AR HVAC key terms

The amount of heat energy needed to raise one pound of water by one degree Fahrenheit. Technicians use it to express equipment capacity and to compare how much heat a system moves or produces.

Heat that changes the temperature of air or another substance without changing its state. Dry-bulb readings and the sensible capacity printed on equipment ratings are based on this kind of heat.

Heat absorbed or released when a substance changes state, such as liquid refrigerant boiling into vapor, with no change in temperature. Most of the heat moved by an evaporator and condenser is latent heat.

The water vapor present in air stated as a percentage of the most moisture that air could hold at the same temperature. It rises and falls as air is heated or cooled even when the actual moisture content stays the same.

The temperature to which air must be cooled before its water vapor starts condensing into liquid. A coil surface colder than the dew point removes moisture and creates condensate.

The reading from a thermometer whose bulb is wrapped in a wet sock and exposed to moving air. It accounts for evaporative cooling and, paired with dry bulb, indicates the total heat content of the air.

An instrument that measures small air or gas pressures, such as gas supply pressure, duct static pressure, or flue draft, by comparing the pressure against a liquid column or a calibrated electronic sensor.

A digital instrument that reads deep vacuum in microns while a system is being evacuated. It confirms that moisture and non-condensable gases have been pulled out before charging begins.

Feeding dry nitrogen through piping while brazing so oxygen is pushed out and no carbon or scale forms inside the tubing. The same gas is also used to pressure test a system for leaks.

A safety procedure in which the power source to equipment is switched off, locked, and marked with a tag so nobody can re-energize it while a technician is working on the unit.

The number of degrees a refrigerant vapor is warmer than its saturation temperature at the measured pressure. Superheat at the evaporator outlet shows whether the metering device is feeding the coil correctly.

The number of degrees a liquid refrigerant is cooler than its saturation temperature at the measured pressure. Subcooling measured at the condenser outlet indicates how much liquid is backed up in the condenser.

The temperature at which a refrigerant boils or condenses at a given pressure. On a pressure-temperature chart it links gauge pressure to the temperature of a saturated liquid or vapor.

The pump that draws low-pressure vapor from the evaporator and squeezes it to a higher pressure and temperature so it can give up heat in the condenser and return as liquid.

The coil where low-pressure liquid refrigerant boils and absorbs heat from the air or product being cooled. Its boiling temperature is set by the suction pressure the compressor maintains.

The coil where hot, high-pressure refrigerant vapor gives up heat to outdoor air or water and condenses into a liquid, rejecting heat picked up in the evaporator plus heat added by the compressor.

A metering device that varies refrigerant flow into the evaporator to hold a set superheat at the coil outlet, using a sensing bulb and the pressure difference across the valve to move its pin.

A device installed in the refrigerant circuit that traps moisture, dirt, and acid so they cannot circulate, plug the metering device, or damage the compressor.

A canister on the suction line that catches liquid refrigerant and oil and lets only vapor travel on to the compressor, protecting it from slugging during low-load or off-cycle conditions.

The federal regulation covering refrigerant handling. It requires technicians who service or dispose of refrigerant-containing equipment to be certified and to follow recovery, recycling, and leak-repair rules.

A four-way valve used on heat pumps that swaps the jobs of the indoor and outdoor coils by redirecting refrigerant flow, letting one system both heat and cool.

An electrically operated switch whose low-voltage coil pulls in high-voltage contacts to start a compressor or fan motor. Pitted, stuck, or welded contacts cause a unit to fail to start or stop.

A capacitor wired into a motor circuit during operation to shift current phase and improve torque and efficiency. A weak, bulged, or open capacitor makes the motor overheat or fail to start.

A meter with jaws that fit around a single conductor and read current from its magnetic field, letting a technician check amp draw without disconnecting any wiring.

A test instrument that applies a high test voltage to motor or compressor windings and measures insulation resistance. Low readings point to moisture or insulation breakdown that can cause ground faults.

A device that steps line voltage down to about 24 volts to feed thermostats and control circuits, keeping the low-voltage controls separated from the higher-voltage power wiring.

A timer or circuit board on a heat pump that starts a defrost cycle when frost builds on the outdoor coil, energizing the reversing valve and usually stopping the outdoor fan until frost melts.

A heat-operated switch used with electric resistance heat that brings elements on one at a time with a delay and keeps the blower running after the elements shut off.

A protective device that trips when it senses current leaking to ground, reducing the chance of shock. Electrical codes require GFCI protection in many places where HVAC equipment is serviced or installed.

An accepted method for calculating a building's heating and cooling load from its size, insulation, windows, doors, and local climate. The calculated load is what determines correct equipment capacity.

An accepted method for designing a duct system that delivers the required airflow to each room while keeping static pressure, velocity, and noise within acceptable limits.

The resistance to airflow created by the ductwork, filter, and accessories, measured at the blower. Excessive static pressure lowers airflow and should be checked on every installation and service call.

A seasonal rating of cooling efficiency calculated with updated test conditions. A higher number means the unit uses less electricity to deliver the same cooling over a typical season.

Annual fuel utilization efficiency, a percentage showing how much of the fuel burned by a furnace becomes usable heat. The remaining energy leaves with the flue gases and other losses.

The spread between return-air and supply-air temperatures across a system. Comparing it with the manufacturer's range helps confirm that airflow and refrigerant charge are in line.

Charging a system by adding a measured weight of refrigerant based on the nameplate charge, adjusted for line-set length. It is the required method for many blends and for units with no sight glass.

A water seal formed in the drain line of a coil that stops air from being pulled or pushed through the drain and allows condensed water to leave the unit properly.

Joining copper refrigerant tubing with a filler metal that melts above roughly 840 degrees Fahrenheit. The joint is made while nitrogen flows through the pipe to prevent internal oxidation.