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Glossary of Terms


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AC or DCAbbreviation for equipment capable of operating on alternating or direct current.
ACAbbreviation for alternating current, a type of electric current in which the polarity is constantly reversing causing the electron flow to reverse.
ACCAAir Conditioning Contractors of America.
ACOILA heat exchanger consisting of two diagonal coils that are joined together in a manner that looks like the letter “A”.
ACOUSTICALOf or pertaining to sound.
AFUEAnnual 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.
AGAAbbreviation for American Gas Association, Inc.
AIR CLEANERAny device that removes undesirable particles from moving air.
AIR CONDITIONERAny device that can change the temperature, humidity or general quality of the air.
AIR FLOW VOLUMEThe 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 HANDLERAn 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.

ARIAir Conditioning & Refrigeration Institute.
ASHRAEAmerican Society of Heating, Refrigeration and Air Conditioning Engineers.
BTUBritish 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.
BTUhBritish thermal units per hour. 12,000 BTUh equals one ton of cooling.
BURNER (SEALED COMBUSTION)A burner that obtains all air for combustion from outside the heated space.
BURNER ORIFICEThe opening through which gas flows to the air/gas mixing chamber of the burner.
BURNERA device that uses fuel to support combustion.
CAPACITYThe output or producing ability of cooling or heating systems. Cooling and heating capacities are referred to in British thermal units (BTUs) per hour.
CELSIUSThe 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).
CFMAbbreviation for cubic feet per minute, a standard measurement of airflow. A typical system requires 400 cfm per ton of air conditioning.
CHARGEAdding refrigerant to a system. This is refrigerant contained in a sealed system or in the sensing bulb to a thermostatic expansion valve.
COMPRESSORThis 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.
CONDENSATEVapor 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 FANThe fan that circulates air over the air cooled condenser.
CONTACTORA switch that can repeatedly cycle, making and breaking an electrical circuit. When sufficient current flows through the coil built into the contactor, the resulting magnetic field causes the contacts to be pulled in or closed.
CRANKCASE HEATERThis 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.
CSACanadian Standards Association.
DAMPERFound 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.
DCDirect current electricity. This type of electricity (as opposed to Alternating Current, or AC) flows in one direction only, without reversing polarity.
DEFROSTTo melt frost; as in from an air conditioner or heat pump coil.
DEGREEDAYA computation that gauges the amount of heating or cooling needed for a building. A degreeday is equal to 65 degrees Fahrenheit minus the mean outdoor temperature.
DEHUMIDIFIERAn air cooler that removes moisture from the air.
DIFFUSERA grille over an air supply duct having vanes to distribute the discharging air in a specific pattern or direction.
DOEDepartment of Energy
DOWNFLOW FURNACEA furnace that intakes air at its top and discharges air at its bottom.
DRAIN PANAlso 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 TEMPERATUREHeat intensity, measured by a dry bulb thermometer.
DRY BULB THERMOMETERAn instrument that measures air temperature independently of humidity.
DUCTWORKA 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.
DXDirect expansion. A system in which heat is transferred by the direct expansion of refrigerant.
EEREnergy Efficiency Ratio (steady state)
EPAEnvironmental Protection Agency
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.
EXPANSION VALVEA refrigerant metering valve with a pressure or temperature controlled orifice.
FAHRENHEITThe temperature scale on which water freezes at 32 degrees and boils at 212 degrees; designated by the letter F. To convert Fahrenheit to Celsius, subtract 32 from the Fahrenheit number, multiply by 5 and divide by 9 (77 32 equals 45, times 5 equals 225, divided by 9 equals 25 degrees Celsius).
FANAny device that creates air currents.
FILTERAny device that removes impurities through a straining process.
FLUEAny vent or passageway that carries the products of combustion from a furnace.
FURNACEThat part of the heating system in which the combustion of fossil fuel and transfer of heat occurs.
FUSEA metal strip in an electrical circuit that melts and breaks the circuit when excessive current flows through it. The fuse is designed to break in order to save more expensive electrical components.
GAMAGas Appliance Manufacturers Association.
GAS FURNACE HEAT EXCHANGERLocated in the furnace, the heat exchanger transfers heat to the surrounding air, which is then pumped throughout your home.
HEAT EXCHANGERAn area, box or coil where heat flows from the warmer to the colder fluid or surface.
HEAT GAINHeat added to the conditioned space by infiltration, solar radiation, occupant respiration and lighting.
HEAT LOSSThe rate of heat transfer from a heated space to the outdoors.
HEAT PUMPA mechanical compression cycle refrigeration system that can be reversed to either heat or cool the controlled space.
HEAT TRANSFERThe movement of heat energy from one point to another. The means for such movement are conduction, convection, and radiation.
HEATING COILAny coil that serves as a heat source.
HERTZIn alternating current (AC electricity), the number of cycles per second.
HSPFHeating 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.
HUMIDIFIERA machine that adds water vapor to the air to increase humidity.
HUMIDISTATA humidity sensing control that cycles the humidifier on and off.
HUMIDITY, ABSOLUTEWeight of water vapor per cubic foot of dry air, expressed as grains of moisture per cubic foot.
HUMIDITY, RELATIVEThe 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.
HUMIDITYThe presence of water vapor in the air.
HVACHeating, Ventilating and Air Conditioning.
IGNITIONThe lighting of a fuel to make it burn.
KILOWATT (KW)1,000 watts.
LATENT HEATA 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.
MEDIAThe material in a filter that traps and holds the impurities.
NECNational Energy Council / National Electric Code
NEMANational Electrical Manufacturing Association
OEMOriginal equipment manufacturer
ORIFICEAn opening or hole; an inlet or outlet.
PACKAGE UNITA 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.
PSIPounds per square inch.
PSIAPounds per square inch, absolute.
PSIGPounds per square inch gauge.
PVCPolyvinyl chloride; a type of plastic.
RECIPROCATING COMPRESSORA compressor whose piston or pistons move back and forth in the cylinders.
REFRIGERANT CHARGEThe required amount of refrigerant in a system.
REFRIGERANTA 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.
SEERSeasonal 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 13.
SELF CONTAINED SYSTEMA refrigerating system that can be moved without disconnecting any refrigerant lines; also know as a package unit.
SENSIBLE HEATThat heat which, when added to or taken away from a substance, causes a rise or fall in temperature.
SENSORAny device that reacts to a change in the conditions being measured, permitting the condition to be controlled.
SETPOINTThe 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 SYSTEMThe 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.
THERMOSTATA series of sensors and relays that monitor and control the functions of a heating and cooling system.
THERMOSTATIC EXPANSION VALVEA refrigerant metering device that maintains a constant evaporator temperature by monitoring suction vapor superheat. Also called a thermal expansion valve.
TONA unit of measurement used for determining cooling capacity. One ton is the equivalent of 12,000 BTUs per hour.
TWOSTAGE HEATING / TWOSTAGE COOLINGTwostage heating and cooling is considered to be more efficient, because it operates at low speed most of the time. However, on days when more heating or air conditioning is required, it switches to the next stage for maximum comfort.
UFACTORThe factor representing resistance to heat flow of various building materials.
ULUnderwriters Laboratories.
UPFLOW FURNACEA furnace in which air is drawn in through the sides or bottom and discharged out the top.
VACUUMA pressure below 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 water soaked cloth. The lowering of temperature that results from the evaporation of water around the bulb indicates the air’s relative humidity.
ZONING SYSTEM A method of dividing a home into different comfort zones so each zone can be independently controlled depending on use and need; an air conditioning system capable of maintaining varying conditions for various rooms or zones.