Glossary of HVAC terms

A
Air changes per hour
The hourly ventilation rate divided by the volume of a space. For perfectly mixed air or laminar flow spaces, this is equal to the number of times per hour that the volume the space is exchanged by mechanical and natural ventilation. Also called air change rate or air exchange rate.

Air conditioner
An appliance, system, or mechanism designed to dehumidify and extract heat from an area.

Air handler
A central unit consisting of a blower, heating and cooling elements, filter racks or chamber, dampers, humidifier, and other central equipment in direct contact with the airflow. This does not include the ductwork through the building.

C
Centrifugal fan
A centrifugal fan is a mechanical device for moving air or other gases

Chiller
A device that removes heat from a liquid via a vapour-compression or absorption refrigeration cycle. This cooled liquid flows through pipes in a building and passes through coils in air handlers, fan-coil units, or other systems, cooling and usually dehumidifying the air in the building. Chillers are of two types; air-cooled or water-cooled. Air-cooled chillers are usually outside and consist of condenser coils cooled by fan-driven air. Water-cooled chillers are usually inside a building, and heat from these chillers is carried by recirculating water to a heat sink such as an outdoor cooling tower.

Coil
Equipment that performs heat transfer to air when mounted inside an air handling unit or ductwork. It is heated or cooled by electrical means or by circulating liquid or steam within it.

Condenser
A component in the basic refrigeration cycle that ejects or removes heat from the system. The condenser is the hot side of an air conditioner or heat pump. Condensers are heat exchangers and can transfer heat to air or to an intermediate fluid (such as water) to carry heat to a distant sink, such as ground (earth sink), a body of water, or air (as with cooling towers).

Constant air volume
A system designed to provide constant airflow. This term is applied to HVAC systems that have a variable supply-air temperature but constant air flow rates. Most residential forced-air systems are small CAV systems with on/off control.

Controller
A device that controls the operation of part or all of a system. It may simply turn a device on and off, or it may more subtly modulate the set point of components. Most controllers are automatic but have user input such as temperature set points, e.g. a thermostat.

D
Damper
A plate or gate placed in a duct to control airflow by increasing friction in the duct.

Damper blades
Components made of multiple smaller blades, sometimes adjustable, placed in ducts or duct entries to control the volume of airflow. When used inside of ducts, their function is similar to that of a damper, but they can be manufactured to fit larger openings than a single-piece damper.

Duct
Specialized housing for the airflow.

Dehumidifier
A dehumidifier is a piece of equipment that extracts and removes humidity from the air. It works by cooling air to the point where water turns to liquid from vapour form and then the liquid is removed.

Diffuser
A diffuser is placed over ductwork, and it separates air with vanes going in differing directions. It evenly distributes airflow in the desired directions.

E
Evaporator
A component in the basic refrigeration cycle that absorbs or adds heat to the system. Evaporators can be used to absorb heat from the air or from a liquid. The evaporator is the cold side of an air conditioner or heat pump.

F
Fan coil
A small terminal unit that is often composed of only a blower and a heating and/or cooling coil.

Fire damper
A damper or adjustable louvre designed to augment the ventilation of a space during a fire.

Fresh air intake
An opening through which outside air is drawn into the building. This may be to replace the air in the building that has been exhausted by the ventilation system or to provide fresh air.

G
Grille
A facing across a duct opening, often rectangular in shape, containing multiple parallel slots through which air may be delivered or withdrawn from a ventilated space. The grille directs the airflow in a particular direction and prevents the passage of large items.

H
Heating coil
A heating coil is the part of the system that conducts heat. It allows electricity to act as fire.

Heat exchanger
A heat exchanger is the part of the system that transfers heat from the hot parts of the machine or a system to the cold parts of the machine or system.

Heat load
A term for the amount of cooling (heat gain) or heating (heat loss) needed to maintain desired temperatures and humidities in controlled air. Regardless of how well-insulated and sealed a building is, buildings gain heat from sunlight, conduction through the walls, and internal heat sources such as people and electrical equipment. Buildings lose heat through conduction during cold weather. Engineers use heat load calculations to determine the HVAC needs of the space being cooled or heated.

Heat pump
A heat pump is a compressor that cycles hot or cold air. It is a device that is designed to move thermal energy in the opposite direction of heat flow by absorbing heat from a cold space which is released to a warmer space.

Heat transfer
Heat transfer happens when heat moves from one area to another. It is an important and vital step in the process of cooling a space.

L
Louvre
Blades in a rectangular frame placed in the doors or walls to permit the movement of air.

O
Outside air temperature
A measure of the air temperature outside a building. The temperature and humidity of the air inside and outside the building are used in enthalpy calculations to determine when outside air can be used for free heating or cooling.

P
Packaged unit
An air-handling unit, defined as either “recirculating” or “once-through” design, made specifically for outdoor installation. They most often include, internally, their own heating and cooling devices. Very common in some regions, particularly in single-story commercial buildings. Also called a Rooftop unit (RTU).

Plenum space
An enclosed space inside a building or other structure, used for airflow. Often refers to the space between the ceiling and the structural roof. Distinct from ductwork as a plenum is part of the structure itself. 

S
Split system
A split system is the combination of an outdoor unit and an indoor unit. This is the most common type of system to cool individual rooms or small open areas. Also called a Wall split.

Superheat
The number of degrees a vapour is above its boiling point at a specific pressure.

Subcooling
The conditions where liquid refrigerant is colder than the minimum temperature required to keep it from boiling which would change it from a liquid to a gas phase. Subcooling is the difference between its saturation temperature and the actual liquid refrigerant temperature.

System
General term used to refer to the set or a subset of components that perform a specific HVAC function within a building.

T
Terminal unit
A small component that contains a heating coil, cooling coil, automatic damper, or some combination of the three. Used to control the temperature of a single room.

Thermal zone
An individual space or group of neighbouring indoor spaces that the HVAC designer expects will have similar thermal loads. Building codes may require zoning to save energy in commercial buildings. Zones are defined in the building to reduce the number of HVAC subsystems, and thus initial cost.

Thermostat
A thermostat is a system that monitors and regulates a heating or cooling system. It can be used to set the desired temperature at which it keeps the environment either heated or cooled.

Thermostatic expansion valve
A thermostatic expansion valve is a piece of equipment that meters the flow of liquid refrigerant into the evaporator while measuring the vapour refrigerant leaving the evaporator. It thereby controls the superheating at the outlet of the evaporator.

V
Variable air volume
An HVAC system that has a stable supply-air temperature, and varies the airflow rate to meet the temperature requirements. Compared to constant air volume systems, these systems conserve energy through lower fan speeds during times of lower temperature control demand. Most new commercial buildings have VAV systems. VAVs may be bypass type or pressure dependent. Pressure dependent type VAVs save energy while both types help in maintaining the temperature of the zone that it feeds.

Z
Zoning system
A zoning system sections a building or space into zones which are controlled independently of each other. This is beneficial when different areas or rooms of a building have different temperatures as well as when the desired temperatures in different rooms are different. Temperature is controlled by different thermostats.