HVAC Excellence Master LC-AC key terms

Heat that changes the temperature of a substance without changing its state. In air conditioning it is the portion of the total load that a thermometer can measure directly.

Heat absorbed or released when a substance changes state at constant temperature, such as water evaporating or refrigerant boiling. It is not visible on a dry-bulb thermometer.

The ordinary air temperature measured by a dry thermometer. It is the value reported in weather forecasts and used to set room thermostats.

The lowest temperature reachable by evaporating water into air, measured with a wetted thermometer in moving air. It indicates how much moisture the air can still absorb.

The percentage of moisture the air currently holds compared with the maximum it could hold at that temperature. Warmer air can hold more water vapor.

The temperature at which air becomes saturated and water vapor begins to condense. A coil operating below dew point removes moisture from the airstream.

The total heat content of air, combining sensible and latent energy per pound of dry air. Technicians use it to compare the energy of air entering and leaving a coil.

The temperature of refrigerant vapor above its saturation temperature at the same pressure. Measured at the evaporator outlet, it confirms the coil is fully fed with liquid.

The temperature of liquid refrigerant below its saturation temperature at the same pressure. Measured at the condenser outlet, it confirms the coil is fully condensing and feeding a solid liquid column.

The temperature at which a refrigerant boils or condenses at a given pressure. On a pressure-temperature chart it is the temperature read directly from the measured pressure.

A chart or app that converts refrigerant pressure to saturation temperature for a specific refrigerant. It is the basis for calculating superheat and subcooling in the field.

The temperature difference between the bubble point and dew point of a zeotropic refrigerant blend as it changes state. Glide affects how superheat and subcooling are calculated.

The pump that raises refrigerant pressure and moves vapor through the system. It creates the pressure difference that allows the evaporator to absorb heat and the condenser to reject it.

The indoor coil where low-pressure liquid refrigerant boils and absorbs heat from the airstream. It removes both sensible heat and latent heat from the conditioned space.

The outdoor coil where high-pressure refrigerant vapor rejects heat and condenses to liquid. It discharges both the heat absorbed indoors and the heat of compression.

A valve or fixed orifice that drops refrigerant pressure and regulates flow into the evaporator. Common types include thermostatic expansion valves, electronic expansion valves, and pistons.

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.

The volume of air moved through a system per unit time, usually stated in cubic feet per minute. Low airflow causes low capacity, high superheat, and possible coil icing.

The resistance to airflow in a duct system, measured in inches of water column. High static pressure reduces airflow and raises blower load.

The difference between return-air and supply-air temperatures across an operating system. It is a quick field check of capacity, airflow, and charge.

Air supplied to a burner for complete fuel combustion. Inadequate combustion air produces carbon monoxide and poor flame quality.

Air supplied beyond the theoretical amount needed for complete combustion, usually expressed as a percentage. Proper excess air balances safety against stack loss.

A colorless, odorless gas produced by incomplete combustion. It is a health hazard and must be tested in flue gas and ambient air during service.

A flame-sensing method in which the burner flame conducts current in one direction between a sensing rod and ground. The control module proves flame before allowing gas flow.

A device made of two dissimilar metals that generates a small voltage when heated. In standing-pilot systems it holds the gas valve open while flame is present.

The timed series of steps a furnace control follows before and during burner operation, including inducer start, pressure switch proof, ignition, and flame proving.

A safety device that proves the inducer is moving combustion air before the gas valve opens. It opens or closes on a set pressure difference.

A small heater in a mechanical thermostat that warms the bimetal slightly near setpoint to end the heating cycle early and reduce overshoot.

The range of temperature around setpoint in which no heating or cooling is called. It prevents short cycling and is adjustable on many digital controls.

A control that senses space temperature and switches heating or cooling equipment to hold a setpoint. Modern versions add scheduling, staging, and remote sensing.

An electrically operated switch that uses a coil to close heavy power contacts for a compressor or other large load. The coil is controlled by low-voltage circuitry.

A protective device that opens a circuit when current or temperature exceeds a safe limit, preventing motor damage. It may reset automatically or require manual reset.

The current a motor draws when the rotor cannot turn. Comparing it with running current helps diagnose seized compressors and starting problems.

A component that stores electrical energy and shifts phase to help start or run a motor. A failed capacitor causes high current draw and poor motor performance.

Setting refrigerant charge in a fixed-orifice or piston system by measuring evaporator outlet superheat against the manufacturer's target for current conditions.

Setting refrigerant charge in a thermostatic expansion valve system by measuring condenser outlet subcooling against the manufacturer's target.

Removing refrigerant from a system into a container for reuse, recycling, or disposal. It is legally required before opening a circuit and protects the atmosphere.

The fan that moves air through the ductwork and across the indoor coil or heat exchanger. Its speed is set to match required airflow for the installed equipment.