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Glossary of Common HVAC Terms

If you’ve often wondered what HVAC (Heating, Ventilation, Air Conditioning) stands for, you’ve come to the right place!  Let us know if we’ve missed a term that interests you, and we’ll be sure to add it.



Accumulator: Storage tank that receives liquid refrigerant from the evaporator and prevents it from flowing into the suction line of the compressor before evaporating.

Air Handler: The indoor component of a split system, that houses the blower fan, evaporator coil and electric heat strips.

Air Purifier: A device that eliminates unwanted contaminants from the air.  The best air purifiers are installed in the duct and the entire home’s air is filtered through it.


Boiler: Closed container in which a liquid may be heated and vaporized.

Brazing: Method of joining metals with a filler(solder) using heat between 800 degrees and the melting point of base metals.

British Thermal Unit (BTU): Quantity of heat required to raise the temperature of one pound of water one degree Fahrenheit.



Capacitor: Electrical storage device used to start and run circuits on many electrical motors.

Capillary Tube System: A refrigerant control system in which the pressure difference is maintained through the use of a thin capillary tube.

Carbon Monoxide: Colorless, odorless, and poisonous gas produced when carbon fuels are burned with an improper air mixture.

Check Valve: Device which permits fluid to flow in only one direction.

Chiller: Air Conditioning system that circulates chilled water to various cooling cools in an installation.

Compressor: The pump of a refrigerating mechanism that draws a low pressure gas on the cooling side of refrigerant cycle and compresses the gas into the high pressure or condensing side of the cycle.

Communicating System: Similar to a computer network where system components communicate over a serial network. In HVAC, each component has a unique address that allows the controller to recognize and coordinate each part’s operation. Typically, a residential communicating system uses 4 wires – 2 for power and 2 for communication. HVAC components respond to commands sent over the communicating system instead of the presence or absence of control voltage. Communication is two-way to allow components know what other components are doing and adjust their own operation accordingly.

Condensate Pump: Device used to remove water condensate that collects beneath an evaporator coil or from a condensing furnace.

Condenser: The part of the refrigeration mechanism which receives hot, high pressure refrigerant gas from the compressor and cools it until it returns to its liquid state.

Condensing Unit: The outdoor component of a split system, that houses the compressor, condenser coil, outdoor fan and other refrigerant components depending on the type of system.

Contactor:  A device that connects or breaks an electric circuit. In an air conditioner, when the contactor gets pitted, it doesn’t make smooth contact and can interrupt the smooth operation of the system.  Contactors have metal moving parts and eventually wear out and need to be replaced. Inspecting the contactor for signs of pitting is a routine part of a good maintenance program.

Controller: Used to monitor indoor factors like temperature and humidity and control heating and cooling to achieve or maintain desired levels. HVAC controllers receive input from thermostats and humidity sensors and send output to respective HVAC systems.

Cubic Feet Per Minute(CFM): This measures how much air is flowing through a unit per minute.



Damper: Device for controlling airflow.

Delta T: Difference in temperature. When the technicians measure the differences in temperature between various areas of the system, they will use the shorthand: △T.

Diffuser: Attachment for duct openings that distribute the air in wide flow patterns.

Drain Line: A pipe that drains condensation from the evaporator.  A clogged drain line is one of the most common issues we repair in early summer, and one of the most easily avoided with routine maintenance.  Unfortunately, if your air handler is located in the attic space and you don’t have an adequate drain pan or an emergency float switch in the drain pain, the water can overflow and cause extensive ceiling damage.  Good drainage is key for the optimal operation of any air conditioner.

Drier: Device used to remove moisture from a refrigeration system.

Duct: Tube or channel through which air is moved.



ECM Motor: electronically commutated motor.  A brushless DC (direct current) motor with built in speed and torque controls that allows it to adjust speed for optimal airflow.

Evaporator: Part of a refrigerating mechanism in which the refrigerant vaporizes and absorbs heat.

Expansion Valve: Device in refrigerating system that reduces the pressure from the high side to the low side causing a temperature difference in the refrigerant.



Flex: A type of flexible duct used in a general HVAC system that enables it to be installed in tight places where standard duct installation would not be possible.

Float Switch: Device that is attached to a drain pan that can shut the system down to prevent water from overflowing.

Furnace: Self-contained appliance designed to supply heated through ducts.



Gas Package: An “all in one” HVAC system which combines gas heating and electrical air conditioning outside of your home. The only part of the unit that is inside your house is the thermostat. A package unit offers the advantage of not having to give up any space inside a closet or the attic or under the house for the air handler or furnace.  The disadvantage is that all the components are outside in the weather.

Gas Valve: Device in gas used for starting, stopping or regulating the flow of gas.



2-Stage Heat: Usually refers to a furnace that has two heating levels. Stage 2 is “high” heat and performs when the temperatures get fairly low in winter.  Stage 1 operates as “low” heat and works at a lower capacity, using less resources, but will still keep you warm in mild weather.

Head Pressure: Pressure that exists in condensing side of refrigerating system.

Heat Exchanger: Device used to transfer heat from a warm or hot surface to a cold or cooler surface.

Heat Load Calculation: Calculation used to determine the HVAC needs of the space to be cooled or heated. The heat load determines the size, or tonnage, of the unit required.  Anytime a new system is being considered, a heat load calculation should be done to make sure the new unit is sized properly to the home and the duct work.

Heat Pump: A refrigerating system used to supply heat to a temperature controlled space. The same system can also remove heat from the same space.

Hybrid System: A system that combines the best of an electric heat pump with the benefits of gas. An electric heat pump works efficiently down to about 40º and below that gas is a better method for creating comfortable heat.  Here in North Alabama, that means when evening temps drop much below 40º, a hybrid or dual-fuel system could provide cost savings and comfort.



Indoor Air Quality (IAQ): The status of indoor air as measured by numerous factors: temperature, humidity, airflow, pollutants,etc.



Leak Detector: Device or instrument, electronic sniffer or soap solution, used to detect leaks.

Line Set: Also known as refrigerant lines, they are typically used in a split system to supply and return refrigerant between the indoor coil and the outdoor condenser.  The size and proper installation of the line set is of key importance when installing a new system.  The distance and height the refrigerant must travel will impact the line set size requirements.  If a line set is too small (a common cost-cutting shortcut used by less reputable installers) the operation of the entire system will be jeopardized.

Liquid Line: Tube that carries liquid refrigerant from the condenser to the evaporator.



Manual J: A specific procedure set by the Air Conditioning Contractors of America to calculate the heating and cooling requirements (heat load calculation) for single-family homes

Mastic: A flexible and waterproof cement used as a sealant. It is used for sealing ducts and is far superior to metal tapes.

Micron Gauge: Instrument for measuring vacuums very close to perfect vacuum.

Mini Split or Ductless System: A cooling system that has a fairly small exterior condenser and a small wall-mounted indoor evaporator unit. It is a smart way to add air conditioning to rooms without opening walls or attic/crawlspace for ductwork installation. We commonly install ductless systems in bonus rooms, home additions, or areas where it would be very hard to run duct.

Nitrogen: A gaseous element used for purging systems and pressure test.



Orifice: Accurate size opening for controlling fluid flow.

Overload Protector: Device that will stop operation of unit dangerous conditions arise.



Package Units: Complete refrigerating system in one cabinet.



R-Value: The thermal resistance of a given material.

Reclaiming: Taking the refrigerant that has been removed from a system and processing it in accordance with EPA rules.

Refrigerant: Substance used in refrigerating mechanism. It absorbs heat in evaporator by change of state from liquid to a gas, and releases its heat in a condenser as the substance returns from the gaseous state back to a liquid state.

Relay: An electromagnetic mechanism moved by a small electrical current in a control circuit. It operates a valve or switch in an operating circuit.

Reversing Valve: Device used to reverse direction of the refrigerant flow, depending on weather heating or cooling is desired.



Seasonal Energy Efficiency Ratio (SEER): A measure of cooling capacity and efficiency.

Sensor: Material or device that goes through physical or electronic change as surrounding conditions change.

Split System: Refrigerating system that places the condensing unit outside or away from the evaporator.

Sub cooling: Cooling of liquid refrigerant below its condensing temperature.

Suction Pressure: Pressure in the low side of the refrigerating system.

Superheat: 1- Temperature of vapor above its boiling temperature as a liquid at that pressure. 2- The difference between the temperature at the evaporator outlet and the lowering temperature of the refrigerant evaporating in the evaporator.



Ton Refrigerant Unit: Used for measuring the capacity of a system. Unit that removes same amount of heat in 24 hours as melting of one ton of ice.

Transformer: Electromagnetic device that transfer electricity energy from the primary circuit into variations of voltage in a secondary circuit.

TXV: abbreviation for thermal expansion valve. A thermal expansion valve controls the rate of refrigerant that enters into the evaporator.  Technicians often call it a metering device.  The TXV is deeply embedded in the heat and air system so when the TXV breaks down, it is a major, time-consuming repair that requires professional expertise.



Vacuum Pump: Device used for creating vacuums for testing or drying purposes.

Valve: Device used for controlling fluid flow.

Variable Speed Motor: A motor that runs at variable capacities (between 20% and 100%) based on programmed algorithms on the furnace’s circuit board. The benefits of a variable speed motor include less cost, quiet operation, and continuous air supply at lower speeds. A variable speed motor is able to adjust optimal output for whatever the situation requires.



Zone Controls: Controls used to maintain each specific area or zone within a building at a desired condition.