AL HVAC key terms

The number of degrees a refrigerant vapor is above its saturation temperature at the pressure being measured. Measured at the evaporator outlet, it confirms that all liquid has boiled off before the vapor reaches the compressor.

The number of degrees a liquid refrigerant is below its saturation temperature at the measured pressure. Measured near the condenser outlet, it shows the liquid seal that keeps flash gas out of the metering device.

The temperature at which a refrigerant changes between liquid and vapor at a given pressure. On a pressure-temperature chart it is the reference point used to figure superheat and subcooling.

A modulating metering device with a sensing bulb that opens or closes a needle valve to hold a target superheat at the evaporator outlet, matching refrigerant flow to the load as conditions change.

A rate of heat removal equal to 12,000 British thermal units per hour, roughly the heat needed to melt one ton of ice in a day. It is the standard sizing unit for cooling equipment.

A small electric resistance heater wrapped around or inserted in the compressor sump. It keeps oil warm so refrigerant does not condense into the oil during the off cycle, which would dilute the lubricant.

In a fuel-fired furnace, the metal chamber where combustion gases heat the indoor air stream without mixing with it. Cracks or holes let flue gases, including carbon monoxide, enter the supply air.

The gas pressure measured at the burner manifold, set by the appliance regulator or orifices. It determines firing rate and must match the nameplate value; too high or too low changes combustion and efficiency.

Annual Fuel Utilization Efficiency, the percentage of fuel energy a heating appliance converts to usable heat over a typical year. Older atmospheric furnaces run near 80 percent; condensing models exceed 90.

A timed or demand-initiated sequence in a heat pump that temporarily stops the outdoor fan or reverses operation to melt frost off the outdoor coil, then returns the unit to normal heating.

A four-way valve in a heat pump that swaps the roles of the indoor and outdoor coils by redirecting refrigerant flow, selecting either heating or cooling mode.

The air supplied to a burner that provides oxygen for burning fuel. It must come from outdoors or a large enough indoor volume; a shortage causes incomplete combustion, sooting, and carbon monoxide production.

A heat exchanger that rejects heat from hot refrigerant gas to outdoor air moved by a fan, condensing the gas to liquid. Coils must stay clean and airflow unobstructed to hold rated capacity.

A U-shaped pipe in the drain line that holds a water seal, preventing air from being pulled or pushed through the drain while still letting condensate flow away from the coil.

An electrically operated switch with a low-voltage coil and high-voltage contacts. The coil pulls the contacts closed to start the compressor or fan motor and opens them when the thermostat ends the call.

A capacitor wired into a motor circuit continuously to shift phase and improve starting torque and running efficiency of single-phase compressor and fan motors. A weak or bulged capacitor causes hard starting and high current.

The current a motor draws when its rotor cannot turn. It is the highest current the motor will draw and is used to size overloads, wiring, and starting components.

The relationship stating that voltage equals current multiplied by resistance. Technicians use it to predict current draw, find resistance in a circuit, or diagnose voltage drop.

The total resistance to airflow, measured in inches of water column, that the blower works against in the duct system and accessories. High readings signal restricted ducts, dirty filters, or undersized returns.

A box-like chamber attached to the supply or return side of an air handler that distributes air into branch ducts. Plenums must be sealed and insulated where they pass through unconditioned space.

The number of times the entire volume of air in a space is replaced in one hour. It is used to set ventilation rates for occupancy and to check whether a building is too tight.

An adjustable plate inside a duct that is set during air balancing to control how much air flows into a branch, so each room receives its designed airflow.

Joining copper tubing with a filler metal that melts above 840 degrees Fahrenheit, forming a strong leak-free joint by capillary action while the tubing itself does not melt.

A slow flow of dry nitrogen through tubing during brazing that displaces oxygen and prevents oxidation inside the pipe, which would otherwise leave scale that clogs valves and driers.

A canister in the liquid line containing a screen and desiccant that traps debris and absorbs moisture from the refrigerant, protecting the metering device and compressor.

The industry-accepted method for calculating residential heating and cooling loads room by room, using building size, insulation, windows, infiltration, and local design conditions.

The ordinary air temperature measured by a standard thermometer, with no effect from humidity. It is used to evaluate sensible heat and the equipment air temperature rise or drop.

The lowest temperature air can reach by evaporating water into it, read with a thermometer whose bulb is kept wet. It reflects moisture content and is needed for load calculations.

The portion of total cooling load made up of sensible heat, found by dividing sensible capacity by total capacity. It guides equipment selection and airflow settings for proper humidity control.

Pulling a system down with a vacuum pump to a low micron level, typically 500 microns or below, to remove moisture and noncondensable gases before charging.

Adjusting dampers, fan speed, and register settings so measured airflow at each terminal matches design values, while total system airflow stays within the equipment's rated range.

The documented process of verifying that installed equipment is started, adjusted, and operating as designed, including checks of airflow, charge, controls, and safety devices before handover to the owner.

The difference between return air and supply air temperatures at the equipment. A steady-state reading outside the manufacturer's expected range points to airflow, charge, or duct problems.

A small window in the liquid line that lets the technician see refrigerant flow. Bubbles suggest a shortage or restriction, and a color indicator shows moisture in the system.

The EPA credential required for technicians who service, repair, or dispose of equipment containing ozone-depleting refrigerants or their substitutes, obtained by passing a written exam.

A refillable, DOT-approved pressure vessel used to collect refrigerant removed from a system. It must never be filled above 80 percent of its capacity and is evacuated before each use.

A number comparing how much heat a gas traps in the atmosphere over a set period relative to carbon dioxide, which is set at one. Higher values mean stricter handling and phase-out rules.

A mildly flammable refrigerant classification. Systems using A2L refrigerants require specific handling, charge limits, ventilation, and ignition-source rules that differ from nonflammable refrigerants.

A safety procedure in which a disconnect or breaker is locked open and tagged so equipment cannot be energized while someone works on it. Only the person who applied the lock removes it.