Glossary

Often called a “doorbell” transformer, this device converts the line voltage of 120V found in most AC circuits into an extra-low-voltage value of 24V which powers the control circuit.

A electromechanical device that controls damper blade position. It has a spring-loaded coil to return dampers to a fail-safe position in case of loss of power.

The process of cooling a room below the ambient temperature.

The component of an HVAC system which drives air through the building.

The unit used to measure electrical current. It is equal to a flow of one coulomb per second. It may also be called "amp."

A resistor that is physically mounted inside a room thermostat, and any heat generated by it will directly affect the temperature-sensitive bi metallic strip inside the thermostat.

The surface of an object or medium. Measured in square units (m2).

Axial fans drive low volumes of air under low static pressure conditions in parallel with their axis of rotation. Mainly used for cooling of coils.

An electric heating load installed at baseboard height and locally controlled.

A motor that drives air in force-air systems.

A phase change where the entire material converts from a liquid to a gas.

The main unit in a central electric heating system. Consists of air handler unit, resistive heating coils and control devices.

A heating system that uses one central electric furnace.

Main unit that converts natural gas into heat via heat exchanger and usually includes a blower motor.

A heating system that uses one central gas furnace.

A centrifugal fan drives lager volumes of air under high static pressure conditions at a right angle to its axis of rotation. Mainly used for the moving air through ductwork.

An automatic device that is designed to safely disconnect circuits under fault conditions. Most circuit breakers provide Overload and Overcurrent protection, and are rated in Volts, Amps and Horsepower.

A motor-driven component that pushes a fluid medium through a system.

A phase change where the entire material converts from a gas to a liquid.

A component which allows a refrigerant to rapidly condense and dissipate thermal energy into the environment. Will incorporate coils to maximize surface area to volume ratio.

The maximum rated value of RMS current that the overcurrent device is designed to handle on a continuous basis without tripping.

In contrast to the Power Circuit, the Control Circuit consists of inputs, in the form of switches, pushbuttons or pilot devices, which when activated, can either directly, or through a magnetic motor starter, energize a load. The Control Circuit often operates at a lower voltage than the Power Circuit for safety and ease of installation.

The rate of flow of an electric charge, measured in amperes (or amps). When one coulomb of charge moves past one point in once second, current is said to flow at a rate of one ampere. Current flows from negative potential to a positive potential through a load.

A mechanical cooling system that uses the evaporation and condensation of a refrigerant to remove heat from an area.

Main unit that converts electrical energy into heat via resistive coils and usually includes a blower motor.

A thermostat with a programmable feature.

Heat energy absorbed from the environment, and added to the heating/cooling system

A phase change where the entire material converts from a liquid to a gas.

A component which allows a refrigerant to rapidly boil and absorb thermal energy from a room or area. Will incorporate coils to maximize surface area to volume ratio.

Heat energy released to the environment from the heating/cooling system.

A valve which allows a refrigerant to expand into a larger volume.

Any voltage not exceeding 30V. Equipment energized at 24 V is commonly referred to as the "low voltage circuit".

An electric motor load that drives air through an area or a ducting system.

Electric heating loads that are permanently installed and hard wired into a buildings electrical system.

A pushing or pulling on an object, that when unopposed will result in a change in that objects velocity.

A heating or cooling system that uses fans and duct work to distribute heated or cooled air throughout a building.

A phase change where the entire material converts from a liquid to a solid.

An insulated tube containing a strip of conductive metal that has a lower melting point than either copper or aluminum. It protects a circuit from damage because it will melt in overload or overcurrent situations and break the connection with the rest of the circuit.

A material in its highest energy state which will expand to fill the full volume or shape of the container, such as steam.

A set of physical laws that describe how gasses and liquids behave as their properties of volume, density and temperature change.

The main unit in a central natural gas heating system. Consists of air handler unit, resistive heating coils and control devices.

The amount of energy added to or removed from a system required to change the temperature or phase of medium.

A component that separates the toxic products of combustion from the heated air in a gas fired furnace.

A mechanical cooling system in which a reversal of the direction of the flow of refrigerant changes the function of the system from either cooling a building in the summer to heating that building in the winter.

Cables installed around liquid piping systems to help reduce environmental heat loss and prevent freezing of the pipes.

A method of preventing freezing in pipes by running resistive wires along their length that produce a small amount of heat.

Heating load installed in floors or under pavement.

How much electric power is consumed by the motor. 1 horsepower is the equivalent of 746 watts.

A cooling system that uses water as the medium to capture and remove heat from the desired area.

A heating or cooling system that uses pumps and pipe work to distribute heated or cooled water throughout a building.

Heating loads installed below a finished floor or driveway.

The initial high value of current produced when an inductive load is first energized.

The maximum fault current that an overcurrent device can interrupt without damage to itself. Most circuit breakers and fuses have an IC rating of 10,000 amps.

The thermal energy required to cause a medium to undergo a phase change.

A common way to refer to a 240V thermostat.

A material in its moderate energy state which will maintain a fixed volume but will move to fill any shape it is contained in, such as a glass of water.

The current drawn by a motor when the motor is not spinning.

Any voltage between 31 and 750 V, though equipment energized at either 120V or 240V are often referred to as "line voltage" loads.

A control circuit which operates below 30V.

Circuits with low-voltage protection will not automatically turn back on when voltage is restored following a power outage. Examples include the microwave or power tools.

Circuits with low-voltage release are designed to re-energize automatically when voltage is restored after a power outage. Examples include lights or the kitchen fridge.

A temperature-activated switch that operates below 30V.

Any method that uses energy to actively remove heat from one area and dissipate it elsewhere.

A material that can carry thermal energy from one place to another, such as air, water, or refrigerants.

A phase change where the entire material converts from a solid to a liquid.

Fossil fuel, comprised mostly of methane and used as the energy source in gas-fired heating systems.

The unit used to measure electrical resistance (Ω). It takes one volt to push one amp through one ohm of resistance.

Current = Voltage divided by Resistance (or I=E/R).

Blades that alternate their direction of rotation.

A sharp and fast rise in current over a short period of time (fractions of a second) where the value of current is far greater than the nominal line current.

Fuses and or circuit breakers that are designed to protect wires and equipment in the event of a fault.

A moderate and gradual rise in the value of current over a relatively long period of time that is caused by excessive amounts of current drawn by a motor due to too much load being put on the motor.

A moderate and gradual rise in the value of current over a relatively long period of time that is caused by excessive amounts of current drawn by a motor due to too much load being put on the motor.

Blades that all rotate in the same direction.

A small flame that is maintained to provide ignition for the main gas feed on many heating appliances.

Electric heating loads that can be easily moved from one location to another, and plug into available AC outlets, commonly 120V 15A receptacles.

The rate at which work is done. It is measured in watts (W), or joules per second (J/s).

In contrast to the control circuit, the power circuit provides the large values of voltage and current used by the motor itself. Must be equipped with overcurrent and overload protection, and horsepower-rated contacts in the control gear equal to the voltage and current ratings of the motor.

The force acted upon an object or medium by the surrounding environment. Often measured in PSI or Pascal’s.

An electric motor load that drives water or other fluids through a piping system.

Meaning the component is designed to be safely operated within certain tolerances. EG: A 240V Rated switch could safely switch a 120V load, but not a 480V load.

A material used in DX cooling systems that boils below room temperature, and is used to transfer thermal energy from one point to another.

The artificial cooling of a space or area by removing heat energy.

An electromechanical switch that can be opened or closed remotely by small sources of voltage.

The opposition to the flow of current in an electric circuit, measured in ohms (Ω).

The main control device.

Thermal energy always flows spontaneously in the form of heat from regions of higher temperature to regions of lower temperature, increasing the entropy of the system.

The thermal energy required to raise or lower the temperature of a medium.

When multiple voltage sources are connected so that their potential differences add together, providing a higher voltage than any one individual device

The simplest electrical circuit. It requires only two lines: one for power to go in and the other is a return path for current. These are often called Line 1 and Line 2, or Line 1 and Neutral. Current only has one path to travel in a single-phase circuit, such as a control circuit.

A material at its lowest energy state which will maintain a fixed volume and shape, such as a block of ice.

The speed or RPM of a fan determines how fast it rotates.

The force resisting the movement of air.

A device for making or breaking the connection in an electric circuit.

A measure of the average molecular kinetic energy of a system on a definite scale. Measured in degrees Fahrenheit, Celsius or Kelvin.

The kinetic energy possessed by an object or mediums atoms and perceived as heat.

A device made of two dissimilar metals, which produce a DC voltage of approximately 25 – 30 millivolts  when one end is subjected to heat.

The branch of science that studies the relations between heat and other forms of energy.

Several thermocouples connected together to provide a higher value of DC voltage. Typically either 250 or 750 DC millivolts.

A temperature activated switch used to control heating or cooling equipment.

An electrical circuit that uses three current carrying conductors, called Line 1, Line 2, and Line 3, which have a 120° phase shift in the voltage and current waveforms between them. The Power Circuit of three-phase motors is an example of a three-phase circuit.

In motor control terminology, a three-wire circuit utilizes a magnetic motor starter with a holding contact, along with momentary contact pushbuttons. A three-wire circuit provides low-voltage-protection.

The sum of static and velocity pressure that must be overcome by an air handling unit.

In motor control terminology, a two-wire circuit utilizes a manual motor starter with a maintained contact. A two-wire circuit provides low-voltage release.

Electric heating load with built-in fan, usually installed at ceiling height.

The force exerted by a moving air stream, in the direction of the moving air.

The difference in electric potential between two points, which is defined as the work needed per unit of charge to move a test charge between the two points. It is measured in volts (V).

The maximum amount of voltage that a fuse, circuit breaker, switch-gear or motor starter can handle. The voltage rating of a fuse or circuit breaker must be equal to or greater than the system voltage.

The amount of space that an object or medium takes up. Measured in cubic units (cm3) or liters.

The unit used to measure power in an electric circuit, equivalent to one joule per second, or the power dissipated when one volt pushes one amp through a circuit.

Power = Voltage times Current (or W=EI)