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HVAC Glossary


14 SEER - This is the new minimum efficiency standard for an air conditioner or heat pump. All new units must now meet this standard. 

80% A.F.U.E. - 80% of the fuel burned is turned into heat delivered at the register. The U.S. government's established minimum AFUE rating for a furnace is 78 percent.

A-Coil - A heat exchanger consisting of two diagonal coils that are joined together in the shape of the letter A.

Accumulator - A storage tank located in the suction line of a compressor.

A.F.U.E (Annual Fuel Utilization Efficiency) - A rating that reflects the efficiency of a gas furnace in converting fuel to energy. A rating of 90 means that approximately 90% of the fuel is utilized to provide warmth to your home, while the remaining 10% escapes as exhaust.

Air Flow - The distribution or movement of air.

Air Handler - The portion of the central air conditioning or heat pump system that moves heated or cooled air throughout a home's ductwork. This unit is usually located in the attic, utility room or closet or in a crawlspace. In some systems a furnace handles this function.

Air Conditioner - A device that modifies the temperature, humidity, cleanliness, or general quality of air.

Air Conditioning - The science of controlling the temperature, humidity, cleanliness or general quality of air. Abbreviated: AC.

Air Duct - Pipes that carry warm air and cold air to rooms and back to furnace or air conditioning system.


Ambient Temperature - The temperature of the immediately surrounding air or area.

Amperage - The rate of electrical current flow in a circuit.

ARI - Air-Conditioning and Refrigeration Institute is a non-profit, voluntary organization comprised of heating, air conditioning and refrigeration manufacturers. ARI publishes standards for testing and rating heat pumps and air conditioners to provide you with a standardized measure of comparison. So, ARI ensures a level of quality within the industry.

Auto-Ignition - A feature of newer gas furnaces that eliminates the need for a standing pilot light. The system shuts off completely when not in use.





Balance Point - An outdoor temperature -- usually between 30°F to 45°F -- at which a heat pump's output exactly equals the heating needs of the house. Below the balance point, supplementary electric resistance heat is needed to maintain indoor comfort.

Balancing or Air Balancing - Adjusting an air conditioning system so that the right amount of air is delivered to the right places in your home in order to achieve the right heating or cooling effect.

Breaker - A heat-activated electrical device used to open an electrical circuit to protect it from excessive current flow.

BTU ( British Thermal Unit) - In scientific terms, it represents the amount of energy required to raise one pound of water one degree Fahrenheit. One BTU is the equivalent of the heat given off by a single wooden kitchen match. For your home, it represents the measure of heat given off when fuel is burned for heating or the measure of heat extracted from your home for cooling.

BTU/H or BTU/HR- The number of BTUs in an hour.

Burner - The device that facilitates the combustion of air and gas.

Burner Orifice - The opening in the burner through which the gas or fuel passes prior to combustion.


Capacity - The output or producing ability of a piece of cooling or heating equipment. Cooling and heating capacity are normally referred to in BTUs. The ability of a heating or cooling system to heat or cool a given amount of space. For heating, this is usually expressed in BTUs. For cooling, it is usually given in tons.

Carbon Monoxide - Carbon Monoxide (CO) is a colorless, odorless gas produced by burning any fuel. CO is poisonous and symptoms of CO poisoning are similar to those of the flu: headaches, fatigue, shortness of breath, nausea and dizziness. The Consumer Product Safety Commission (CPSC) recommends a yearly, professional inspection.

Central Forced-Air Heating System - A piece of equipment that produces heat in a centralized area, then distributes it throughout the home through a duct system.

CFC - Chlorofluorocarbons, used as a refrigerant in air conditioners and heat pumps, linked to the depletion of the ozone layer.

CFM - A standard of airflow measurement. Cubic feet per minute. A typical system produces 400 CFM per ton of air conditioning. The abbreviation for cubic feet per minute, commonly used to measure the rate of air flow in an air conditioning system.

Charge - Amount of refrigerant in a system.

Coefficient of Performance (COP) - A ratio calculated by dividing the total heating capacity provided by the heat pump, including circulating fan heat but excluding supplementary resistance heat (Btu's per hour), by the total electrical input (watts) x 3.412. (See Heating Seasonal Performance Factor, below.) Coefficient of Performance, COP, is the ratio of energy input to heating capacity. This is the instantaneous measurement of the heating performance of your heat pump. It is comparable to knowing how many miles per gallon of gasoline your car gets when it is going full speed. You can find the COP on the nameplate of your heat pump.

Compressor - The heart of an air conditioning or heat pump system. It is part of the outdoor unit that pumps refrigerant. The compressor maintains adequate pressure to cause refrigerant to flow in sufficient quantities to meet the cooling requirements of the system. Part of a split-system heat pump or air conditioner’s outdoor unit that controls the pressure applied to the refrigerant, necessary for taking in heat to warm your home or getting rid of heat to keep your home cool.

Condensate/Condensation - Water vapor that liquefies due to the lowering of its temperature to the saturation point. The process by which a gas is changed into a liquid at constant temperature by heat removal.

Condensate Drain - An outlet for the moisture collected on an evaporator coil.

Condensate Pump - A water pump that disposes of or recirculates collected condensate.

Condenser - A device that transfers unwanted heat out of a refrigeration system to a medium (either air, water, or a combination of air and water) that absorbs the heat and transfers it to a disposal point. There are three types of condensers: air-cooled condensers, water-cooled condensers, and evaporative condensers. The evaporative condenser uses a combination of air and water as its condensing medium. Most residential systems have an air-cooled condenser.

Condensing Unit - Part of a refrigerating mechanism which pumps vaporized refrigerant from the evaporator, compresses it, liquefies it in the condenser and returns it to the refrigerant control. The outdoor portion of a split system air conditioner contains the compressor and outdoor coil ignoring the reverse cycle operation, also the outdoor in a heat pump system.

Condenser Coil or Outdoor Coil - Located in the outdoor unit, the coil dissipates heat from the refrigerant, changing the refrigerant from vapor to liquid. Part of the outdoor portion of a split-system air conditioner or heat pump. By converting refrigerant that is in a gas form back to a liquid, the coil sends heat carried by the refrigerant to the outside.

Condenser Fan - The fan that circulates air over an air cooled condenser. (usually located outside)

Contactor - A switch that can repeatedly cycle, making and breaking an electrical circuit. When sufficient current flows through a coil built into the contactor, the resulting magnetic field causes the contacts to be pulled in.

Cooling Capacity - A measure of the ability of a unit to remove heat from an enclosed space.



Damper-Found in ductwork, this movable plate opens and closes to control airflow. Dampers can be used to balance airflow in a duct system. They are also used in zoning to regulate airflow to certain rooms.

Defrost Cycle - The process of removing ice or frost buildup from the outdoor coil during the heating season.

Defrost Mode - During the heating cycle of a heat pump, frost may build up on the outdoor coil. To remove the frost and maintain efficiency, the system will automatically defrost itself. This usually only takes a few minutes, then the system automatically switches back to heating. It is normal to see steam rising from the outdoor unit when this happens.


Ductwork-Pipes or channels that carry air throughout your home. In a home comfort system, ductwork is critical to performance ­ in fact, it's as critical as the equipment.

Ductless Mini-Split System - A system where there is no ductwork involved, simply a single indoor unit (usually wall mounted) or two or more indoor units and a single outdoor unit (condensing unit/heat pump) Used to heat and cool areas where traditional ductwork is not feasible such as additions, workshops, warehouse offices, enclosed porches, etc.


Electronic Leak Detector - A test instrument that detects the presence of refrigerants in small air samples.

Energy Efficiency Ratio (EER) - A ratio calculated by dividing the cooling capacity in Btu's per hour (Btuh) by the power input in watts at a given set of rating conditions, expressed in Btuh per watt (Btuh/watt). (See Seasonal Energy Efficiency Ratio,below.)

Efficiency - The amount of usable energy produced by a machine, divided by the amount of energy supplied to it. A rating on comfort equipment is similar to the miles per gallon rating on your car. The higher the rating number, the more efficient the system and the lower your fuel consumption will be.

Emergency Heat (Supplementary Electric Heat) - The back up electric heat built into a heat pump system. The same as an auxiliary heater, except it is used exclusively as the heat source when the heat pump needs repair.


Energy Star® - EPA rating given to products that exceed a standard efficiency level of operating costs of H.V.A.C. equipment. When properly installed, Energy Star-labeled products can save consumers 10-40% on heating and cooling bills each year. A government supported branding used to identify energy efficient products. The branding was developed by the US Department of Energy and the US Environmental Protection Agency.

Evaporator Coil or Indoor Coil-The other half of your air conditioning system located inside your home is the indoor unit. This is where the refrigerant evaporates as it absorbs heat from the air that passes over the coil.

Expansion Valve - A valve that meters the levels of refrigerant through a temperature or pressure control.


Filter - A device used to remove dust and other particles from air for the purposes of reducing the load on the respiratory system and to protect the HVAC equipment. Filters vary greatly in particle arrestance; the higher the MERV rating, the better the filter.

Flexible Duct - A duct that can be gradually bent to go around obstacles.

Flue - A vent that carries the products of combustion from a boiler or furnace.

Furnace - That part of an environmental system which converts gas, oil, electricity or other fuel into heat for distribution within a structure.


Gas Furnace Heat Exchanger - Located in the furnace, the heat exchanger transfers heat to the surrounding air, which is then pumped throughout your home.

Grille - A cage-like panel that covers the end of a duct, permitting the passage of air but not foreign objects.


HCFC - Hydrochlorofluorocarbons, used as a refrigerant in air conditioners and heat pumps. HCFCs were thought to contribute to the depletion of the earth’s ozone layer.

Heat Exchanger - A device for the transfer of heat energy from the source to the conveying medium.

Heat Gain - The amount of heat gained, measured in BTU's, from a space to be conditioned, at the local summer outdoor design temperature and a specified indoor design condition.

Heat Loss - The amount of heat lost, measured in BTU's from a space to be conditioned, at the local winter outdoor design temperature and a specified indoor design condition.

Heat Pump - An electric air conditioning and heating system that captures existing outdoor heat and transfers or "pumps" this heat into your home. The most common type is an "air to air" meaning heat is captured from the outside air and transferred to the air inside your home. A heat pump is basically an air conditioner with a reversible valve that allows it to operate in reverse, removing heat from your house and shunting it outdoors in the summer, and removing heat from outdoor air and shunting it into your house in the winter. Because heat pumps do not actually create heat - they just move it from one place to another - heat pumps are more efficient than other forms of heating.

Humidity - The amount of moisture in the air. Air conditioners remove moisture for added comfort.

HVAC - Industry acronym for the general category of Heating, Ventilation, and Air Conditioning


Ignition - Elevating the temperature of a substance to the point of causing a combustion reaction.

Indoor Air Quality - The purity of indoor air or a lack of polluting gases or particles given off by equipment and furnishings in a space that can affect human health.

Indoor Coil - The portion of a heat pump or central air conditioning system that is located in the house and functions as the heat transfer point for warming or cooling indoor air.

Indoor Unit - This is usually located inside the house and contains the indoor coil, fan, motor, and filtering device, sometimes called the air handler.

Induced-Draft Furnace - A furnace in which a motor-driven fan draws air from the surrounding area or from outdoors to support combustion.

Infiltration - Air flow into a space usually through walls and leaks around doors and windows. Air that enters your home through holes, gaps, and cracks, (e.g., plumbing or electrical holes, the heating and air conditioning system, doors, and windows).

Insulation - Any material that slows down the transfer of heat.



Kilowatt (Kw) - Equal to 1,000 watts.

Kilowatt-hour (Kwh) - A common unit of electrical consumption measured by the total energy created by one kilowatt in one hour.


Latent Cooling Capacity - An A/C system's capability to remove moisture from the air.

Latent Heat - The heat energy needed to change the state of a substance (i.e.: from a liquid to a gas) but not it's temperature.

Line Drier - An accessory that removes moisture from a refrigeration system.

Line Set - A term used for tubing sets furnished by the manufacturer for connecting parts of an air conditioning system.

Liquid Line - The refrigerant tubing extending from the condenser outlet to the metering device. Usually the small warm line.

Load Calculation - A mathematical design tool used to determine the heat gain and heat loss in a building so that properly sized air conditioning and heating equipment may be installed.

Locked Rotor Amperage (LRA) - The current drawn by a motor at start-up, before the rotor starts turning.


Matched System - A heating and cooling system comprised of products that have been certified to perform at promised comfort and efficiency levels when used together, and used according to design and engineering specifications.

Media - The fine material of a filter that traps dirt, dust, mildew or bacteria.



Natural Gas - A fossil fuel formed over millions of years from dead vegetation and animals that were deposited or washed deep into the earth.



OEM - Original equipment manufacturer.

Operating Cost - The cost of running your heating and air conditioning system, day-to-day, based on energy use.


Payback - The amount of time required (usually in years) for positive cash flows to equal the total investment costs. This is often used to describe how long it will take for energy savings resulting from using more energy-efficient equipment to equal the premium paid to purchase the more energy-efficient equipment.

Payback Analysis - An overall measurement of the efficiency and value of your heating and air conditioning system. Payback analysis is used to measure the period of time required to add up the energy saving on higher efficient equipment against the purchase price of that equipment. Or to calculate the monthly energy savings against the monthly payments to purchase an upgrade.

Plenum - A chamber which can serve as a distribution area for heating or cooling systems, generally between a false ceiling and the actual ceiling. A sealed chamber at the inlet or outlet of an air handler. The duct attaches to the plenum.

Programmable Thermostat - A thermostat with the ability to record different temperature settings for different times for your heating and/or cooling equipment. Programmable thermostats can be electronic, or mechanical.


Pump Down - To use the compressor to pump all of the system's refrigerant into the receiver and/or condenser prior to opening the system for service. This is also used in commercial systems to prevent refrigerant migration in the off cycle.


R-22 - A refrigerant containing chlorine used in air conditioning systems. The EPA has mandated that new R-22 cannot be manufactured after 2020 because it has been linked to the depletion of the ozone layer and global warming. Most commonly referred to by its trademarked name, Freon.

R-410A - The refrigerant that will replace R-22 in the HVAC equipment manufacturing process. It does not contain chlorine and is not hazardous to the environment.

R Value - Term used to measure the level of effect of insulating products that lower temperature transfer.

Reciprocating Compressor - A type of compressor used in air conditioners that compress refrigerant by using a type of "piston" action.

Refrigerant - A chemical that produces a refrigerating effect while expanding and vaporizing. Most residential air conditioning systems contain R-22 refrigerant. R-22 is regulated by international controls under the Montreal Protocol and in the United States by the Environmental Protection Agency. It is scheduled to be in production until the year 2020. It's used in approximately 95 percent of air conditioning equipment manufactured in the U.S. today.

Refrigerant Charge (or, “charging the refrigerant”) - The procedure an HVACR technician performs to ensure that the system has enough of the right kind refrigerant for proper and peak operating performance.

Refrigerant Lines - Two copper lines that connect the outdoor air conditioner or heat pump to the indoor evaporator coil.

Refrigerant Recovery - The process of removing refrigerant from a system and placing it in a container.

Register - Combination grille and damper assembly covering an air opening or end of an air duct.

Relative Humidity (RH) - The ratio of the amount of vapor contained in the air to the greatest amount the air could hold at that temperature. Normally expressed as a percentage. The percent of moisture actually in the air compared to the maximum amount of moisture the air can hold at that temperature.

Relay - An electrical switch controlled by the flow of current in a separate or parallel circuit.

Refrigerant Charge (or, “charging the refrigerant”) - The procedure an HVACR technician performs to ensure that the system has enough of the right kind refrigerant for proper and peak operating performance.

Refrigerant Lines - Two copper lines that connect the outdoor air conditioner or heat pump to the indoor evaporator coil.

Refrigerant Recovery - The process of removing refrigerant from a system and placing it in a container.

Register - Combination grille and damper assembly covering an air opening or end of an air duct.

Relative Humidity (RH) - The ratio of the amount of vapor contained in the air to the greatest amount the air could hold at that temperature. Normally expressed as a percentage. The percent of moisture actually in the air compared to the maximum amount of moisture the air can hold at that temperature.

Relay - An electrical switch controlled by the flow of current in a separate or parallel circuit.

Reversing Valve - An electrical, four-way valve in a heat pump that diverts refrigerant flow according to whether cooling or heating is needed; also called a four-way valve.


Scroll Compressor - A specially designed compressor that works in a circular motion vs. an up and down piston action.

SEER - Seasonal Energy Efficiency Ratio - means the total cooling output of a central air conditioner in British thermal units during its normal usage period for cooling divided by the total electrical energy input in watt-hours during the same period as determined using the ARI-specified test procedure. This rating is only for units with cooling capacity less than 65,000 Btu/hr. The higher the SEER number, the more efficient the system is at converting electricity into cooling power. The more efficient the unit, the lower the operating cost (electric bill).

Sensor - A device that reacts to a change in conditions.

Sensible Heat - The temperature of the air. This type of heat is measured with a thermometer.

Setback Thermostat - A state-of-the-art electronic thermostat with a built-in memory that can be programmed for different temperature settings at different times of the day. A device, containing a clock mechanism, which can automatically change the inside temperature maintained by the HVAC system according to a preset schedule. The heating or cooling requirements can be reduced when a building is unoccupied or when occupants are asleep.

Setpoint - The temperature to which a thermostat is set for desired comfort level.

Split System- A central air conditioner consisting of two or more major components. The system usually consists of a compressor-containing unit and condenser, installed outside the building and a non-compressor -containing air handling unit installed within the building. This is the most common type of system installed in a home. Refers to a comfort system configuration consisting of components in two locations. Common examples include an outside unit, such as an air conditioner and an indoor unit, such as a furnace and coil.


Temperature Drop - The amount of supply air (outlet air) temperature under the return air (inlet air) temperature.

Temperature Rise - The amount of supply air (outlet air) temperature over the return air (inlet air) temperature.

Thermostat - A temperature sensitive switch for controlling the operation of a heater or furnace. Typically found on a wall inside the home, that consists of a series of sensors and relays that monitor and control the functions of a heating and cooling system. A device that allows you to control the temperature inside your home by telling the heating or cooling system how much air to produce. An automatic control device designed to be responsive to temperature and typically used to maintain set temperatures by cycling the HVAC system.

Time Delay - Usually refers to a device that will not allow the condenser to restart for an average of 5 minutes.

Ton or "Ton of Cooling" - A cooling unit of measure. Each ton equals the cooling effect of 12,000 Btuh. Heat pumps and air conditioners are generally sized in tons. Typical sizes for single family residences are between two and five tons. It is important to note that actual capacity is not constant and will change based on outdoor or indoor temperatures. The published capacity rating of air conditioners and heat pumps is based on performance at the ARI standard temperature levels of 95 F outside, 80 F inside, and 50% relative humidity. The number of tons a system has is the total BTU capacity of the system. The size of the area to be cooled will determine the correct size of the system in tons. While an air conditioner may be called a three ton unit, it may not produce 36,000 Btu/h in cooling. There is a wide variety of actual capacities that are called "three tons." It is important to note that actual capacity is not constant and will change based on outdoor or indoor temperatures.

Transformer - A set of coils that increases or decreases voltage by induction.

TXV (Thermostatic Expansion Valve) - A valve that controls the flow of refrigerant. It is operated by evaporator temperature and pressure.


UL - Underwriters' Laboratories

Unconditioned Space - A space that is neither directly nor indirectly conditioned space, which can be isolated from conditioned space by partitions and/or closeable doors.

Undercharged - A refrigeration system that is short of refrigerant.

Upflow Furnace - A heater in which air is drawn in through the sides or bottom and discharged out the top.

UV - Ultra Violet Light. Tested and proven to be excellent source of air purification.


Vacuum - Any pressure below atmospheric pressure.

Vacuum Pump - A vapor pump capable of creating the degree of vacuum necessary to evaporate moisture near room temperature. A pump used to remove air and moisture from a refrigeration system at a pressure below atmospheric pressure.

Voltage - The force pushing electrical current along wires and cables.


><b>Watt<b> - A unit of power that equals one joule per second. Named after James Watt.

Weather Stripping - Specially designed strips, seals and gaskets installed around doors and windows to limit air leakage.