Air Conditioning Glossary

Absolute Humidity:  The amount of water vapor in a given amount of air space. Grains per cubic foot measure absolute humidity.

Accurator:  A piston type metering device that feeds the proper amount of liquid refrigerant into the evaporator.

Air Change:  The amount of air required to completely replace the air in a room or building; not to be confused with re-circulated air.

Air Conditioner:  A device or system which delivers simultaneous control of air temperature, relative humidity, purity, and motion.

Air-Cooled System:  A type of air conditioning system that uses Freon as a refrigerant and air as a condensing medium. A fan discharges heat from the condenser coil to the outdoors. Typically, the air-cooled condenser is located outside and refrigerant lines are piped to it from the indoor unit.

Air Diffuser:  An air distribution outlet or grille designed to direct airflow into desired patterns.

Air Handler:  The portion of the central air conditioning or heat pump system that forces heated or cooled air through your home’s ductwork. In some systems a furnace handles this function. Also known as a fan-coil.

Ambient Temperature:  The air temperature (usually the outdoor air temperature) surrounding the heating or cooling equipment.

ARI: Stands for Air-Conditioning and Refrigeration Institute, a non-profit, voluntary organization comprised of heating, air conditioning and refrigeration manufacturers. ARI publishes quality standards for testing and rating heat pumps and air conditioners to provide you with a standardized measure of comparison.

Atmospheric Pressure:  The weight of a 1 square inch column of the earth’s atmosphere. At sea level, atmospheric pressure is 14.696 pounds per square inch.

Auto Changeover:  A control package that allows you to automatically switch from a primary air conditioning system to a backup in the event of the primary system’s failure.

Balance Point:  An outdoor temperature, usually between 30° F and 45° F, at which a heat pump’s output exactly equals the heating needs of the home. Below the balance point, supplementary electric heat is needed to maintain indoor comfort.

Blower:  An air handling device for moving air in a distribution system.

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

Capacity:  The ability of a heating or cooling system to heat or cool a given amount of space. For heating, this is usually expressed in BTUs. For cooling, it is usually given in tons.

CFM:  Stands for Cubic Feet per Minute. Commonly used to measure the rate of air flow in an air conditioning system. The higher the number, the more air is being forced through the system.

Central Air Conditioner System:  System in which air is treated at a central location and carried to and from the rooms by one or more fans and a system of ducts.

Centrifugal Compressor:  A type of compressor used in vapor compression refrigeration cycles where a rotating impeller is the device which compresses the refrigerant vapor. The vapor is drawn into the impeller axially, and is discharged radically after energy is added to the vapor within the impeller.

Centigrade:  A temperature scale with the freezing point of water at 0 degrees and the boiling point at 100 degrees at sea level. Also known as the Celsius scale.

Charge:  The amount of refrigerant in a system.

Comfort Air Conditioning:  An A/C system designed for the comfort of people, not the protection of computer-based electrical systems. Computers and other high-tech equipment usually require an extremely low humidity environment — far lower than people require in their homes.

Compressor:   Part of a split-system heat pump or air conditioner’s outdoor unit that controls the pressure applied to the refrigerant so that it can circulate. It plays a vital role in taking in heat to warm your home with a heat pump or getting rid of heat to keep your home cool.

Compression:  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 (pounds per square inch).

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

Condenser:  A device that transfers unwanted heat out of a refrigeration system to a medium that absorbs the heat and transfers it to a disposal point. There are three types of condensers: air-cooled condensers, water-cooled condensers, and evaporative condensers. Most residential systems have an air-cooled condenser.

Condenser Coil:  A series of tubes filled with refrigerant, normally located outside the home that removes heat from the hot, gaseous refrigerant so that it becomes liquid again.

Condensing Unit:  Part of a refrigerating mechanism which pumps vaporized refrigerant from the evaporator, compresses it, liquefies it in the condenser and returns it to the refrigerant control. It’s the outdoor section of a split system air conditioner or heat pump.

Cooling Capacity:  A measure of the ability of a unit to remove heat from an enclosed space.

Cooling Load:  Heat which flows into a space from outdoors and/or indoors.

COP:  Stands for Coefficient of Performance. A ratio that compares a heat pump system’s heating efficiency to that of electric heat. For example, a heat pump system with a COP of 3.0 provides heat at 3 times the efficiency of electric heat. A heat pump’s system COP will decrease as outdoor temperatures drop, eventually providing little or no efficiency advantage over electric heat – and that’s the point when your auxiliary heat kicks in to heat your home.

Damper:  A valve or movable plate used in ductwork that opens or closes to control airflow. Used in zoning to regulate the amount of warm or cool air entering certain rooms in your home.

DB:  Stands for Decibels. A unit measuring the intensity of noise.

Dehumidification:  The reduction and removal of water vapor in air by cooling the air below the dew point.

Direct Expansion Systems:  One of two types of basic cooling media (the other is chilled water). Direct expansion systems utilize Freon for cooling and dehumidification. The three most common methods of heat rejection are air-cooled, water-cooled and glycol cooled.

Downflow Air Conditioner:  A type of precision air conditioning system that discharges air downward, directly beneath a raised floor. Commonly found in computer rooms and modern office spaces.

Duct:  A hollow pipe or closed conduit made of sheet metal, fiberglass board, or other suitable material used for carrying heated or cooled air to and from an air handling unit to the vents in your home.

EER:  Stands for Energy Efficiency Ratio — a measurement of the efficiency with which a product uses energy to function. It is calculated by dividing a product’s BTU output by its wattage.

Efficiency:  A rating on comfort equipment, similar to the miles per gallon rating on your car. The higher the rating, the more efficient the system and the lower your fuel consumption will be.

Electronic Air Cleaner:  An electronic device that filters out large particles and contaminants in indoor air. 

Emergency Heat:  The back-up heat built into a heat pump system. Also known as supplemental or auxiliary heat.

Energy Saver Switch: A switch that causes an air conditioner’s fan and compressor to cycle on and off together, reducing energy consumption.

ENERGY STAR:  A government program that helps businesses and individuals protect the environment through superior energy efficiency. Products earn the Energy Star rating by meeting strict anti-pollution and energy-efficiency guidelines set by the Environmental Protection Agency and the  Department of Energy. Products with the Energy Star rating will help you use less energy and save on utility bills.

Evaporator Coil:  Part of a split-system air conditioner or heat pump located inside the home. A series tubes filled with refrigerant cools and dehumidifies the air by converting liquid refrigerant into a gas (or vice-versa). A blower motor then moves air over the coil to either heat or cool your home. It is also known as a cooling coil, blower coil, chilling unit or indoor coil.

Fahrenheit:  The scale of temperature measurement most commonly used in the United States of America. In this scale, water freezes at 32 degrees and boils at 212 degrees.

Fan Coil:  An indoor component of an air conditioner or heat pump system, used in place of a furnace and evaporator coil, to provide change the refrigerant from a gas to a liquid (or vice-versa) and blow air over the coil to cool or heat your home.

Filter:  A device used to remove dust and other particles from air to provide healthier air quality and to protect the HVAC equipment. The higher the MERV rating, the better the filter.

Free Cooling System:  A water- or glycol-cooled system with an additional coil that provides chilled water-cooling when the outdoor ambient temperature is cold, thereby reducing or eliminating compressor operation. Provides efficient cooling in Northern climates.

Freon:  A general term used to identify, any of a group of partially or completely halogenated simple hydrocarbons containing fluorine, chlorine or bromine, which are used as refrigerants. R-22 is the most well-known and used of these refrigerants; it will be phased out in the coming years.

Glycol-Cooled System:  A type of air conditioning system that uses Freon as a refrigerant and a water/glycol solution as a condensing medium. Typically, the glycol-cooled condenser is located inside the air conditioner with the rest of the refrigeration components. The glycol keeps the solution from freezing during winter operation.

Ground Water-Source:  Water from an underground well is being used as the heat source or heat sink for a heat pump.

Heat Pump:  A unit that both cools and heats your home.  It works just like an air conditioner in cooling mode; however, in heating mode, the refrigerant flow is reversed and heat is extracted from the outside air too heat your home. A heat pump system can be either a split system or a packaged system.

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

Humidifier:  A piece of equipment that adds water vapor to heated air as it moves out of the furnace. This adds necessary moisture to protect your furnishings and reduce static electricity.

Humidistat:  A device designed to regulate humidity input by reacting to changes in the moisture content of the air.

HVAC:  Stands for Heating, Ventilating and Air Conditioning. The initials are often used to describe the industry that produces and services home comfort equipment.

Hybrid Heat System:  A fuel-saving alternative to traditional heating and cooling systems that combines a furnace with a heat pump. A Hybrid heat system reacts to changing temperatures and automatically adjusts to the most efficient energy saving method available to heat or cool a home. Hybrid Heat is adaptable for gas or electric systems and by using a variable speed fan hot or cool air is distributed consistently and evenly throughout a home.

Indoor Unit:  This is usually located inside the house and contains the indoor coil, fan, motor, and filtering device, sometimes called the air handler.

Insulation:  Any material that slows down the transfer of heat.

Integrally Controlled Motor:  A variable-speed motor that operates at low RPM when possible for maximum efficiency and quiet operation. ICM motors are more than 90% efficient versus 60% efficiency for conventional motors.

Isolation Valves:  Valves used for the transfer and isolation of refrigerant charge in the cooler or condenser, allowing refrigerant to be stored inside a chiller during servicing.

(K) Factor:  The insulating value of any material. Also known as conductivity.

Kilowatt (kW):  A measurement of the energy output of a device. Equal to 1,000 watts.

Kilowatt-Hour (kWh):  A common unit of electrical consumption measured by the total energy created by one kilowatt in one hour. 

Load Calculation: A series of measurements and studies used to analyze and determine the heating or cooling requirements of your home so that properly sized air conditioning and heating equipment may be installed. This calculation uses information such as the square footage of your home, window or door areas, insulation quality and local climate to determine the heating and cooling capacity needed by your furnace, heat pump or air conditioner.

Low Boy:  A type of furnace configuration in which the furnace is lower in height and occupies more floor space.

Matched System:  A heating and cooling system comprised of products that have been certified to perform at promised comfort and efficiency levels when used together, and used according to design and engineering specifications.

MERV:  Stands for Minimum Efficiency Reporting Value. A measurement of air filter’s ability to remove particles from 3 to 10 microns in size. The MERV scale ranges from 1 (least efficient) to 16 (most efficient).

Microprocessor Controls:  A air conditioning control system that uses computer logic to maintain precise control of temperature and humidity and to monitor the unit’s operation.

Outdoor Coil/Condensing Unit:  The portion of a heat pump or central air conditioning system that is located outside the home and functions as a heat transfer point for collecting heat from and dispelling heat to the outside air.

Packaged System:  A self-contained heating and/or air conditioning system which has all components located in one cabinet. It is placed outside the home – on the ground, on the roof, or sometimes mounted through the walls of the building.

Payback Analysis:  An overall measure of the efficiency and value of your home comfort system. By combining your purchase price and ongoing operating costs, a payback analysis determines the number of years required before monthly energy savings offset the purchase price.

Purge Device:  A device which removes air and water vapor from the refrigerant inside a chiller.

Puron:  An environmentally sound refrigerant designed not to harm the earth’s ozone layer. Federal law requires that all manufacturers phase out ozone-depleting refrigerants in the next few years. Puron Refrigerant has been approved by the US Environmental Protection Agency as a replacement for Freon R-22 and other ozone depleting refrigerants.

R-22 Refrigerant:  An ozone-depleting, hydro chlorofluorocarbon (HCFC) refrigerant that has been the refrigerant of choice for residential heat pump and air-conditioning systems for decades. Because of its potentially harmful environmental effects, production of R-22 and systems that use it are being phased out and will cease in 2015.

Reciprocating Compressor:  A type of compressor used in air conditioners that compresses refrigerant through a piston action.

Recycling:  The removing, cleaning and reusing of refrigerant.

Refrigerant:  A substance that produces a re-cooling effect while expanding or vaporizing.

Refrigerant Lines:  Two copper lines that connect an air conditioning system’s condenser coil (the outdoor unit) to the evaporator coil (the indoor unit).

Register:  A 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 greatest amount the air could hold at that temperature. Normally 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 is switched from cooling to heating.

Scroll Compressor:  A specially designed compressor that works in a circular motion vs. an up and down piston action.

SEER:  Stands for Seasonal Energy Efficiency Ratio. A rating that denotes the efficiency of air conditioning equipment. It is the amount of cooling your equipment delivers for every dollar spent on electricity. It is the ratio of cooling delivered by a system (measured in BTUs) to the dollar cost of the electricity to run the system (measured in, as measured in watt-hours.  The higher the SEER, the more efficient the unit. The more efficient the unit, the lower the operating cost. The U.S. Government’s minimum SEER is 10 for split systems and 9.7 for packaged units.

Setback Thermostat:  A state-of-the-art electronic thermostat with a built-in memory that can be programmed for different temperature settings at different times of the day.

Single Package Product:  A year-round heating and air conditioning system that has all of the components completely encased in one unit outside the home.

Split System:  A central air conditioning or heat pump system consisting of two or more major components. The system usually consists of a compressor-containing unit and condenser (installed outside the house) and a non-compressor-containing air handling unit (installed within the building). This is the most common type of system installed in a home.

Supercooled Liquid:  Liquid refrigerant cooled below its saturation point.

Subcooler: This is a section of some condensers in which the temperature of the condensed refrigerant liquid is reduced. This improves the energy efficiency of the chiller.

Superheating:  Creating a rise in temperature by adding heat energy to a refrigeration vapor.

Switchover:  A device in a heat pump that reverses the flow of refrigerant as the system is switched from cooling to heating.

Temperature:  The measure of the intensity of heat that a substance possesses.

Thermidistat:  A sophisticated, programmable thermostat that senses the outdoor temperature, indoor air temperature, and indoor relative humidity. A built-in microprocessor determines the most efficient way to achieve ideal comfort.

Thermostat:  A temperature control device, typically found on a wall inside that consists of a series of sensors and relays that monitor and control the functions of a heating and cooling system.

Thermostatic Expansion Valve:  A precision device used to meter the flow of liquid refrigerant entering the evaporator at a rate that matches the amount of refrigerant being boiled off in the evaporator.

Ton (or Tonnage):  Not 2000 lbs, but a unit of measure used to describe the cooling capacity of an air conditioning system. One ton of cooling is based on the amount of heat needed to melt one ton (2000 lbs.) of ice in a 24 hour period (equal to 12,000 BTUh).

Two Stage Compressor:  Two Stage Compressors are capable of two levels of operation, a low stage and a high stage. Properly sized equipment will operate 80% of the time in low stage, enhancing efficiency and comfort with lower humidity levels and quieter operation. It’s like getting two air conditioners or heat pumps in one system.

UL:  Stands for Underwriters Laboratories, an impartial, non-profit organization that tests and rates electrical products for public safety.

Upflow Air Conditioner:  A type of air conditioning system that discharges air into the conditioned space through a top-mounted discharge plenum or an overhead duct system.

Vacuum Pump:  A pump used to remove air and moisture from a refrigeration system.

Vapor Seal:  A barrier that prevents air, moisture, and contaminants from migrating through tiny cracks or pores in the walls, floor, and ceiling into a room. Vapor seals may be created using plastic film, vapor-retardant paint, vinyl wall coverings and vinyl floor systems.

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.

Ventilator:  A devices that captures heating or cooling energy from stale indoor air and transfers it to fresh incoming air.

Water Cooled System:  A type of air conditioning system that uses freon as a refrigerant and water as a condensing medium. Typically, the water-cooled condenser is located inside the air conditioner with the rest of the refrigeration components. Water is piped to the unit from an external source.

Zoning:  A way to increase your home comfort and energy efficiency by controlling when and where heating and cooling occurs. Programmable thermostats control the timing of when your equipment operates; dampers are used to direct air flow to certain sections or “zones” of your home.


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