Glossary of Terms

HVAC systems have a tendency to be very complex. In order to make the best decision about your HVAC service provider, it is important to understand all the terms and concepts related to your system and its functionality.

That’s where we come in. We have compiled this Glossary of Terms to help you navigate both our website and your daily HVAC interactions. Click one of the letters below to search for a specific term, or scroll down to browse through the entire list.


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Absolute humidity
The weight of water vapor in a given amount of air.

Absolute pressure
Pressure measured with the base of zero.

Absolute temperature
A temperature scale expressed in degrees F or C using absolute zero as a base. Referred to as the Rankin or Kelvin scale.

Absolute zero
The temperature at which molecular activity theoretically ceases: -456.69 degrees F or -273.16 degrees C.

An electrically or pneumatically driven device that adjusts the position of a valve or damper based upon a signal from a remote controller.

Air Balance
Measuring air flow from supply diffusers to confirm actual flow rate to design conditions. Measured with a digital flow hood, and is expressed in CFM (cubic feet per minute).

Air conditioning
The process of controlling the temperature, humidity, cleanliness and distribution of the air.

Air handler
A generic name for a device consisting of a blower and finned tube heat exchanger, for the  purpose of heat exchange.

Air, standard conditions
Conditions at which capacity ratings for air conditioning equipment is rated.

Air vent
A manual or automatic device that releases air bubbles from a hydronic piping system.

Refers to the temperature surrounding a body or unit under test.

The maximum electrical current that can be carried by a conductor in an electrical system as determined by the national electrical code.

A device that measures the temperature of a liquid at some point in a hydronic system and enables the operation of equipment thru an electrical circuit.

Atmospheric pressure
The weight of a 1 unit column of the earth’s atmosphere.


BACnet is a data communication protocol for building automation and control networks. A data communication protocol is a set of rules governing the exchange of data over a computer network. The rules take the form of a written specification – in BACnet’s case they are also on compact disk – that spells out what is required to conform to the protocol.

Backflow Preventer
A plumbing device used to protect drinking water systems from contamination by other building water systems, especially fire protection water systems.

Ball Valve
A valve containing a rotating ball with a hole thru it for the use of equipment water flow isolation and flow limiting.

Building Automation System (BAS)
Controls your HVAC system to maximize efficiency.

Two metals with different rates of expansion fastened together. When heated or cooled they will warp and can be made to open or close a switch or valve.

Boiler, hot water
A pressure vessel designed to store hot water at pressures greater than 15 PSIG. Also serves as a source of hot water for heating or other purposes.

Boiler, steam
A pressure vessel designed to store high temperature, high pressure steam at pressures greater than 15 PSIG. Also provides a source of steam for heating and other purposes.

Boiling point
The temperature at which the addition of any heat will begin a change of state from a liquid to a vapor.

British Thermal Unit (BTU)
The amount of heat necessary to change the temperature of 1 pound of pure water 1 degree Fahrenheit.

Building Automation System (BAS)
Controls your HVAC system to maximize efficiency.

Burner, oil fired
This may include a domestic type, rotary cup burner, steam atomizer or burner water and fan assembly.

Burner, gas or propane fired
This may include a natural gas burner and liquefied petroleum (propane) gas burner.

Burner, combination
This is a dual fuel burner that allows for quick changeover when required. Most common combinations are gas/oil and gas/propane.


Capillary tube
A refrigerant control consisting of a small diameter tube which controls flow by restriction. They are carefully sized by inside diameter and length for each particular application.

Carbon monoxide monitor/controller
A device which monitors and/or controls the carbon monoxide levels in parking garages. It is tied into the garage exhaust ventilation system.

A temperature scale with the freezing point of water (0 degrees Celsius) and the boiling point (100 degrees Celsius) at sea level.

Centrifugal pump
An electrical device that uses a rotating impeller to add mechanical energy to a fluid for the purpose of water movement.

Check valve
A valve designed to permit flow in one direction only.

Chiller, absorption
A low pressure refrigeration machine that uses chemicals and a source of heat to generate chilled water.

Chiller, centrifugal
A self contained, low pressure power driven refrigeration unit, consisting of a refrigerant evaporator, compressor and water cooled condenser. Used to provide chilled water to air conditioning air systems.

Chiller, reciprocating
A high pressure piston type compressor usually operated by an electric motor combining a compressor and chilled water evaporator as one unit. The air or water cooled condenser may be remote or be part of the assembly.

Combustion efficiency
The efficiency of a heat source in converting the chemical energy content of its fuel into heat, based upon measurement of exhaust gas temperature and carbon dioxide content.

The reduction of volume of a vapor or gas by mechanical means.

Compression ratio
The ratio determined by dividing the discharge pressure by the suction pressure in PSI (Pa).

A mechanical device used to compress gases. Three main types: reciprocating, centrifugal and rotary.

Condensation point
The temperature at which the removal of any heat will begin a change of state from a vapor to a liquid.

Condensing Boiler
A boiler designed to achieve optimum efficiency. Fuel is burned and the hot gases produced are passed through a heat exchanger, where much of their heat is transferred to water, thus raising the water’s temperature.

Condensing medium
The substance, usually air or water, to which the heat in a condenser is transferred.

Condensing unit
The portion of a refrigeration system where the compression and condensation of refrigerant is accomplished. Sometimes referred to as the ‘high side’.

The transfer of heat from molecule to molecule within a substance.

An electromagnetic actuated relay. Usually used to refer to the relay which closes the circuit to a compressor.

The transfer of heat by a moving fluid.

Cooling anticipator
A resistance heater (usually not adjustable) in parallel with the cooling circuit. It is ‘on’ when the current is ‘off”, adding heat to shorten the off cycle.

Cooling coil
A device that is designed to remove heat from a surrounding medium, usually air or water, and is generally fabricated with piping attached.

Cold room
A refrigerated space, a cooler or freezer that is part of a permanent structure. For example, a wine cellar or a meat freezer. Does not include walk in coolers and freezers.

Condenser, evaporative
This is a device which uses open spray or spill water and air to cool the refrigerant. It usually consists of a condenser, sump, make up water pump, spray bars, wooden slots, etc. May or may not include a fan assembly.

Compressed air dryer
A device which utilizes a refrigeration system to remove moisture from the ambient air prior to distributing through a pneumatic network.

Compressor refrigeration
Equipment designed to add the heat of compression to a refrigerant and circulate it throughout the refrigeration system, which consists of a condenser metering device, evaporator and associated piping. This includes reciprocating open, hermetic or semi-hermetic and rotary compressors.

Condenser, air cooled
This is part of a refrigeration system, a heat exchanger, that uses surrounding air as a medium to convert the high pressure, high temperature vapor received from the compressor to a high pressure high temperature liquid through the heat of rejection.

Condenser, water cooled
Part of refrigeration system, heat exchanger, that uses water as a medium to reject heat from the refrigerant.

Controls, combustion
A system of controls on combustion equipment for the safe and efficient operation of the combustion equipment.

Controls, electric and electronic
Electric and electronic control devices.

Controls, system
Devices used with a computer or multiplex monitor for alarm, security, fire, energy, environmental or control system.

Controls, pneumatic
A control system which uses compressed air as the operational medium.

Controls, feed water
Devices that monitor and control the flow of feed water to a boiler.

Controls, gas analyzer
A device for sampling and analyzing exhaust gases which may be used to give alarms, record, indicate, or control.

Convector, hot water
A heat transfer unit using the principle of either natural or forced convection. Natural convection is used for heating, and forced convection may be used in both heating and cooling applications.

Heat transfer equipment that functions with two different mediums. For example, steam and water, or brine and chilled water.

Equipment for moving material from one point to another. May be manually or power operated.

Cooling tower
Equipment which cools by water evaporation in air.

Ratio of work performed or accomplished as compared to the energy used.

Cubic feet per minute (CFM)
A common means of assigning quantitative values to volumes of air in transit.

The complete course of operation of a refrigerant back to a selected starting point in a system.


A mechanical device which may be of a fin type for controlling the flow of gases including air. They are positioned by actuators which may be electric or pneumatic driven.

Equipment that removes the entrained gases from the circulating medium of heating or cooling systems.

Equipment designed to remove moisture from air, accomplished by cooling air below its dew point.

Equipment which removes excess heat or energy from superheated steam.

Mass or weight per unit of volume.

Discharge line
A tube used to convey the compressed refrigerant vapor from the compressor to the condenser inlet.

Discharge pressure
The pressure read at the compressor outlet. Also called head pressure or high side pressure.

May be floor, roof, trench or sump types which provide a means by which excessive water is channeled out of a building.

Dry air
Air which contains no moisture vapor.

Dry bulb temperature
Temperature read with an ordinary thermometer.

Dust collectors
Equipment that is used to remove dust and particulates from the air and is usually in wood working and metal working shops.


A heat transfer device located within the breaching of a boiler stack.

A ratio calculated by dividing the cooling capacity in Btus per hour (Btu/h) by the power input in watts at a given set of rating conditions, expressed in Btu/h per watt. The EER of a product also determines the product’s Seasonal Energy Efficiency Ratio (SEER) which is the measure of cooling efficiency for air conditioning products. The higher the SEER rating number, the more energy efficient the unit is.

The ratio of a desired output quantity or rate divided by the necessary input quantity rate.

Effective temperature
An arbitrary concept which combines into a single value the effect of temperature, humidity, and air movement as sensed by the human body.

Electrical Disconnect Switch
A switch that can interrupt the power to an electrical circuit or to a group of electrical circuits.

End devices
Devices which are used to transmit signals in pneumatic, electric and electronic control systems. Includes sensors, transducers, transmitters, receivers, relays, actuators, valves and air flow devices.

Energy Management System (EMS)
Controls your HVAC system to maximize efficiency.

Total amount of heat in one pound (kg) of a substance calculated from accepted temperature base, expressed in BTU’s per pound mass (J/kg).

Equivalent length
That length of straight tubing which has the same pressure drop as the fitting, valve or accessory (of the same nominal size) being considered.

Evaporative cooling
The cooling effect of vaporization of a liquid in a moving air stream.

A device in which a liquid refrigerant is vaporized. Some superheating usually takes place.

Evaporator superheat
The actual temperature of the refrigerant vapor at the evaporator exit as compared to the saturated vapor temperature indicated by the suction pressure.

Exhaust air valve
A device used for controlling the flow of exhaust air from a building.

Expansion joint (piping)
A device installed in a steam or water pipe which is used to compensate for expansion or contraction caused by temperature or pressure fluctuations.

Expansion Tank
A tank specifically designed and sized to accommodate the increased volume of a system  liquid when heated.

External static pressure
The sum of the static and velocity pressures of a moving air system at the point of measurement.


A temperature scale with the freezing point of water at 32 degrees F and the boiling point at 212 degrees F at sea level.

Feet per minute (FPM)
A term assigned to a velocity of a moving air stream.

A device that removes moisture, acid and foreign matter from the refrigerant.

Flash gas
Instantaneous evaporation of some liquid refrigerant at the metering device due to pressure drop which cools the remaining liquid refrigerant to desired evaporation temperature.

Fire Damper
A device that prevents the migration of fire, where air ducts penetrate fire barriers such as floors, walls and partitions. The construction of the fire damper includes a galvanized steel frame and a fusible link, which is a heat-sensitive device.

Fire/Smoke Dampers
Similar to fire dampers in fire resistance rating, and also prevent the spread of smoke inside the ducts. When a rise in temperature occurs, the fire damper closes, usually activated by a thermal element which melts at temperatures higher than ambient but low enough to indicate the presence of a fire, allowing springs to close the damper blades.

Freezing point
The temperature at which the removal of any heat will begin a change of state from a liquid to a solid.


Gauge pressure
Pressure measured with atmospheric pressure as a base.


A form of energy causing the agitation of molecules within a substance.

Heat Emitter
A generic term for a device that releases heat from a circulating stream of heated water into the space to be heated. Example would be wall baseboard heaters and fan coils.

Heat exchanger
A device for the transfer of heat energy from the source to the conveying medium.

Heat flow
Heat flows from a warmer to a cooler substance. The rate depends upon the temperature difference, the area exposed and the type of material.

Heat of compression
The heat added to a vapor by the work done on it during compression.

Heat of the liquid
The increase in total heat (Enthalpy) per pound of a saturated liquid as its temperature is increased above a chosen base temperature (usually -40 degrees F for refrigerants).

Heat transfer
The three methods of heat transfer are conduction, convection and radiation.


Inches of mercury
Atmospheric pressure is equal to 29.92 inches of mercury.


Latent heat
Heat that produces a change of state without a change in temperature. For example, ice to water at 32 degrees F, or water to steam at 212 degrees F.

Latent heat of condensation
The amount of heat energy, in BTU’s, that must be removed to change the state of one pound of vapor to one pound of liquid at the same temperature.

Latent heat of fusion
The amount of heat energy, in BTU’s, required to change the state of one pound of liquid to one pound of solid at the same temperature.

Latent heat of melting
The amount of heat energy, in BTU’s, that must be removed to change the state of one pound of solid to one pound of liquid at the same temperature.

Latent heat of vaporization
The amount of heat energy, in BTU’s, required to change the state of one pound of liquid to one pound of vapor at the same temperature.

To elevate a fluid from one level to a higher level.

Liquid line
A tube used to convey the liquid refrigerant from the condenser outlet to the refrigerant control device of the evaporator.

LonWorks (local operating network)
A networking platform specifically created to address the needs of control applications. The platform is built on a protocol created by Echelon Corporation for networking devices over media such as twisted pair, powerlines, fiber optics, and RF.


A tube filled with a liquid used to measure pressures.

One MBH is equivalent to 1,000 BTU’s per hour.

Mean temperature differences
The mean of difference between the temperature of a fluid receiving and a fluid yielding heat.

Melting point
The temperature at which the addition of any heat will begin a change of state from a solid to a liquid.

Mercury manometer
Used to measure vacuum in inches of mercury.

A unit used to measure high vacuums. One micron equals 1/25,400 of one inch mercury.

An integrated DDC controller capable of being electrically programmed to execute specific logical and mathematical operations.

Mixing Valve
A valve that blends two fluid streams entering at two different temperatures to achieve a desired outlet temperature.

Mollier chart
A psychrometric chart. See: how-to convert between Mollier and Psyhrometric charts?

Device installed in hot gas line to silence discharge surges.


Oil separator
A device for separating out oil entrained in the discharge gas from the compressor and returning it to the crankcase.


Package Terminal Air Conditioner (PTAC)
Provides heating and cooling for small spaces, and is g
enerally used in hotels and motels.

Partial pressure
The pressure exerted by any individual gas in a mixture.

The slope of a pipe line for the purpose of improving drainage.

Pitot tube
A device comprising of a small diameter orifice projecting directly into an air stream, measuring total pressure, and surrounded by an annular section with small diameter entrances normal to the flow, measuring static pressure. Both sections are usually connected to a manometer to indicate velocity pressure.

Precharged lines
Refrigerant lines which are filled with refrigerant and are sealed at both ends. The seals are broken when the lines are installed and the line charge becomes part of the total system charge.

Pressure drop
The decrease in pressure due to friction of a fluid or vapor as it passes through a tube or duct and/or lift.

Pressure – temperature relationship
The change affected in temperature when pressure is changed or vice versa. Only used at saturated conditions. An increase in pressure results in a temperature increase. A decrease in temperature results in a pressure decrease.

Process of pumping refrigerant out of the evaporator and suction line at the end of the on- cycle by closing a solenoid valve in the liquid line and letting the compressor shut-off by the low pressure control.

A devices having both a dry and wet bulb thermometer. It is used to determine the relative humidity in a conditioned space. Most have an indexed scale to allow direct conversion from the temperature readings to the percentage of relative humidity.

Psychrometric chart
A chart on which can be found the properties of air under varying conditions of temperature, water vapor content, volume, etc.


Quick connect
Name given to the end connections on precharged lines which screw on to mated fittings of the outdoor and indoor sections. Tightening the quick connections ruptures the seals on the fittings and the line charge becomes part of the total system charge.


The transfer of heat without an intervening medium. It is absorbed on contact with a solid surface.

A vessel for holding refrigerant liquefied by the condenser.

A substance which produces a refrigerating effect while expanding or vaporizing.

Refrigerant control
A device used to meter the amount of refrigerant to an evaporator. It also serves as a dividing point between the high and low pressure sides of the system.

Refrigerant distributor
A device which meters equal quantities of refrigerant to independent circuits in the evaporator coil.

Refrigerant migration
The movement of refrigerant through the system to the compressor crankcase during the off-cycle, caused by its attraction to oil.

Refrigerant operation charge
The total amount of refrigerant required by a system for correct operation.

Refrigerant velocity
The rate at which refrigerant is moving at a given point in a system, usually given in feet per minute (FPM).

The transfer of heat from a place where it is not wanted to a place where its presence is not undesirable.

Refrigeration effect
The amount of heat a given quantity of refrigerant will absorb in changing from a liquid to a vapor at a given evaporating pressure.

Relative humidity
The percentage of water vapor present in a given quantity air compared to the amount it can hold at its temperature.

A device used to open and close an electrical circuit. The relay may be actuated by a bimetal electrically heated strip, a rod wrapped with a fine resistance wire causing expansion when energized, a bellows actuated by expansion of a fluid or gas, or an electromagnetic coil.

Reversing valve
A device in a heat pump that is electrically controlled to reverse the flow of refrigerant as the system is switched from cooling to heating. Also called a four-way valve.

A vertical tube or pipe which carries refrigerant in any form from a lower to a higher level.

Roof Curb
Raised frame used to mount mechanical units (such as air conditioning or exhaust fans), skylights, etc. on a roof.

Roof Top Unit (RTU)
A heating and cooling unit that is located on the roof, providing conditioned air to occupied space via air distribution duct work.


Saturated vapor
Vapor in contact with a liquid.

A condition of stable equilibrium of a vapor and a liquid.

Seasonal Energy Efficiency Ratio (SEER)
The efficiency of air conditioners is often rated by SEER, which is defined by the Air Conditioning, Heating, and Refrigeration Institute in its 2008 standard AHRI 210/240, Performance Rating of Unitary Air-Conditioning and Air-Source Heat Pump Equipment.

Sensible heat
Heat that can be measured or felt. Sensible heat always causes a temperature rise.

Sight glass
A glass installed in the liquid line permitting visual inspection of the liquid refrigerant for the purpose of detecting vapor in the liquid. They also generally have a device included to monitor moisture content of the refrigerant.

A condition in which a quantity of liquid enters the compressor causing hammering and possible compressor damage.

Specific heat
The amount of heat necessary to change the temperature of one pound of a substance one degree F.

Specific volume
The volume of a substance per unit of mass (such as, standard air 13.33 cubic feet per pound). The reciprocal of density.

Standard air density
0.075 pounds per cubic foot. Equivalent to dry air at 70 degrees F and at sea level pressure.

State condition
Substances can exist in three states: solid, liquid or vapor.

Static pressure
The normal force per unit area at a small hole in the wall of a duct.

Static tap
A means by which static pressures of a duct system may be read directly, usually consisting of a small diameter hole in the side of the duct connected to a manometer.

Sub cooling
Cooling of a liquid, at a constant pressure, below the point at which it was condensed.

A condition where a substance changes from a solid to a gas without becoming a liquid.

Suction line
A tube used to convey the refrigerant vapor from the evaporator outlet to the suction inlet of compressor.

Suction line accumulator
A device located in the suction line that intercepts quantities of a liquid refrigerant and thereby prevents damage to the compressor.

Heat added to a vapor after all liquid has been vaporized.


Tank, chemical holding
A storage facility designed to store chemicals on a temporary basis, or to accommodate spills.

Tank, fuel above ground
A storage vessel designed to contain petroleum fuels that is to be supplied to a distribution network.

Tank, fuel underground
A storage vessel designed to contain petroleum fuel to a distribution network.

Tank, gravity storage, fire protection
A non-pressurized storage vessel designed to supply a fire protection distribution network.

Tank, gravity storage, HVAC
A non-pressurized storage vessel designed to act as a storage tank or expansion tank for chilled water, hot water heating, glycol or condenser loops.

Tank, pressurized storage, fire protection
This is a pressurized storage vessel designed to supply a fire protection distribution network.

Tank, pressurized storage, HVAC
This is a pressurized storage vessel designed to act as a storage tank or expansion tank for chilled water, hot water heating, glycol or condenser loops.

A measurement of heat intensity.

A semiconductor which has electrical resistance that varies inversely with temperature.

A bimetal actuated switch to close and open a circuit to indicate or terminate operation of a heating or air conditioning system.

Thermostatic expansion valve
Refrigerant control which monitors the flow rate according to the superheat at the evaporator outlet.

Ton of refrigeration
The amount of heat of fusion absorbed by melting 1 short ton (0.893 long ton or 0.907 t) of pure ice at 0 degrees C (32 degrees F) in 24 hours. Equivalent to the consumption of one ton of ice per day during the transition from stored natural ice to mechanical refrigeration.

Total heat (enthalpy)
Total heat energy in a substance. The sum of sensible and latent heat.

Total pressure
The sum of all partial pressures in a mixture of gases.

A depression or dip in refrigerant piping in which oil will collect. A trap may be placed at the base of a suction or hot gas riser to improve oil return up the riser.

Traps and strainers
An apparatus used in pipe systems to capture impurities and particulates to prevent their circulation throughout the system.

Turbine, steam
A turbine operated by high pressure steam directed against vanes on a rotor.


Unit heater
A fan coil that is usually mounted near the ceiling to direct heated or cooled airflow to the floor level. Also commonly used for natural gas appliances.


Any pressure below atmospheric pressure.

Valves, control
Includes all control valves that my be pneumatic or electric.

Valves, steam pressure reducing
A device used to regulate and control steam pressure for delivery throughout the system.

Vapor barrier
The term applied to an impervious layer of material superimposed upon a layer of insulation. Vapor barriers are always applied on the warm side of the insulation layer.

Vapor pressure
The pressure exerted by vapor.

Variable Frequency Drive (VFD)
A type of motor controller that drives an electric motor by varying the frequency and voltage supplied to the electric motor. Other names for a VFD are: variable speed drive, adjustable speed drive, adjustable frequency drive, AC drive, microdrive, and inverter.

Variable Refrigerant Flow (VRF)
An HVAC technology that comes in two system formats. First, Heat Pumps (HP) are a two pipe system that have a heating and cooling mode. Since a VRF system will generally have 2 or more Indoor Units (IDU), HP IDUs will all cool or heat together. The second style system is the Heat Recovery (HR) three pipe system, which has a special box that controls the flow of suction gas and hot gas through the evaporator, indoor, and coils.

Velocity pressure
In a moving fluid, the pressure capable of causing an equivalent velocity as applied to move the same fluid through an orifice such that all pressure energy expanded is converted into kinetic energy.


Water heater, domestic
Includes a storage tank and a heater section, either electrical, gas, steam or hot water.

Water manometer
Used to measure pressure in inches of water.

Water softener, demineralizer
This is external equipment for the removal of minerals by chemical, ion exchange or other means. Usually includes a distiller.

Water treatment systems
Water treatment for potable water, glycol boilers, feed water, chilled water, condenser water, humidifier and spray water. Includes chemical mixing tanks, pot feeders and associated components.

Well pump
A pump designed to supply water from an underground reservoir or well and could include floor mounted, submersible or turbine types.

Wet bulb temperature
Temperature read with a thermometer whose bulb is encased in a wetted wick.