Unit 1
Alternator|A device that generates alternating current by means of conductors rotated in a magnetic field.
Armature|The rotating component of a generator.
Capacitor|An electrical storage device containing two metal plates separated b an insulating(dielectric) material.
Commutator|The movable contact surface on an electric generator or motor.
Dielectric|A material that strongly resists passage of current.
Effective voltage|See root-mean-square (rms) voltage.
Frequency|The number of complete cycles of an alternating current, sound wave, or vibrating object that occur in a period of time.
Fusible link|A circuit protective device that melts, opening the circuit, when the current is excessive.
Hertz(Hz)|The unit of measure for the frequency of alternating current. One Hertz equals one cycle per second.
Induction|To generate a current in a conductor by placing it in a moving magnetic field.
Induction motor|An AC motor.
Inertia|The tendency of a body in motion to remain in motion and a body at rest to remain at rest.
Isolation transformer|A transformer with a one-to-one turns ratio. It is used for personnel safety and to prevent electrical interference.
Megohmmeter(megger)|A test instrument used to test high-resistance circuits.
Microfarad|One-millionth of a farad. Used to rate capacitors.
Qualified person|One who has the skills and knowledge related to the construction and operation of electrical equipment and installations, and has received safety training to recognize and avoid the hazards involved.
Root-mean-square(rms) voltage|The value of AC voltage that will produce as much power when connected across a load as an equivalent amount of DC voltage. Also known as effective voltage.
Rotor|The rotating component of an induction motor.
Run capacitor|A capacitor that remains in the motor circuit while the motor is running to improve running efficiency.
Run winding|The stator winding of am otor that draws current during the entire running cycle of the motor.
Sinusoidal(sine) wave|The waveform created by an AC generator.
Start winding|The stator winding of a motor that is used to provide starting torque.
Stator|The stationary windings of a motor.
Synchronous speed|The maximum rated speed of a motor.
Torque|The force that must be generated to turn a motor.
Turns ratio|The ratio between the number of turns in the primary and secondary windings of a transformer.

Unit 2
Burnout|A condition in which the breakdown of the motor winding insulation causes the motor to shout out or ground electrically.
Capacity control|Methods used in cooling systems to adjust system operations to match changes in the system cooling load.
Clearance volume|The amount of clearance between a piston at the top dead-center position of travel and the cylinder head.
Compliant scroll compressor|A version of the scroll compressor that allows the orbiting scroll to temporarily shift from its normal operating position if liquid refrigerant enters the compressor.
Compression|The reduction in volume of a vapor or gas by mechanical means.
Floodback|A condition in which there is a continuous return of liquid refrigerant in the suction vapor being returned to the compressor during operation.
Flooded starts|A condition in which slugging, foaming, and inadequate lubrication occur at compressor startup as a result of the oil in the compressor having absorbed refrigerant during shut-down. When the compressor starts and the pressure in the crankcase drops very quickly, the refrigerant boils out rapidly, creating a foam.
Line-duty device|A motor protection device that senses current flow and temperature in the motor windings. If an overload occurs, it opens the motor winding circuit to remove the line voltage.
Pilot-duty device|A motor protection device that senses current overload or temperature within the motor. If an overload occurs, it opens the motor contactor control circuit to remove power from the motor.
Positive-displacement compressor|Any compressor where the pumping action is created by pistons or moving chambers.
Pressure (force-feed) lubrication system|A method of compressor lubrication that uses an oil pump mounted on the end of the crankshaft to draw oil from the crankcase and force it to the compressor main bearings, lower connecting rod bearings, and piston pins.
Pump-down|The process of operating a compressor to clear most or all refrigerant from the low pressure side of the system, or a portion thereof. A pump-down cycle is used on some systems to clear remaining refrigerant from the evaporator and suction line that could condense to a liquid following each cycle. For service and repairs on the low side of the system, a valve on the high-pressure side can be closed and the compressor operated manually to pump the refrigerant out of the low-pressure side and into the high-pressure side.
Short-cycling|A condition in which the compressor is restarted immediately after it has been turned off;sometimes accompanied by very short run periods as well.
Single phasing|A condition in which a three-phase motor continues to run after losing one of the three phases while operating.
Slugging|The entrance of liquid refrigerant and/or a significant amount of oil into a compressor cylinder. Slugging may occur at startup or if there is a rapid change in system operating conditions.
Splash lubrication system|A method of compressor lubrication in which the crankcase oil is splashed up from the crankcase and onto the cylinder walls and bearing surfaces during each revolution of the crankshaft.

Unit 3
Alkylbenzene|A type of synthetic hydrocarbon oil that is compatible with mineral oil but has better oil return properties. It is widely used in low-temperature applications. it is not compatible with synthetic oils.
Azeotrope|A blended refrigerant that behaves like a pure refrigerant. The refrigerant evaporates and condenses at one give pressure and temperature. From the technician's standpoint, working with an azeotropic blend is like working with a pure refrigerant compound.
Binary blend|A blended refrigerant consisting of two refrigerants.
Bubble point|The point at which refrigerant starts evaporating when heat is added to a zeotropic refrigerant. It is also the point that refrigerant finishes condensing when heat is removed from the refrigerant. See dew point.
Chlorofluorocarbon(CFC)|The most dangerous refrigerants to the environment since they contain more chlorine atoms in their structure. Production of CFCs in the United States ended in 1995.
Compound|A substance made up of different elements. A refrigerant that is not a blend is a compound.
Dew point|The point at which refrigerant stops evaporating when heat is added to a zero-tropic refrigerant. It is also the point that refrigerant starts condensing when heat is removed from the refrigerant. See bubble point.
Dielectric strength|The ability of refrigerant oil or any material to resist breaking down in the presence of voltage.
Flash point|The temperature at which heated oil vapors burst into flame. It is a measure of the flammability of the lubricant. Ideally, compressor lubricant oils should have a high flash point for safety reasons. The temperature at which oil gives off ignitable vapors.
Floc point|The temperature at which a 90/10 mixture of oil/refrigerant forms a cloudy or flocculent precipitate of wax in the mixture.
Fractionation|A process in which the component refrigerants of a blended refrigerant boil off into a vapor state at different temperatures.
Glide|The temperature range in which a zeo-tropic refrigerant blend evaporates and condenses.
Global Warming Potential(GWP)|A measure of a substance's ability to contribute to global warming that is expressed as a number. The higher the number, the greater the warming potential.
Greenhouse effect|An effect in which atmospheric gases such as carbon monoxide trap solar heat in the atmosphere in the same way that a greenhouse captures and holds solar heat.
Halogen|A class of elements that include cholorine and fluorine that are used in the manufacture of refrigerants. A fully halogenated refrigerant is one in which all hydrogen atoms of a hydrocarbon molecule are replaced by halogen atoms.
Hydrocarbon|A compound composed only of carbon and hydrogen atoms. When some or all of the hydrogen atoms in a hydrocarbon are combined with a halogen such as chlorine or fluorine, the end result is a compound known as a halogenated hydrocarbon or halocarbon. Most common refrigerants are halocarbons.
Hydrogenated chlorofluorocarbon(HCFC)|A refrigerant that contain chlorines atoms in its structure, but fewer than found in CFCs. They are kinder to the environment than CFCs, allowing them to be phased out over a longer period of time. By 2030, all HCFC refrigerants will no longer be manufactured.
Hydrogenated fluorocarbon(HFC)|A refrigerant that contains no chlorine atoms in its structure, which makes this group of refrigerants the most environmentally friendly of the three classifications of halocarbon refrigerants.
Miscible|Having the necessary properties to mix with or dissolve into another material. A number of refrigerants, for example, are quite miscible in refrigeration oil.
Near-azeotrope|A zeotrope with azeo-tropic properties. The refrigerant HFC-410A is an example of a near-azeotrope. Like zeo-tropes, near-azeotropes exhibit glide characteristics but they are usually minimal to the point that they are not a factor for technicians to consider when servicing a unit containing such a refrigerant.
Ozone depletion potential(ODP)|A meausre of a substance's ability to deplete atmospheric ozone that is expressed as a number. CRC refrigerants have greater ozone depletion potentials than HFC refrigerants.
Polyalkylene glycol(PAG)|Synthetic refrigerant oil used with HFC redrigerants. It is very hygroscopic.
Polyolester(POE)|Synthetic refirgerant oil used with HFC refrigerants. It is very hygroscopic.
Polyvinyl either(PVE)|A synthetic refrigerant oil with properties similar to mineral oils which can be used with HFC refrigerants. It has an advantage over POE and PAG oil in that it is not hygroscopic.
Pour point|A oil quality related to viscosity. It can be defined as the temperature at which oil first starts to flow. See viscosity.
Ternary blend|A blended refrigerant consisting of three refrigerants.
Viscosity|The thickness of a liquid or its ability to flow at a specific temperature.
Zeotrope| A blended refireant that never mixes chemically. As a result, it evaporates and condenses over a temperature range, called the glide.

Unit 4
Active recovery|A recovery process using a self-contained refrigerant recovery unit to draw or push the refrigerant out of the circuit.
Passive recovery|A recover process that makes use of the system's pressure and/or the system compressor to push out the refrigerant out. The refrigerant must be captured in a non-pressurized container.
Phosgene gas|A poisonous, colorless gas that can be generated by the burning of refrigerant gases. The gas has an odor much like that of freshly cut hay. As the gas enters the lungs, it reacts with water to produce both hydrochloric acid and carbon monoxide. Phosgene first gained notoriety as a lethal choking gas used in chemical warfare during World War I.

Unit 5
Direct-expansion(DX)system|A system in which the required cooling effect is achieved by the expansion of a refrigerant from a liquid to a vapor state.
Externally equalized|Describes an expansion valve that has a connection to the suction line downstream of the evaporator coil to serve as a closing force. External equalizers are used when there is a significant pressure drop through the evaporator.
Flash gas|The refrigerant vapor that forms as the results of a rapid change from its liquid state due to a reduced pressure condition.
Hunting|Describes the operation of an expansion valve opening and closing excessively and repeatedly, unable to stabilize at a consistent flow rate.
Internally equalized|Describes an expansion valve that uses pressure value at its own outlet to the evaporator coil to serve as a closing force.
Piston|A component of a fixed-orifice metering device that contains the orifice. The piston is free to move back and forth inside its housing. At one end of its travel, it meters refrigerant flow through the orifice; at the other end, it allows refrigerant to flow both through the orifice and around its outside perimeter.
Stepper motor|A type of motor used to open and close an electronically controlled expansion valve. The motor rotates in very small increments called steps, based on signals received from an external controller.
Thermostatic element|An assembly filled with refirgerant in saturation with an attached sensing bulb that converts the heat from the suction into a pressure value. This pressure acts as the opening force in a thermostatic expansion valve. It is also referred to as a power element.

Unit 6
Balance point|The outdoor temperature at which the heating capacity of the heat pump is equal o the heat loss of the building. The balance point varies depending on the climate, building design, type of construction, and other factors that affect heat loss and gain.
Coefficient of Performance(COP)|The outdoor temperature at which the heating capacity of the heat pump is equal to the heat loss of the building. The balance point varies depending on the climate, building design, type of construction, and other factors that affect heat loss and gain.
Dual-fuel or hybrid system|A system in which a heat pump is combined with a furnace.
Ground-source or geothermal system|A system in which the outdoor coil is buried in the ground and the heat exchange occurs between the earth and the refrigerant flowing through the coil.
Heating Season Performance Factor(HSPF)|A heat pump performance rating that has been adjusted for seasonal operation. It is the total heating output of a heat pump (in Btus) during its normal annual usage period for heating divided by the total electric power input in watt-hours during the same period.
Reversing valve|A valve that changes the direction of refrigerant flow in a heat pump. Also referred to as a four-way valve.
Seasonal Energy Efficiency Ratio(SEER)|The standard developed by the ARI for measuring cooling efficiency. It is the total cooling production of a heat pump or cooling unit (in Btus) during its normal annual cooling period divided by the total electric input in watt-hours during the same period.

Unit 7
Axial load|An external load that acts lengthwise along a shaft.
Belt whip|A condition where excess slack in a drive belt causes it to begin riding up and out of the sheave grooves, primarily on the slack side. The slack side is the side of the belt that exits the drive pulley as it rotates.
Borescope|An optical device that typically uses fiber optics to display an image through a narrow tube, allowing for the inspection of otherwise difficult or impossible areas to see. For example, a borescope is used to examine the inside of a furnace heat exchanger.
Centistoke (cSt)|A metric unit of measurement related to a lubricants resistance to flow, identifying the relationship between simple viscosity and the density of the lubricant.
Diserpsants|In lubricants, an additive used to disperse or disband groups of particles to prevent them from coagulating or joining together.
Dropping point|The temperature at which grease is soft enough for a drop of oil to fall away or flow from the bulk of the grease.
Dynamic seal|A seal made where there is movement between two mating parts, or movement between one of the parts and the seal.
Fire point|The temperature at which oil will burn if ignited.
Flash point|The temperature at which oil gives of ignitable vapors.
Hantavirus|A virus carried primarily by rodents and causing various diseases which often lead to kidney failure or significant damage.
Inhibitors|In lubricants, an additive that slows down or presents an undesirable chemical reaction or similar process.
Ionizing wires|Thin wires in an electronic air cleaner that cause passing particles to become positively charged. The charged particles are then attracted to the negatively charged collector plates, where they then land and remain.
Journal|The part of a shaft, axle, spindle, etc. which is supported by and revolves in a bearing.
Lapped|Honed and/or polished until extremely smooth.
Oxidation|The process of combining with oxygen. All petroleum products react with oxygen to some degree, and the process accelerates as temperature increases.
Pour point|An oil quality related to viscosity. It can be defined as the temperature at which oil first starts to flow. 
Radial load|The side or radial force applied at right angles to a bearing and shaft.
Safety drop time|The time required for a gas pilot valve to recognize the absense of a pilot flame and electrically lock out the main gas valve.
Sheave|A wheel with a groove for a belt to run in; often considered equivalanet to the term pulley. However, the term is also used by some to identify one groove of a pulley with multiple grooves.
Static seal|A seal made where there is no movement between the two adjoining parts or between the seal and the mating part.
Stuffing box|The housing used to control leaking along a shaft or rod. Typically composed of three parts: the packing chamber (also called the box); the packing rings; and the gland follower (also called the stuffing gland).
Thrust|The force acting lengthwise along the acis of a shaft, either toward it or away from it.
Viscosity|The thickness of a liquid or its ability to flow at a specific temperature.
Viscosity index (VI)|A measure of how an oil's viscosity varies with temperature.

Unit 8
Aldehydes|substances that are highly reactive organic compounds that are characterized by a carbonyl group (CHO) attached to a hydrogen atom. 
Complete|_________ combustion is burning in which there is enough oxygen to prevent the formation of carbon monoxide.
Dilution air|added to the flue gases in a natural-draft furnace to aid flue gas removal
Heat anticipator|a resistive heating element in a thermostat that shuts off the furnace before the space temperature reaches the set-point.
Incomplete combustion|burning in which there is not enough oxygen to prevent the formation of carbon monoxide. 
Vent|the vertical section of the vent pipe
Vent connector|the horizontal section of the vent system that connects the appliance(s) to the vent pipe or chimney.

Unit 9
Alloy|a metal mixed with other metals or elements to create strength and durability.
Angle bracket|an L-shaped metal supporting member used to support vertical risers. 
Brake|a sheet metal-working machine, either manually operated or powered, used to bend metal.
Channel|a U-shaped piece of structural steel used as a supporting device.
Cold-rolled steel|metal that has been formed by rolling at room temperature, usually to obtain an improved surface or higher tensile strength.
Lap-joined|a condition in which one piece is joined to another by partly covering one piece with the other before joining. 
Non-ferrous|a metal that contains little or no iron.
Shear load|the amount of weight or pressure that causes an exposed piece of metal to break or shear off.
Sheet steel|an uncoated sheet of steel with a bluish-black surface; commonly referred to as black iron in the trade.
Sound attenuation|the reduction in the level of sound that is transmitted from one point to another.
Tensile load|the weight required t ocause metal to stretch or compress.
Tensile strength|the resistance of a material against rupture when placed under tension.
Torsion|the twisting of a building’s structural member along its length by two equal and opposite forceful rotations at each end.
Vapor barrier|something like foil, plastic, or vinyl placed between insulation and the surrounding air to prevent moisture.

Unit 10
Closure system|any one of the three types of sealing products used with fiberglass duct
Dispersion|the act of breaking up and spreading a substance widely, such as the dispersion of air.
EI rating|a strength and rigidity rating assigned to rigid ductboard that is based on a calculation called Young’s modulus of elasticity.
Foil-scrim-kraft(FSK)|a vapor barrier made from layers of aluminum foil, fiberglass yarn or scrim, and kraft paper. 
Hanger extension|refers to the total distance between the wall of the duct and the hanger wires or straps on each side.
Plenum|in an air supply system, and enclosed volume of air that is at a higher pressure than the atmosphere and is connected to several supply ducts. 
R-value|a measure of the ability of a material such as insulation to slow heat transfer, expressed as a numerical rating.
Shiplap|a groove cut into ductboard that allows interior edges to overlap when the duct-board is folded or fit to another piece.
Staple flap|a piece of ductboard facing from which the insulation has been removed that is pulled over a joint or seam and then stapled into place.
Tap-in|square or round fittings made of fiberglass ductboard, flexible duct, or sheet metal that tap into the main trunk duct to connect the main duct to a branch line.
Vertical riser|in an air distribution system, a duct that extends vertically one full story or more to deliver air to branch ducts on different floors of a building.
Young's modulus of elasticity (E)|a mathematical ratio that describes tensile elasticity, or the tendency of an object to deform along an axis when opposing forces are applied along that axis. 

Unit 11
Constant-volume system|a constant-volume system maintains a constant airflow while varying the air temperature in response to the space load.
Control zone|in HVAC, a building, group of rooms, single room, or part of a room controlled by its own thermostat.
Sick building syndrome|a contribution of symptoms attributed to flaws in the building construction or HVAC systems. 
VAV|a system that controls the temperature within a control zone by varying the quantity of supply air rather than by varying the supply air temperature. 
VVT|a system that delivers a variable volume of air to each controlled zone, as the load dictates. 
YAC|provides both cooling and heating. Its heating capability is provided by a natural or LP gas heating section.
Zoned system|a system that has more than one thermostat used to control the areas it conditions.

Unit 12
Arrestance efficiency|a value representing the ability of a filter to capture ASHRRAE synthetic dust from test air.
Atmospheric dust spot efficiency|a measure of a filter’s ability to remove atmospheric dust from air.
Energy recovery ventilator (ERV)|HVAC equipment to supply fresh air and recover both heating and cooling energy from the exhausted airstream year-round.
Heat recovery ventilator (HRV)|HVAC equipment that saves energy by using a heat exchanger to transfer heat from the exhausted airstream to the cold ventilation air that is entering the building.
Micron|a precise measurement of pressure used with electronic vacuum measuring instruments and vacuum pumps. 
Off-gas|a process that occurs in various materials where volatile chemicals from non-metallic slowly evaporate to the atmosphere at normal atmospheric pressures and temperatures. 

Unit 13
Aquastat|a control that works basically the same way as a thermostat with the exception that it is designed to control water temperature instead of air temperature.
Cavitation|the result of air formed due to a drop in pressure in a pumping system.
Corrosion|the breaking down or destruction of a material, especially a metal, through chemical reactions.
Gravity hot-water system|a hot-water heating system in which the circulation of hot water through the system results from thermal conduction. 
Head pressure|a measure of pressure drop, expressed in feet of water or psig. 
High/low pump head|trade terms used to indicate the relative magnitude of the height  to a column of water that a circulating pump is moving, or must move, in a water system. 
Hydronic system|a system that uses water or water-based solutions as the medium to transport heat or cold from the point of generation to the point of use. 
MBH|equals 1,000 Btus per hour.
Pressure drop|in a water system, the result of power being consumed as the water moves through pipes, heating units, and fittings. 
Redundancy|a valve that changes the direction of refrigerant flow in a heat pump. 
Specific heat|the amount of heat required to raise the temperature of one pound of substance one degree Fahrenheit. 
Static pressure|in a water pressure, created by the weight of the water in a system. 
Thermal circulation|the circulation of water in a hydronic system resulting indirectly from differences in temperature. As the weight of water changes with temperature, pressure differences between water masses are created, resulting in water movement.