HVAC Glossary

HVAC Glossary

Activated carbon — granular processed carbon, commonly used in air filter media to clean the air.

Air conditioner — equipment used to control indoor air temperature, relative humidity, purity, and movement.

Air conditioning — making indoor air more comfortable by transferring heat and humidity to the outside air.

Air-cooled — using a fan to move heat away from the condenser coil to the outside air.

Air filter— in heating and air conditioning, the media, element, or device used to remove unwanted particles, vapors, or odors from indoor air. Air filters serve two functions: to protect the equipment from damage and improve indoor air quality for people.

Air handler — refer to “air handling unit” below.

Air handling unit — equipment to moving and/or mixing air for heating and/or cooling. Air handlers includes a blower-fan, heating and/or cooling coils, and related equipment such as controls, drain pans, and air filters contained in a housing or cabinet.

Air source — the use of ambient air as the heat source or heat sink with heat pumps.

Air vent — a general term that describes the port where air enters (or exits) a duct, room or area. Since warm or cool air exists from them, the supply air vents that serve a room are the points most often described by the term. Also see “vent”.

Ambient temperature — the temperature of the air surrounding HVAC equipment.

ARI — Air Conditioning and Refrigeration Institute – a non-profit organization of heating, air conditioning and refrigeration manufacturers. They test and rate heat pumps and air conditioners and publish standards.

ASHRAE — American Society of Heating Refrigeration and Air Conditioning Engineers — professional organization for HVAC that publishes standards in the areas of HVAC and refrigeration.

Blower coil — refer to “evaporator coil”.

BTU — British Thermal Unit — a standard unit of measure of heat used in heating and air conditioning. It is defined as the amount of energy required to raise the temperature of one pound of water one degree Fahrenheit. In heating, a BTU measures the heat given off during the combustion of fuel. In air conditioning, it is a unit that represents the capacity to remove that amount of heat from inside space. Also: refer to “Tons” below

BTUh — British Thermal Units per hour – one ton of cooling capacity equals 12,000 BTUh.

Capacity — ability of a cooling or heating system to cool or heat a specific amount of space. Cooling capacity is usually expressed in tons, while in heating, it is usually rated in BTUs.

Carbon monoxide (CO) — a colorless, odorless, and highly toxic gas produced when carbon-containing fuels are burned. When a furnace becomes worn or is not properly maintained, CO can enter indoor space.

Central air conditioning system — a system in which indoor air is cooled and dehumidified at a central location. The air is delivered to and from the rooms by an air handler and a system of ductwork. Contrast with “window air conditioner” and “ductless air conditioners”.

CFC — abbreviation for chlorofluorocarbons — these are used as the refrigerant in air conditioning systems and heat pumps. They have been linked to the depletion of the ozone layer of the atmosphere.

Condenser — the outside unit of an air conditioning or heating system. This unit condenses the refrigerant from a gas to a liquid thus releasing unwanted hot or cold air from the building to the outside air. Sometimes called the “heat exchanger”.

Coefficient of Performance(COP) — an efficiency ratio that expresses the capacity of a system divided by electrical energy consumed or compares the amount of heat delivered to the amount of energy used. A higher the score means the equipment is more efficient. This ratio is used to rate heat pumps.

Coil — a general name for part of a heating or cooling system. A coil unit is made of tubing or pipe, and often has fins for cooling.

Compressor — a pump in an air conditioning system that moves refrigerant from the evaporator coils indoors to the outdoor condenser and back again in a closed system. The compressor is like the heart of the system because it circulates the refrigerant throughout the loop. Compressors come in many designs, which are described below.

Compressor, centrifugal — a type of compressor that employs centrifugal force to supply its compression. They often use a rotating disk or an impeller in a shaped housing to provide pumping action.

Compressor, reciprocating – a type of compressor that uses a piston cylinder mechanism to provide its pumping action.

Compressor, rotary screw – a type of compressor that employs two meshed rotating positive-displacement helical screws to force the gas to move. These are often used for commercial and industrial applications

Compressor, scroll — a compressor that uses two spiral-shaped scrolls to pump or compress liquids and gases. One scroll is fixed, while the other moves in an eccentric way without rotating. This traps and compresses pockets of fluid or gas between the scrolls. These are also called a “scroll pump” or a “scroll vacuum pump”,

Condenser – the part of an air conditioning system that transfers unwanted heat to a medium that absorbs the heat and dissipates it. Here, the refrigerant is condensed from a vapor to liquid. The medium can be air, water, or a combination of the two. Most residential HVAC systems use air-cooled condensers.

Condensate pan — refer to “drip pan”.

Condenser coil —  a network of tubes outdoors filled with refrigerant that remove heat from the heated gas refrigerant and convert the refrigerant into a liquid form again. The excess heat escapes into the outside air.

Condenser fan — the fan in the outside unit that circulates air over the air-cooled condenser to change the refrigerant vapor into a liquid.

Condenser, air-cooled — a heat exchanger that transfers heat into the surrounding air.

Condenser, water-cooled — a heat exchanger that transfers heat into water.

Condensing temperature — the temperature at which a gas or vapor will condenses into liquid.

Cooling capacity — the ability of a unit to remove heat from an enclosed space. Also see “capacity”.

Cooling coil — refer to “evaporator coil

Damper — a movable plate in ductwork that can be opened and closed to modulate airflow. Dampers are used to balance airflow in a HVAC system. They are also used in zoned systems to regulate airflow into specific rooms or areas.

Dehumidify — to remove moisture from indoor air. Air conditioning helps to dehumidify the air, and it can be done separately with a dehumidifier.

Dehumidifier — a device used to remove moisture from indoor air.

Drain pan or Drip pan — the tray or pan used to collect water as it condenses from the evaporator. The pan should have a properly working cutoff switch to stop the system and prevent overflow in case the drain lines clog up. This helps prevent damage to ceilings or indoor areas below the pan.

Duct — in air conditioning and heating systems, the conduits that carry cooled, warmed, or ventilated air from the air conditioning system or furnace to the rooms and back. These come in many varieties, including flexible, sheet metal and more.

Ductless air conditioning — an air conditioning unit with no ductwork. These are often seen in motels and in one-room applications such as a garage apartment or loft.

Ductwork – the system of ducts in a building that deliver air to and from the air conditioning and heating system to the rooms and back.

EER — Energy Efficiency Ratio — the ratio of cooling capacity of an air conditioning unit (in British thermal units per hour) to the total electrical input (in watts) under specified test conditions. A Higher EER rating means the equipment is more efficient. Compare with SEER.

Efficiency — a general term used to describe the ratio of output of a device, appliance, system or activity, to the input necessary to create that output. Refer to “SEER”, and “HSPF” for a more specific definition.

Electric heating — heating that relies on electric resistance heaters. An example of electric heating is a central heater that uses “stirip heating” for its heat source.

Electrostatic filter — a filter device for cleaning or purifying of air that employs electricity to charge plates and attract dust particles.

ERV— Energy Recovery Ventilator — this is a device that provides ventilation by pulling fresh air indoors and sending stale air out as exhaust. ERVs pre-heat or pre-cool incoming fresh air to help conserve energy required for heating and air conditioning. Also refer to HRS and HRV below.

Evaporator coil — a network of tubes indoors filled with refrigerant that removes heat and moisture from the air as the refrigerant evaporates into a gas again.
These are also called “cooling coil”, “blower coil”, or “indoor coil”.

Filter – a shortened term for “air filter”. Refer to “air filter” above for HVAC specifics. Otherwise, a general term used to describe the removal of particles from a gas or liquid by straining.

Flexible duct — a type of air duct material, often continuous, that is easy to route around obstacles in attics, etc, by bending it gently.

Freon — a brand name of refrigerant manufactured by the Dupont Company.

Ground source – the use of ground or soil below the frost line as the heat source or heat sink for a heat pump.

Ground source heat pump (GSHP) — a type of heat pump that uses the stored energy in the soil. These systems use the relatively constant temperature of soil to provide heating, cooling, and hot water for homes and buildings.

Ground water-source – the use of water from an underground well as the heat source or heat sink for a heat pump.

HCFC — abbreviation for hydrochlorofluorocarbons — these compounds are used as refrigerants in air conditioners and heat pumps. HCFCs are believed to contribute to depletion of the ozone layer in the stratosphere. Contrast HFC below.

HFC – abbreviation for hydroflorocarbon — these are compounds used as a refrigerant in air conditioners and heat pumps. These are believed to have little to no effect on the ozone layer.

Heat exchanger — this term has two meanings: 1. A major part of a furnace that transfers heat created in the fuel combustion area to the surrounding air that is pumped into the rooms. This component can develop rust holes or other leaks and problems. To prevent carbon monoxide gas from entering the living areas and for other reasons, the heat exchanger and heating system should be professionally checked regularly. 2. A condenser or evaporator device in which heat is added or extracted to heat or cool indoor space.

Heat recovery system (HRS) — a system that uses exhaust air to preheat the supply air when the outside air is cooler than the inside air. When the outside air is warmer than the inside air, the exhaust air cools the supply air.

Heat recovery ventilator (HRV) — a device that brings fresh air into a home or building while recovering heat from the exhaust. This preheats the air and saving on utility bills.

Heating coil — a general term for a coil device that serves as a heat source. . A coil unit is made of tubing or pipe.

Heating, ventilation, and air conditioning (HVAC) — control of the indoor environment (temperature, humidity, air flow, and air filtering) of a building. In commercial buildings, this can include data center components such as computing hardware, cabling, data storage, fire protection, physical security systems, and power.

Heat loss — the rate of heat transfer from a heated space to the outdoors. Contrast with “heat gain” and cross-reference “envelope”.

Heat pump — a type of mechanical compression cycle refrigeration system capable of heating. Heat pumps come in many varieties. Some can be reversed to either heat or cool indoor space, while others are for heating only. One main benefit of heat pumps is their ability to use naturally occurring temperature differentials in air, ground, or water as a heat source or sink to provide indoor comfort.

Heat transfer — the movement of heat energy from one place to another. Types of heat transfer are through convection, conduction and radiation. Cross reference “envelope” and “whole house”.

Humidifier — a device that adds water vapor to the indoor air to increase humidity. Contrast with dehumidifier

Humidity — refer to “relative humidity”.

Humidistat — a type of indoor comfort control device that senses humidity and cycles a humidifier on and off.

Indoor air quality (IAQ) — an overall measure, from the human point of view, of temperature, humidity, chemical pollutants, biological contaminants, movement and sensory qualities (such as fresh smell) of indoor air. As buildings are being built to “tighter” standards, IAQ has become more important as an issue. In addition to proper air conditioning, advanced air filtration and ventilation are two of the keys to better indoor air quality.

Insulation — as it applies to air conditioning and heating, any material that slows down the transfer of heat. In homes and buildings, insulation is commonly used in attics, walls, and under floors. Also used in electrical and other types of wiring.

Kilowatt (kW) — equal to 1,000 watts.

Kilowatt-hour (kWh) — a common unit of electrical consumption. It is the equivalent of one kilowatt (1 kW) of power expended for one hour (1 h).

Load — the required rate of heat removal.

Load calculation — a computation of the quantity of cooling and heating (in British Thermal Units-BTUs) an air conditioning and heating system must produce for indoor comfort. The load is influenced by many factors: climate, square footage of conditioned area, orientation and layout of the building, number of human occupants, room count and dimensions, window and door count and characteristics, insulation quantity and quality, and more.

Media — as used in air conditioning and heating, the material in an air filter that traps and holds the impurities until it is cleaned or replaced.

OEM – Original Equipment Manufacturer- generally used in contrast with aftermarket, or equipment produced by someone other than the original.

Open loop — a system where water is pumped from a well, pond, lake or river for use in a heat pump. Package unit. A type of air conditioning and heating system that is contained in one (usually) outdoor unit. Package units are often installed beside or on the roof of buildings. Also see “self-contained system”. Contrast with “split system”.

Package heat pump — a refrigeration system that has all its components in one cabinet. Contrast with “split system”.

Programmable thermostat — an electronic control device for air conditioning and heating equipment. These are one of the least expensive improvements for increasing comfort and saving on monthly heating and cooling bills.

R-22 — an air conditioning refrigerant that contains chlorine. According to the terms of the Montreal Protocol, the U.S. EPA agreed to a phaseout schedule for HCFCs, Including R-22, between the years 2004 and 2020.

R-410A — the refrigerant that is replacing R-22. It contains no chlorine and is not associated with ozone depletion.

Refrigerant — a fluid with physical ability to absorb heat at low temperatures and release heat at higher temperatures.

Refrigerant charge – as a noun, the amount of refrigerant required to be in an air conditioning or refrigeration system. Used as a verb, the process of adding or “charging the refrigerant”. Keeping the correct amount and type of refrigerant is critical for peak operating performance.

Relative humidity — the amount of moisture in the air compared to the maximum amount of moisture the air can hold at that temperature, expressed as a percentage.

Return side — the path of air as it goes to the air-handler or furnace so it can be cooled or heated. The Return side should be “balanced” with the supply side to achieve optimum airflow and comfort. Sometimes described as “return”, or “return air”. Contrast with “supply side” below.

Reversing heat pump – a type of heat pump in which the condenser and evaporator coils exchange roles when the direction of the flow of refrigerant in the machine changes.

Scroll compressor — refer to “compressor, scroll” above.

SEER – Seasonal Energy Efficiency Ratio – a rating of the efficiency of air conditioner or heat pump. A higher number means the equipment is more efficient at converting electricity to cooling. As of January 2006, U.S. government regulations require all air conditioning manufacturers to produce residential central air conditioners with 13 SEER or higher.

Self-contained system — a refrigerating system that can be moved without disconnecting any refrigerant lines. Also called a “package unit” or “single package”.

Sick building syndrome — a combination of problems and complaints about indoor building environments and related human health issues. The syndrome has become more common since buildings are built to “tighter” standards for energy efficiency. In addition to proper air conditioning and heating, advanced air filtration and ventilation are two of the keys to better indoor air. Refer to “Indoor air quality” above.

Single package — a central air conditioner unit that contains both the condenser and air handling components in one packaged unit.

Sound rating number (SRN) — a unit of sound based on ARI standard tests for air conditioners and heat pumps. SRN is measured in bels (one bel equals 10 decibels). A lower rating means quieter equipment. Most air conditioners operate at 8 to 9 bels. The quietest units are rated to operate at 6.8 bels. A small difference in numerical rating translates into a large sound difference.

Split system –. the most common type of air conditioning and heating systems found in homes. It is a central system with two or more major components located apart from each other. The condensing unit is located outside the structure, and the air handling unit is located inside. Refrigerant lines and electrical wiring run between the two. The components of split systems must be matched for optimum efficiency. Contrast with “package unit”.

SRN – refer to “sound rating number” above.

Supply side – the part of an air conditioning and heating system that moves the conditioned air from the air handler or furnace to the rooms. The supply side should be balanced with the return side for optimum performance. Refer to “return side” above.

Thermostat — a control device that consists of a series of sensors and relays that monitor and activate the functions of an air conditioning and heating system. Generally, older thermostats are mechanical and more recent ones are electronic. See also “programmable thermostat”.

Ton — the unit of measurement for capacity of air conditioning systems. One ton of air conditioning removes 12,000 BTUs of heat per hour. See also BTU and BTHh above.

Variable-speed motor — in HVAC fans and blowers, a type motor that runs at a wide range of speeds rather than just one. When properly coordinated with other equipment such as the compressor and thermostat, variable speed fans circulate air at a flow that enhances temperature control, filtration, humidity control, and more, while reducing noise and energy consumption.

Vent — as a verb, to bring in fresh air or remove unwanted air – to ventilate. As a noun, the port where air enters or exits a duct, room, or area. Refer to “air vent”.

Ventilation — a mechanical system that brings in and circulates fresh air into a building. The letter “V” in the acronym “HVAC” stands for ventilation.


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