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Glossary of HVAC Terms
Confused about all the technical terms? See our glossary below for simple explanations of some of the more common terms used in Heating, Ventilation & Air Conditioning.
AFUE
Annual Fuel Utilization Efficiency. Used to express the efficiency of gas furnaces. The higher the AFUE rating, the more efficient the unit. Federal law has required that all new residential furnaces built after January 1992 operate with an AFUE of 78% or higher. If your furnace was built before 1992, chances are it is operating with an average efficiency of around 60%. Most of the heat is lost up the chimney or out the exhaust vent - devices used by older furnaces to expel dangerous fumes created by the furnace, such as carbon monoxide, carbon dioxide, aldehydes, and even soot. Modern furnaces use more technologically advanced venting techniques to achieve greater energy efficiency. An important note: Be sure to ask your installing contractor if your chimney or exhaust vent is suitable for use with your new equipment.
AHRI
The Air-Conditioning, Heating, and Refrigeration Institute (AHRI) is the trade association representing manufacturers of air conditioning, heating and commercial refrigeration equipment. AHRI’s 300 member companies account for more than 90 percent of the residential and commercial air conditioning, space heating, water heating, and commercial refrigeration equipment manufactured and sold in North America.
Air changes per hour (ACH)
The number of times per hour that the volume of a specific room or building is supplied or removed from that space by mechanical and natural ventilation
Air handler or air handling unit (AHU)
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.
British thermal unit (BTU)
Any of several units of energy (heat) in the HVAC industry, each slightly more than 1 kJ. One BTU is the energy required to raise one pound of water one degree Fahrenheit, but the many different types of BTU are based on different interpretations of this “definition”. In the United States the power of HVAC systems (the rate of cooling and dehumidifying or heating) is sometimes expressed in BTU/hour instead of watts.
British thermal unit per hour (BTUH)
The heat transfer rate of HVAC equipment is measured in British Thermal Units per Hour.
Capacity
Usually measured in BTUs or tons, capacity refers to an air conditioning or heating unit's ability to cool or heat a space. For instance, a 20-ton air conditioning unit has twice the capacity of a 10-ton unit.
CFM
A unit to express movement of volume, including air, in Cubic Feet per Minute. A 400 CFM air handler moves 400 cubic feet in one minute.
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.
Compressor
The compressor plays an integral role in cooling your home. It is the device responsible for pumping refrigerant through the refrigerant lines and the coil, making the transfer of heat from inside your house to the outdoors possible.
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 or an aqueous solution of ethylene glycol) to carry heat to a distant sink, such as ground (earth sink), a body of water, or air (as with cooling towers). The coil responsible for dissipating heat to the surrounding, outside air. Also called the condenser coil, or outdoor coil, its role is reversed when a heat pump is used in heating mode.
Constant air volume (CAV)
A system designed to provide a constant air flow. This term is applied to HVAC systems that have 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. Controls may be analog or digital.
COP
The Coefficient of Performance rates a heat pump's ability to efficiently use electricity in its operation. The Air Conditioning and Refrigeration Institute provides the Coefficient of Performance at 47 degrees Fahrenheit and 17 degrees Fahrenheit. This is because a heat pump is more efficient at higher, outside-air temperatures.
ΔT (pronounced “delta T”)
A reference to a temperature difference. It is used to describe the difference in temperature of a heating or cooling fluid as it enters and as it leaves a system.
dB
The relative loudness of a sound is expressed in dB, short for decibel. As an example, the sound of a human voice talking is around 70 dB. (See also SRN.)
Damper
A plate or gate placed in a duct to control air flow by increasing friction in the duct.
DOE
A federal agency, the Department of Energy, sets the standards for efficiency throughout the HVAC industry and monitors consumption of energy sources.
Downflow
A term used to describe the direction of airflow through a furnace. A downflow furnace takes return air from the top, heats it, and then delivers the warm air from the bottom.
Duct / Ductwork
A central heating and air conditioning system uses many components to heat or cool air. This warm or cool air is then transferred to different registers throughout the house via special, flexible, large-diameter pipes or ducts. The system of ducts throughout your house is often referred to as ductwork or ducting.
EER
Energy Efficiency Ratio. The ratio of the cooling capacity of the air conditioner in BTUs per hour to the total electrical input in watts. This measure is determined by comparing test units to the Air Conditioning and Refrigeration Institute specifications.
Efficiency
A general term used to describe how effectively a heat pump, air conditioning system, or furnace converts incoming energy to outgoing energy. The higher the number, the more efficient the unit and the lower the operating cost.
Evaporator coil / Evaporator
A component in the basic refrigeration cycle that absorbs or adds heat to the system. Evaporators can be used to absorb heat from air or from a liquid. The evaporator is the cold side of an air conditioner or heat pump.
Fan coil unit (FCU)
A small terminal unit that is often composed of only a blower and a heating and/or cooling coil, as is often used in hotels, condominiums, or apartments.
Flow
A transfer of fluid volume per unit time.
Fresh air intake (FAI)
An opening through which outside air is drawn into the building. This may be to replace air in the building that has been exhausted by the ventilation system, or to provide fresh air for combustion of fuel.
Furnace
A component of an HVAC system that adds heat to air or an intermediate fluid by burning fuel (natural gas, oil, propane, butane, or other flammable substances) in a heat exchanger.
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 air flow in a particular direction and prevents the passage of large items.
Heat exchanger
Responsible for transferring heat from furnace burners to the blower.
Heat load, heat loss, heat gain
Terms for the amount of heating (heat loss) or cooling (heat gain) 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 a heat load calculations to determine the HVAC needs of the space being cooled or heated.
Horizontal flow
A term used to describe the direction of airflow through a furnace. A horizontal flow furnace takes return air from one side, heats it, and then delivers the warm air from the other side.
HSPF
Heating Seasonal Performance Factor. It measures the efficiency of the heating portion of your heat pump. The Department of Energy minimum is 6.8. (Similar to SEER.)
Humidifier
Usually available as an optional accessory, a humidifier is used to inject water vapor into the dry, heated air expelled from a furnace/air handler. The benefits can be improved efficiency and a more comfortable living environment.
HVAC
Heating, Ventilating and Air Conditioning. Used to refer to the industry at large, particularly dealers of heating and air conditioning equipment.
Indoor coils
Split-system home comfort systems use two main components to deliver air for a comfortable living environment. The indoor coil is the device responsible for transferring heat from indoors to the outdoors (or the reverse in the case of a heat pump in heating mode). Most modern systems are designed to achieve maximum efficiency when the indoor unit (coils and blower) is properly matched with the outdoor unit (air conditioner or heat pump). For best results, be sure to replace both the indoor and outdoor units at the same time.
Intermediate fluid
A liquid or gas used to transfer heat between two heat exchangers. An intermediate fluid is used when the hot and cold fluids are too bulky (such as air) or difficult to handle (such as halocarbon refrigerant) to directly transfer the heat.
Kilowatt
A unit used to express 1,000 Watts. Denoted as "kW." Note that the "W" in "kW" is always capitalized because the Watt unit was named after a person.
KWH
If a unit uses 1,000 Watts in 1 hour, it is said to have an energy rating of 1kWh.
Louver
Blades in a rectangular frame placed in doors or walls to permit and direct the movement of air.
Makeup air unit (MAU)
An air handler that conditions 100% outside air. Typically used in industrial or commercial settings, or in "once-through" (blower sections that only blow air one-way into the building), "low flow" (air handling systems that blow air at a low flow rate), or "primary-secondary" (air handling systems that have an air handler or rooftop unit connected to an add-on makeup unit or hood) commercial HVAC systems.
Minimum outside air
The lowest amount of fresh air flow that can be allowed into a recirculating system. This limit is sent to ensure that the interior air remains safe and comfortable to breathe.
Modulating Furnace
Furnaces are designed to deliver maximum heat for comfort on the coldest of days. In most cases, those days account for fewer than three percent of winter days. The rest of the time, your furnace is providing more heat than necessary. Because conventional furnaces are either providing no heat, or at full capacity, the temperature in your house goes up and down by several degrees, adversely affecting your comfort and your energy bills. Modulating furnaces solve this problem by varying the amount and temperature of air delivered between different capacities, so that the air flowing out of the registers is always at the temperature you determine. This results in lower operating costs, more comfortable temperatures throughout the house and quieter operation.
Outside air damper
An automatic louver or damper that controls the fresh air flow into an air handler and modulates to the most energy efficient setting.
Packaged terminal air conditioner (PTAC)
An air conditioner and heater combined into a single, electrically-powered unit, typically installed through a wall and often found in hotels.
Package Unit
Equipment in which all heating and cooling components are located in one cabinet. Installed either beside or on top of a home or business.
Plenum space
An enclosed space inside a building or other structure, used for airflow. Often refers to the space between a dropped ceiling and the structural ceiling or a raised floor and the hard floor. Distinct from ductwork as a plenum is part of the structure itself. Cable and piping within a plenum must be properly rated for its fire and smoke indices.
Refrigerant
The liquid used to absorb and transfer heat from one part of the home comfort system to another.
SEER
Seasonal Energy Efficiency Rating. Used to express the efficiency of an air conditioning unit, or a heat pump in cooling mode. The higher the SEER rating, the more efficient the unit. The Department of Energy minimum is 13 SEER.
Smoke damper
A damper or adjustable louver designed to augment the ventilation of a space during a fire.
Split System
A home comfort system that uses an indoor and an outdoor component to deliver comfortable air to a living environment.
SRN (Sound Number Rating)
The Air Conditioning and Refrigeration Institute performs tests and assigns a Sound Rating Number (SRN) to units. A lower SRN rating indicates a quieter unit with average SRNs of between 74dB and 80dB..
System
General term used to refer to the set or a subset of components that perform a specific HVAC function within a building.
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.
Thermostat
A temperature-measuring device used to control the operation of home comfort systems to maintain a comfortable temperature within the house. Programmable thermostats allow you to program different temperatures for different times of the day.
Thermal zone
An individual space or group of neighboring 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. For example, for perimeter offices, rather than one zone for each office, all offices facing west can be combined into one zone. Small residences typically have only one conditioned thermal zone, plus unconditioned spaces such as garages, attics, and crawlspaces, and basements.
Ton
The ton ratings you see here have nothing to do with the weight of the unit. In fact a ton is simply 12,000 BTUs (see BTU definition on this page). A typical home cooling/heating system uses heat pumps or air conditioners with a capacity of between 1.5 and 5 tons.
Upflow
A term used to describe the direction of airflow through a furnace. An upflow furnace takes return air from the bottom, heats it, and then delivers the warm air from the top.
Variable air volume (VAV)
An HVAC system that has a stable supply-air temperature, and varies the air flow 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 temperature of the zone that it feeds.
Watt / watts
Electrical power, also expressed as 'W'. For example, a 100W globe consumes 100 Watts of electrical power. The W in Watt is always uppercased, because it is named after a person.
Zone / zoning
A home may be divided into several different areas, or zones, to better control the temperatures throughout the house. The process of dividing your home into different zones is called zoning.
Source: Wikipedia and http://www.rheem.com/
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