The trades, A to Z.

A

Actuator

A device that converts supplied energy into movement to position a valve, damper, or other controlled component.

Air separator

A hydronic component that collects air from circulating water and allows that air to leave the system.

B

Backdraft damper

A damper designed to allow airflow in one direction while resisting flow in the opposite direction.

Backflow preventer

A protective assembly intended to keep system water from flowing backward into the drinking-water supply.

Balancing damper

An adjustable airflow restriction used to distribute air among branches or outlets at the intended rates.

Balancing valve

A valve used to adjust water flow through a branch so different circuits receive their intended flows.

Barometric damper

A counterweighted damper that moves with pressure changes to help maintain steady draft in a venting system.

Bearing

A component that supports relative movement between machine parts while reducing friction at their moving connection.

Blowdown vessel

A vessel that receives hot boiler blowdown and accommodates steam released as its pressure drops.

Bottom blowdown

The controlled removal of water from a boiler’s low point to carry away settled solids.

Brazed plate heat exchanger

A compact stack of joined metal plates that transfers heat between separate fluid streams without mixing them.

Building automation system (BAS)

A network of controls, sensors and software that monitors and manages building equipment.

C

Capacitor

An electrical component that stores energy in an electric field between conductive surfaces separated by insulating material.

Centrifugal pump

A pump that uses a rotating impeller to add energy to liquid and move it through the system.

Check valve

An automatic valve that permits fluid to move in the intended direction and resists reverse flow.

Chiller

Equipment that removes heat from water or another liquid for cooling.

Compressor

A component that compresses refrigerant vapor, raising its pressure.

Condensate

Liquid water formed when steam or water vapor cools and condenses.

Condenser

A heat exchanger where refrigerant releases heat and changes from vapor to liquid.

Conduction

Heat transfer through a material or between touching materials without bulk movement of the material itself.

Contactor

An electrically operated switch for turning a power circuit on or off.

Control error

The difference between a measured process value and its setpoint, used to determine corrective control action.

Control relay

An electrically operated switch that lets one control signal switch another circuit.

Controller

A device that compares a measured condition with a desired value and determines a corrective output.

Convection

Heat transfer between a surface and a moving liquid or gas, involving both fluid motion and conduction.

D

Damper

A movable blade or blade assembly that regulates or shuts off airflow through a duct or opening.

Deadband

A range of input or error values within which a control produces no change in its output.

Deaerator

Equipment that uses heat and steam to remove dissolved gases from boiler feedwater.

Derivative action

Control action that responds to how quickly the measured condition or control error is changing.

Digital multimeter

An electronic measuring instrument that displays readings such as voltage, current, and resistance on a digital screen.

Diode

A two-terminal electronic component that passes current much more readily in one direction than the other.

E

Economizer

In a boiler system, a heat exchanger that uses exhaust heat to warm incoming feedwater.

Electrical coil

Wire wound into repeated turns, often around a core, used in components such as relays and transformers.

Evaporator

A heat exchanger where refrigerant absorbs heat and changes from liquid to vapor.

Expansion tank

A tank that accommodates expanding system water, using a compressible gas cushion to limit pressure changes.

F

Feed check valve

A one-way valve that helps prevent boiler water from flowing back into the feedwater line.

Filter drier

A refrigeration component that removes moisture and captures particles from refrigerant.

Fire damper

A protective damper that closes automatically during a fire to restrict flame passage through an air opening.

Firetube boiler

A boiler with hot combustion gases inside tubes and water around the tubes.

Flame safeguard

A burner safety control that monitors flame and safety conditions and stops firing when a monitored fault occurs.

Flash steam

Steam formed when hot liquid water enters a lower pressure where some of that water boils.

Flue damper

A device that opens an appliance's flue during operation and closes it during standby when designed to do so.

G

Gas train

An assembly of valves, regulators and safety devices that supplies fuel gas to a burner.

Gauge glass

A transparent indicator that shows the water level inside a boiler.

Gland packing

Compressible sealing rings arranged around a shaft and held under pressure to limit fluid leakage.

Globe valve

A valve that controls flow by moving a disc toward or away from a seat inside its body.

H

Hunting

Repeated oscillation of a controlled value around its target rather than settling at a stable condition.

Hydronic feedwater valve

A pressure-reducing valve that admits makeup water to a hydronic system when downstream pressure falls below its setting.

I

Impeller

A rotating wheel with blades or vanes that transfers mechanical energy to the fluid moving through a pump.

Induction motor

An AC motor whose rotating magnetic field induces rotor current, producing the torque that turns its shaft.

Integral action

Control action that responds to accumulated error over time, helping remove a persistent difference from setpoint.

L

Latent heat

Energy absorbed or released during a phase change; a pure substance can change phase at constant pressure without changing temperature.

Low-water cutoff (LWCO)

A safety control that stops boiler heat input when water falls to an unsafe level.

M

Mechanical seal

A shaft seal with closely fitted stationary and rotating faces that limit leakage around the shaft.

Mixing valve

A valve that blends hotter and cooler fluid streams to produce an outlet temperature suited to the system.

Morrison tube

A large corrugated furnace tube in a firetube boiler, with flame and hot gases inside and boiler water around it.

Motor slip

The difference between an induction motor's rotor speed and rotating magnetic-field speed, often expressed as a percentage.

Motor starter

A device or assembly that starts and stops a motor and provides overload protection.

O

On-off control

A control method that switches an output between two states, such as fully on and fully off.

Overload relay

A protective device that stops a motor through its control circuit when a sustained overload occurs.

Overshoot

A temporary excursion beyond the target value as a controlled process responds to a change.

P

PID control

A control method combining proportional, integral, and derivative actions to regulate a measured condition toward its setpoint.

Plain bearing

A bearing in which one surface slides relative to another, rather than using rolling balls or rollers.

Pressure switch

A device that changes an electrical switching state when pressure reaches a specified operating threshold.

Pressure transducer

A sensor that converts applied pressure into an electrical signal for a controller, display, or monitoring system.

Pressure-relief valve

A protective valve that opens at a specified pressure to discharge fluid and limit pressure buildup.

Proportional control

A control method that changes output in proportion to the difference between the measured value and setpoint.

Pump shaft

The rotating member that carries components such as the impeller and transfers turning force from the drive.

R

Radial bearing

A bearing designed to carry forces acting across a shaft, perpendicular to its length.

Refrigerant sight glass

A window in refrigerant piping for viewing flow; some versions also indicate moisture.

Register

An air-opening assembly that combines a grille with a damper for adjusting the airflow through it.

Relay

An electrically operated switch; an electromechanical relay uses a magnetic coil to move separate electrical contacts.

Resistance temperature detector (RTD)

A temperature sensor that uses the predictable change in a metal element's electrical resistance as temperature changes.

Resistor

An electrical component that deliberately opposes current flow, helping establish the behavior of a circuit.

Reversing valve

A valve that reverses refrigerant flow so a heat pump can switch between heating and cooling.

Rotor

The rotating assembly inside a motor that interacts with the magnetic field to deliver mechanical motion.

S

Safety valve

A protective valve that opens automatically to relieve excessive pressure.

Saturation temperature

The temperature at which liquid and vapor can coexist in equilibrium at a given pressure.

Sensible heat

Heat transferred that changes a substance's temperature without changing its phase, such as warming liquid water.

Sensor

A device that detects a physical condition, such as temperature or pressure, for measurement or control.

Setpoint

The target value a control system is intended to maintain, such as a selected temperature or pressure.

Shaft coupling

A mechanical connection that transmits turning force from one shaft to another, such as between motor and pump.

Shaft seal

A sealing arrangement that limits leakage where a rotating shaft passes through a stationary equipment casing.

Silicon-controlled rectifier (SCR)

An electronic switching device whose gate signal starts conduction, which continues until current falls sufficiently low.

Solenoid valve

An electrically controlled valve that uses a magnetic coil to operate its flow-opening mechanism, directly or through a pilot.

Spring check valve

A check valve that uses a spring to return its closing element toward the seat when forward flow stops.

Squirrel-cage rotor

An induction-motor rotor with conductive bars joined by end rings, forming closed paths for induced current.

Stator

The stationary magnetic assembly of an electric motor; in an induction motor, its windings produce the rotating field.

Steam dryness fraction

The fraction of a wet-steam mixture's mass that is vapor rather than suspended liquid water.

Steam trap

An automatic device that drains condensate from a steam system while limiting steam loss.

Step-down transformer

A transformer with fewer secondary turns than primary turns, used to provide a lower output voltage.

Strainer

A pipeline component that catches solid debris in a screen or perforated element before it reaches downstream equipment.

Subcooling

How far a liquid’s temperature is below its saturation temperature at the same pressure.

Superheat

The amount a vapor’s temperature is above its boiling temperature at the same pressure.

Swing check valve

A check valve with a hinged disc that swings open with forward flow and closes against reverse flow.

Switching hysteresis

The gap between a control's turn-on and turn-off thresholds, which helps prevent rapid repeated switching.

T

Thermal conductivity

A material property describing how readily heat passes through it when a temperature difference exists.

Thermal radiation

Heat transfer carried by electromagnetic waves, allowing energy exchange without direct contact or a moving fluid.

Thermistor

A temperature-sensitive resistor whose resistance changes substantially with temperature, allowing electronic equipment to sense heating or cooling.

Thermocouple

A temperature sensor made from different metals that develops a voltage related to the temperature difference between its junctions.

Thermostatic expansion valve (TXV)

A valve that meters refrigerant into an evaporator in response to outlet superheat.

Thermostatic radiator valve (TRV)

A temperature-responsive valve that varies water flow through a radiator to help control the surrounding room temperature.

Thrust bearing

A bearing designed to carry forces acting along a shaft's length, helping control its axial position.

Transformer

A device that transfers electrical energy between windings through a magnetic field, commonly changing AC voltage.

Transistor

A semiconductor component used to amplify electrical signals or switch current in an electronic circuit.

V

Variable frequency drive (VFD)

An electronic device that controls an AC motor’s speed by varying its supply frequency and voltage.

Volute

The spiral-shaped pump casing passage that collects liquid from the impeller and directs it toward the discharge.

W

Watertube boiler

A boiler with water inside tubes and hot combustion gases outside them.

Wet-rotor motor

A motor whose rotor operates inside liquid, a design commonly used in seal-less hydronic circulating pumps.

Z

Zone valve

A valve that controls water flow to an individual heating or cooling zone in a building.