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Glossary of Heating, Ventilation and Air Conditioning Terms

Air Conditioner

A system or an assembly comprised of certain system components which are designed for the control of air temperature, relative humidity and air flow in a living or working space.

Air Cooled System

A type of air conditioning system that uses air as a condensing medium and R-22 or R-410A as a refrigerant. In most air cooled systems, the condenser is located outside and the refrigerant is piped to it from the indoor unit. In air conditioning, the heat from the indoor space is transferred to the outside air. In a heat pump, the heat is drawn from the outdoor air and is used to heat the indoor air.

Air Diffuser

An air distribution outlet or grille that directs airflow into desired patterns.

Air Handling Unit (or Air Handler)

The portion of the central air conditioning system that moves heated or cooled air throughout a home’s ducts, though it does not include the ductwork. Typically it is located inside and includes a blower, dampers and other equipment in direct contact with air flow.

Annual Fuel Utilization Efficiency (AFUE)

A rating that measures the amount of heat your equipment delivers for every dollar spent on fuel. A higher rating indicates more efficient equipment.

Blower

A piece of equipment designed to move air through a system. Usually refers to the air handling unit or air handler.

BTU (British Thermal Unit)

The standard of measurement used for the amount of heat required to raise the temperature of one pound of water by one degree (Fahrenheit). BTUH = the number of BTUs per hour.

Central Air Conditioner System

A system that treats air at a central location and carries it to and from the rooms by one or more fans and a system of ducts.

Charge

The amount of refrigerant in a system.

Coil (Indoor Coil)

Equipment that enables heat transfer to and from the refrigerant and the air when installed inside an air handling unit. The indoor coil often features two rectangular coil surfaces connected at the top and open at the bottom. From the side, this configuration looks like the letter "A."

Compressor

The ‘pump’ that circulates vapor refrigerant throughout the system from the indoor evaporator to the outdoor condenser and back.

Condenser Coil

A series of tubes filled with gas (vapor) refrigerant that carries heat from the home and removes it outdoors allowing the refrigerant to condense or liquefy and start the process again.

Condensing Unit

A device that condenses a substance from a gaseous to a liquid state, typically by cooling it. In the process the latent heat is given up by the substance and will transfer to the condenser coolant.

Cooling Capacity

A measure of a unit’s ability to remove heat from an enclosed space. The COP, or Coefficient of Performance, of a heat pump measures the ratio of the rate of useful heat output that the pump delivers (excluding supplementary heating) to the corresponding rate of energy input.

Damper

Found in ductwork, this movable plate opens and closes to control airflow and is used in zoning to regulate airflow to certain rooms.

Defrost Cycle

The removal of ice or frost buildup from the outdoor coil during the heating season.

Dehumidification

The reduction of water vapor by cooling the air below the dew point, as well as the removal of water vapor from air by chemical means, refrigeration, etc.

Downflow Furnace

Draws in return air from the top and expels warm air at the bottom.

Dual Fuel System

A heating solution that combines a furnace and a heat pump to provide an economical way to heat a home. A heat pump is activated for moderate heating needs and a furnace is switched on when higher levels of heat are needed. This system helps maximize the energy efficiency of each unit.

Duct

Any pipe or closed chamber, usually made of sheet metal or fiberglass, that is used for housing and conducting air flow from an air handling unit to the conditioned space.

Ductless Mini-Split

A type of air conditioning system that does not use ducts to transfer cool air and instead the outdoor condenser unit is connected directly to an interior air handler. Because of this direct connection, generally only one room or space can be cooled at a time making ductless mini-splits a viable option for room additions or add-ons, but potentially troublesome for whole-home solutions.

Ductwork

Building-wide air delivery conducted through pipes or channels.

Efficiency

A fuel-efficiency rating similar to the miles-per-gallon rating on your car.

Emergency Heat (Supplemental or Auxiliary Heat)

The backup heat built into a heat pump system.

Energy Efficiency Ratio (EER)

The ratio of an air conditioning unit’s cooling capacity in BTUs per hour to the total electrical energy consumed in watts.

Evaporator

Absorbs heat or liquid from the surrounding air and moves it outside the refrigerated area by means of a refrigerant. Also known as a cooling coil, blower coil, chilling unit or indoor coil.

Evaporator Coil

Located inside the home, a series or network of tubes filled with refrigerant that remove heat and moisture from indoor air as liquid refrigerant evaporates.

Filter

A device that removes dust and other air particles to comfort the respiratory system and protect the heating and cooling equipment. The higher the MERV rating, the better the filter.

Freon

R22 refrigerant, also known as Freon, has been the HVAC industry standard refrigerant used in the manufacture of central air conditioning systems.

Furnace

The part of an environmental system that converts gas, oil, electricity or another fuel into heat for distribution within a structure.

Heat Exchanger

Transfers heat energy from its source to the conveying medium.

Heat Gain

As measured in BTUs, the amount of heat gained from a space to be conditioned, at the local summer outdoor design temperature and specified indoor design condition.

Heat Loss

As measured in BTUs, the amount of heat lost from a space to be conditioned, at the local summer outdoor design temperature and specified indoor design condition.

Heat Pump

An air conditioner with a valve that permits alternate heating and cooling.

Heat Source

A body of air or liquid from which heat is collected. With any heat pump, the air outside the home is the source for the heating cycle.

Heat Transfer

The movement of heat from one place to another, between two substances, or within a substance.

Heating Capacity

The rate at which a specific device can add substantial heat to a substance, expressed in BTUH (British Thermal Units per hour).

Heating Seasonal Performance Factor (HSPF)

The total heating output of a heat pump in BTUs during its normal usage period for heating, as divided by the total electrical energy input in watt-hours during the same period.

HEPA Filter

 Refers to a filter that is manufactured, tested, certified, and labeled in accordance with current HEPA filter standards.

Horizontal Furnace

A sideways furnace that draws in return air from one side and expels warm air from the other.

Humidification

The process of adding moisture to the air within a space.

Humidistat

Regulates humidity input by reacting to moisture content changes in the air.

Humidity

The amount of moisture in the air. Air conditioners remove moisture for added comfort.

HVAC

Heating, Ventilation and Air Conditioning.

Indoor Unit

Usually located inside the house, it houses the indoor coil, fan, motor and filtering device, sometimes called the air handler.

Indoor Coil

A refrigerant containing a portion of a fan coil unit similar to a car radiator, typically made of several rows of copper tubing with aluminum fins.

Infiltration

Airflow into a space usually through walls and leaks around doors and windows.

Insulation

Any material that reduces the speed of heat transfer.

Kilowatt (kW)

Equal to 1,000 watts of electricity, a kilowatt-hour (kWh) is a common unit of electrical consumption measured by the total energy that one kilowatt creates in an hour.

Latent Cooling Capacity

An air conditioning system’s capability to remove moisture from the air.

Latent Heat

The heat energy needed to change the state of a substance (e.g., from a liquid to a gas), but not its temperature.

Load Calculation

Determines a building’s heat gain and loss to ensure installation of properly sized air conditioning and heating equipment.

Matched System

A heating and cooling system consisting of products certified to perform at promised comfort and efficiency levels when used together, in accordance with design and engineering specifications.

Operating Cost

The day-to-day cost of running your home comfort equipment, based on daily energy use.

Outdoor Coil/Condensing Unit

The portion of a heat pump or central air conditioning system located outside the home. It functions as a heat transfer point for collecting heat from and dispelling heat to the outside air.

Packaged Unit

All-in-one heating and cooling system for homes that don’t have a lot of room indoors for either a furnace and coil, or an air handler.

Payback Analysis

A general measure of your home comfort system’s efficiency and value. By combining your purchase price with ongoing operating costs, it determines the number of years required before monthly energy savings offset the purchase price.

Refrigerant

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

Refrigerant Lines

Set of two copper lines connecting the outdoor unit and the indoor unit.

Register

Combination grille and damper assembly covering an air opening or end of an air duct.

Relative Humidity

The ratio of the amount of vapor contained in the air to the maximum amount the air could hold at that temperature, usually expressed as a percentage.

Return Air

Air drawn into a heating unit after having been circulated from the heater’s output supply to a room.

Reversing Valve

A device in a heat pump that reverses the flow of refrigerant as the system switches from cooling to heating.

SEER (Seasonal Energy Efficiency Ratio)

A rating that denotes the efficiency of air conditioning equipment in terms of the amount of cooling your equipment delivers for every dollar spent on electricity. The higher the SEER, the more efficient the unit and the lower the operating cost.

Sensible Cooling Capacity

An air conditioning system’s capability to reduce the temperature by removing heat from the air.

Sensible Heat

Heat energy that raises or lowers the temperature of a gas, liquid or solid when added or removed from that material.

Setpoint

The temperature at which a thermostat is set for desired comfort level.

Single Package

A year-round heating and air conditioning system with all components encased in one unit outside the home.

Split System

The most common type of home central air conditioner, it consists of a compressor (the unit and condenser, installed outside the building) and a non-compressor (the air-handling unit installed within the building).

Supplementary Heat

Auxiliary or emergency heat, usually electrical resistance heat, provided at temperatures below a heat pump’s balance point.

Thermostat

A temperature control device, typically found on an inside wall, that consists of a series of sensors and relays for monitoring and controlling a heating and cooling system.

Tonnage

The unit of measure used in air conditioning to describe the cooling capacity of a system. One ton of cooling is based on the amount of heat needed to melt one ton (2,000 lbs.) of ice in a 24-hour period, and equals 12,000 BTUH.

Upflow

A type of air conditioning system that discharges air into the conditioned space via a top mounted discharge plenum or through an overhead duct system.

Upflow Furnace

A furnace that pulls return air in from the bottom and expels warm air from the top.

Vapor Barrier

A moisture-resistant layer applied to the surfaces of humid spaces that prevents moisture from traveling to a point where it might condense due to lower temperature.

Vapor Seal

A barrier essential to prevent moisture from infiltrating into, or migrating from, a data processing center or other "critical space" that contains sensitive electronic instrumentation. Vapor barriers may be created using plastic film, vapor-retardant paint, vinyl wall coverings and vinyl floor systems, in combination with careful sealing of all openings (doors and windows) into the room.

Ventilation

The process of supplying or removing air, by natural or mechanical means, to or from any space. Such air may or may not have been conditioned.

Watt

A unit of power that equals one joule per second. 

Zone System

A method of dividing a home into zones that makes it possible to control the amount of comfort provided to each.

Zoning

The practice of providing independent heating and/or cooling to different areas within a structure.

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