KY HVAC key terms

The number of degrees a refrigerant vapor is warmer than its saturation temperature at the same pressure. Technicians measure it at the evaporator outlet to check charge and expansion-valve operation.

The number of degrees a liquid refrigerant is cooler than its saturation temperature at the condensing pressure. It is measured at the condenser outlet and helps confirm proper charge.

The temperature at which a refrigerant boils or condenses at a given pressure. Liquid and vapor coexist at this temperature, and changing the pressure changes the value.

Heat energy absorbed or released when a substance changes phase, such as liquid boiling to vapor, without a change in temperature. Refrigeration depends on this energy transfer.

A pump that raises the pressure and temperature of refrigerant vapor so it can reject heat in the condenser, then returns to the evaporator through the metering device.

A heat exchanger that removes heat from hot, high-pressure refrigerant vapor, causing it to condense into a liquid. It rejects heat to outdoor air, water, or another medium.

A heat exchanger where low-pressure liquid refrigerant boils and absorbs heat from the air or water being cooled. The vapor then travels to the compressor.

A metering device that regulates refrigerant flow into the evaporator by opening and closing in response to evaporator outlet temperature and pressure, maintaining a set superheat.

The process of removing refrigerant from a system and storing it in an approved container without cleaning or processing it. Recovery is required before opening a system to the atmosphere.

Air supplied to a burner so fuel can burn completely. It must come from outdoors or from a space large enough to avoid negative pressure and incomplete combustion.

Air supplied to a burner beyond the theoretical amount needed for complete combustion. Some excess is necessary for safety, but too much lowers efficiency and raises stack losses.

The pressure difference, usually created by a chimney's buoyancy or a fan, that moves combustion gases from the appliance through the flue to outdoors.

A colorless, odorless, toxic gas produced by incomplete combustion. It can cause illness or death, so appliances must be vented properly and tested for it.

In a furnace, the metal passages that separate hot combustion gases from the supply air, transferring heat without mixing the two airstreams.

A safety device that detects the presence of a burner flame and signals the control board to keep the gas valve open, shutting it off if no flame is proven.

Annual Fuel Utilization Efficiency, a rating that expresses how much of the fuel's energy a heating appliance converts to heat over a typical year. Higher numbers mean greater efficiency.

The outward push of air against duct walls, measured in inches of water column. It indicates the resistance the blower must overcome to move air.

A box-like chamber connected to the air handler that distributes supply air into branch ducts or collects return air before it enters the unit.

The number of times the total volume of air in a room or building is replaced with outdoor or filtered air in one hour. It is used to judge ventilation and filtration adequacy.

The study of the properties of moist air, including dry-bulb temperature, humidity, enthalpy, and dew point, used to analyze heating, cooling, and moisture processes.

The amount of moisture in air expressed as a percentage of the maximum moisture the air could hold at its current temperature. It changes as air is heated or cooled.

The temperature to which air must be cooled, at constant pressure and moisture content, to become saturated and begin condensing water vapor.

Minimum Efficiency Reporting Value, a number that rates how well an air filter captures particles of specified sizes. Higher ratings capture smaller particles.

The relationship in an electrical circuit where voltage equals current multiplied by resistance (V = I × R). It is used to troubleshoot motors, contacts, and wiring.

An electrically operated switch that uses a low-voltage coil to close higher-voltage contacts, commonly switching power to compressors, heaters, or condenser fans.

An electrical component that stores charge and provides a phase shift or extra starting torque to some motors. Failed capacitors cause motors to hum or fail to start.

The current a motor draws at the instant it is energized while its rotor is not turning. It is much higher than running current and is used for overload and protection sizing.

A control device that senses space temperature and switches heating, cooling, or fan operation to maintain the setpoint chosen by the occupant.

An industry method for calculating a building's heating and cooling loads from envelope, window, infiltration, and internal-gain data. Results guide equipment sizing.

An industry method for designing a duct system, using room airflows and available static pressure to select duct sizes and fittings that meet the blower's capacity.

An industry method for selecting heating and cooling equipment that matches the calculated loads, accounting for capacity, efficiency, and operating range.

The amount of heat a building gains from sun, occupants, equipment, and outdoor air that must be removed to hold the desired indoor temperature and humidity.

A diagnostic test that mounts a calibrated fan in an exterior doorway to measure how much air leaks through a building's envelope at a specified pressure difference.

An official authorization from a local building department allowing specified work to proceed, usually requiring plans, inspections, and payment of a fee.

The adopted local or model regulations governing installation, alteration, and inspection of HVAC equipment, ductwork, venting, and related systems.

A legal claim against a property for unpaid work or materials, which a contractor or supplier may file to secure payment for a project.

A written agreement that modifies the original contract scope, price, or schedule for a project, signed by the owner and contractor before the extra work proceeds.