying outdoor conditions.

The systematic process of verifying that installed equipment and controls perform as designed. It includes airflow measurement, refrigerant charge verification, control checks, and documentation of results.

A structured assessment of a building's energy use that identifies inefficiencies and cost-effective improvements. Levels range from walk-through surveys to detailed modeling with measured data.