AC Abbreviation for alternating current, a type of electric current in which the polarity is constantly reversing causing the electron flow to reverse.

ACCA Air Conditioning Contractors of America.

AC or DC Abbreviation for equipment capable of operating on alternating or direct current.

AFUE (pronounced ‘A’-‘Few’) Annual Fuel Utilization Efficiency. A measure of a gas furnace’s efficiency in converting fuel to energy. The higher the rating, the more efficient the unit.

AGA Abbreviation for American Gas Association, Inc.

Air Conditioner is a home appliance, system, or mechanism designed to dehumidify and extract heat from an area. The cooling is done using a simple refrigeration cycle. In construction, a complete system of heating, ventilation, and air conditioning is referred to as “HVAC.” Its purpose, in a building or an automobile, is to provide comfort during either hot or cold weather.

Air Filter is a device composed of fibrous materials which removes solid particulates such as dust, pollen, mold, and bacteria from the air.

Air flow Volume The amount of air the system circulates through your home, expressed in cubic feet per minute (cfm). Proper airflow depends on the outdoor unit, the indoor unit, the ductwork and even whether the filters are clean.

Air handler An air moving and/or mixing unit. Residential air handlers include a blower, a coil, an expansion device, a heater rack and a filter. Heaters for air handlers are sold as accessories. In some models heaters are factory installed.

ARI Air Conditioning & Refrigeration Institute.

ASHRAE American Society of Heating, Refrigeration and Air Conditioning Engineers.


BTU British thermal unit. The amount of heat required to raise or lower the temperature of one pound of water one degree Fahrenheit. The heat extracted from your home by an air conditioner is measured in BTUs.

BTUh British thermal units per hour. 12,000 BTUh equals one ton of cooling.

Burner A device that uses fuel to support combustion.

Burner Orifice The opening through which gas flows to the air/gas mixing chamber of the burner.

Burner (sealed combustion) A burner that obtains all air for combustion from outside the heated space.


Coil A heat exchanger consisting of two diagonal coils that are joined together.

Capacity The output or producing ability of cooling or heating systems. Cooling and heating capacities are referred to in British thermal units (BTUs) per hour.

Celsius The metric temperature scale in which water freezes at zero degrees and boils at 100 degrees, designated by the symbol “C”. To convert to Fahrenheit, multiply a Celsius temperature by 9, divide by 5 and add 32 (25 x 9 equals 225, divided by 5 equals 45, plus 32 equals 77 degrees Fahrenheit).

CFM Abbreviation for cubic feet per minute, a standard measurement of airflow. A typical system requires 400 cfm per ton of air conditioning.

Charge Adding refrigerant to a system. This is refrigerant contained in a sealed system or in the sensing bulb to a thermostatic expansion valve.

Compressor This is the heart of an air conditioning or heat pump system. It is part of the outdoor unit and pumps refrigerant to meet the cooling requirements of the system.

Condensate Vapor that liquefies due to the lowering of its temperature to the saturation point.
Condenser coil (or outdoor coil) In an air conditioner, the coil dissipates heat from the refrigerant, changing the refrigerant from vapor to liquid. In a heat pump system, the coil absorbs heat from the outdoors.

Condenser Fan The fan that circulates air over the aircooled condenser.

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

Crankcase Heater This is the electric resistance heater installed on compressor crankcases to boil off liquid refrigerant that may have combined with compressor oil. Many newer cooling systems do not require crankcase heaters, however heat pumps do require crankcase heaters.

CSA Canadian Standards Association.


DC Direct current electricity. This type of electricity (as opposed to Alternating Current, or AC) flows in one direction only, without reversing polarity.

Damper Found at the exit point of ductwork, this plate usually contains grates that can be opened or closed to control the flow of air into a zone.

Defrost To melt frost, as in from an air conditioner or heat pump coil.

Degree-Day Calculated by subtracting the average outdoor temperature for an area from 65ᄚ Fahrenheit. This measurement is used to estimate the amount of heating or cooling a home or building will need.

Dehumidifier A device that removes humidity, or moisture, from the air.

Desuperheater An energy saving device in a heat pump that, during the cooling cycle, recycles some of the waste heat from the house to heat domestic water.

Diffuser A grille over an air supply duct having vanes to distribute the discharging air in a specific pattern or direction.

DOE Department of Energy.

Downflow Furnace A furnace that intakes air at its top and discharges air at its bottom.

Drain Pan Also referred to as a condensate pan. This is a pan used to catch and collect condensate (in residential systems vapor is liquefied on the indoor coil, collected in the drain pan and removed through a drain line).

Dry Bulb Temperature Heat intensity, measured by a dry bulb thermometer.

Dry Bulb Thermometer An instrument that measures air temperature independently of humidity.

Ductwork A pipe or conduit through which air is delivered. Ducts are typically made of metal, fiberboard or a flexible material. In a home comfort system, the size and application of ductwork is critical to performance and is as important as the equipment.


EER Energy Efficiency Ratio (steady state).
ENERGY STAR® qualified Heat pumps, boilers, air conditioning systems, and furnaces are available. In addition, cooling and heating bills can be significantly lowered with air sealing and duct sealing. Air sealing reduces the outdoor air that penetrates a building, and duct sealing prevents attic or basement air from entering ducts and lessening the heating/cooling system’s efficiency. Energy Star qualified room air conditioners are at least 10% more energy efficient than the minimum U.S. federal government standards.

EPA Environmental Protection Agency.

Expansion Valve A refrigerantmetering valve with a pressure or temperature controlled orifice.

Evaporator Coil (or Indoor Coil) The other half of an air conditioning system, located inside your home in the indoor unit. This is a tubing coil in which a volatile liquid evaporates and absorbs heat. This is where the refrigerant evaporates as it absorbs heat from the indoor air that passes over the coil.


Fahrenheit A temperature scale in which water freezes at 32 degrees and boils at 212 degrees at normal atmospheric pressure.

Fan Any device that creates air currents.

Filter Any device that removes impurities through a straining process.

Flue A vent that removes the byproducts of combustion from a furnace.

Furnace The major component in heating a home. A device that facilitates the combustion of fuel and air to create heat.

Fuse A delicate metal strip connecting two parts of an electrical circuit. This strip breaks, or melts, in the event of excess electrical charge, breaking the electrical circuit.


GAMA Gas Appliance Manufacturers Association.

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


Heat Exchanger A device through which heat is transferred to a cold area or surface

Heat Gain Heat added to the conditioned space by infiltration, solar radiation, occupant respiration and lighting.

Heating Coil A coil that acts as a heat source for a heating system.

Heat Loss The rate of heat transfer from a heated space to the outdoors.

Heat Pump A device used for either the heating or cooling of a space by transferring heat between two reservoirs.

Heat Transfer The movement of heat energy from one point to another. The means for such movement are conduction, convection, and radiation.

Hertz In alternating current (AC electricity), the number of cycles per second.

HSPF Heating Seasonal Performance Factor. This rating is used in measuring the heating efficiency of a heat pump. The higher the number, the more efficient the heat pump system.

Humidifier A machine that adds water vapor to the air to increase humidity.

Humidistat A humidity sensing control that cycles the humidifier on and off.

Humidity Dampness in the air caused by water vapor.

Humidity – Absolute Weight of water vapor per cubic foot of dry air, expressed as grains of moisture per cubic foot.

Humidity – Relative The amount of moisture in the air expressed as a percentage of the maximum amount that the air is capable of holding at a specific temperature.

HVAC Heating, Ventilating and Air Conditioning.


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


Kilowatt (kW)1,000 watts.


Latent Heat A type of heat, which when added to or taken from a substance, does not change the temperature of the substance. Instead, the heat energy enables the substance to change its state.


Media The material in a filter that traps and holds the impurities.


Natatorium Industry term for indoor pools, whirlpools, or spas ranging in size from small residential installations to large commercial indoor waterparks.

NEC National Energy Council / National Electric Code.

NEMA National Electrical Manufacturing Association.


OEM Original equipment manufacturer.

Orifice An opening or hole; an inlet or outlet.


Package Unit A heating and cooling system contained in one outdoor unit. A package unit is typically installed beside, on the roof, or sometimes in the attic of a home.

PSI Pounds per square inch.

PSIA Pounds per square inch, absolute.

PSIG Pounds per square inch gauge.

PVC Polyvinyl chloride; a type of plastic.


Reciprocating Compressor A compressor whose piston or pistons move back and forth in the cylinders.

Refrigerant A chemical that produces a refrigerating effect while expanding and vaporizing. Most residential air conditioning systems contain R22 refrigerant. R22 is regulated under the Montreal Protocol and in the United States by the Environmental Protection Agency. R22 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 The required amount of refrigerant in a system.


SEER Seasonal Energy Efficiency Ratio. A measure of cooling efficiency for air conditioners and heat pumps. The higher the SEER, the more energy efficient the unit. The U.S. Government’s minimum SEER rating is 10.

Self Contained System A refrigerating system that can be moved without disconnecting any refrigerant lines; also know as a package unit.

Sensible Heat That heat which, when added to or taken away from a substance, causes a rise or fall in temperature.

Sensor Any device that reacts to a change in the conditions being measured, permitting the condition to be controlled.

Setpoint The temperature or pressure at which a controller is set with the expectation that this will be a nominal value depending on the range of the controller.

Split System The combination of an outdoor unit (air conditioner or heat pump) with an indoor unit (furnace or air handler). Split systems must be matched for optimum efficiency.


Thermostatic Expansion Valve A refrigerant metering device that maintains a constant evaporator temperature by monitoring suction vapor superheat. Also called a thermal expansion valve.

Thermostat A series of sensors and relays that monitor and control the functions of a heating and cooling system.

Ton A unit of measurement used for determining cooling capacity. One ton is the equivalent of 12,000 BTUs per hour.


U Factor The factor representing resistance to heat flow of various building materials.

UL Underwriters Laboratories.

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


Vacuum A space where the pressure is significantly below that of standard atmospheric pressure. A perfect vacuum is 30 inches Mercury (periodic symbol “Hg”).
Volt The unit of measure used to describe a difference in electrical potential. Abbreviated by the symbol “v”.

Voltage The force that pushes electrical current along wires and cables.


Watt The unit of electrical power equal to the flow of one amp at a potential difference of one volt.

Wet Bulb Thermometer A thermometer whose bulb is covered with a piece of watersoaked cloth. The lowering of temperature that results from the evaporation of water around the bulb indicates the air’s relative humidity.


Zoning A system that divides a home, office or space into different regions in order to better control the temperature and effectiveness of a heating and cooling system.
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