Air Flow:

The distribution or movement of air.

Air Handler:

The portion of the central air conditioning or heat pump system that moves heated or cooled air throughout a home’s ductwork. In some systems a furnace handles this function.




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 resistance heat is needed to maintain indoor comfort.


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

BTU (British Thermal Unit):

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


The abbreviation for British thermal units per hour. The amount of heat required to raise one pound of water one degree Fahrenheit per hour, a common measure of heat transfer rate.





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

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.

CFM (Cubic Feet per Minute):

The abbreviation for cubic feet per minute, commonly used to measure the rate of air flow in an air conditioning system.


The pump that moves the refrigerant from the indoor evaporator to the outdoor condenser and back to the evaporator again. The compressor is often called “the heart of the system” because it circulates the refrigerant through the loop.


The process by which a gas is changed into a liquid at constant temperature by heat removal.


A device that transfers unwanted heat out of a refrigeration system to a medium (either air, water, or a combination of air and water) 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. The evaporative condenser uses a combination of air and water as its condensing medium. Most residential systems have an air-cooled condenser.

Condenser Coil:

A series or network of tubes filled with refrigerant, normally located outside the home, that removes heat from the hot, gaseous refrigerant so that the refrigerant 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. The outdoor portion of a split system air conditioner contains the compressor and outdoor coil ignoring the reverse cycle operation, also the outdoor in a heat pump system.

COP (Coefficient of Performance):

This is a measure of the energy efficiency of a chiller.

Cooling Capacity:

A measure of the ability of a unit to remove heat from an enclosed space. COP - Coefficient of Performance of a heat pump means the ratio of the rate of useful heat output delivered by the complete heat pump unit (exclusive of supplementary heating) to the corresponding rate of energy input, in consistent units and under operating conditions.




Defrost Cycle:

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


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

Direct Vent:

Pulls outside air for combustion and vents combustion gases directly outside.


Refers to 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.


A pipe or closed conduit made of sheet metal, fiberglass board, or other suitable material used for conducting air to and from an air handling unit.


Pipes or channels that carry air throughout a building.





Energy Efficiency Ratio (EER):

Means the ratio of the cooling capacity of the air conditioner in British Thermal Units per hour, to the total electrical input in watts under ARI-specified test conditions.


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

Evaporator Coil:

A series or network of tubes filled with refrigerant located inside the home that take heat and moisture out of indoor air as liquid refrigerant evaporates.




Fahrenheit (Represented as degrees “F”):

The scale of temperature measurement most commonly used in the United States of America.


A device used to remove dust and other particles from air for the purposes of reducing the load on the respiratory system and to protect the HVAC equipment. Filters vary greatly in particle resistance; the higher the MERV rating, the better the filter.


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.




Heat Gain:

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

Heat Loss:

The amount of heat lost, measured in BTU’s from a space to be conditioned, at the local winter outdoor design temperature and a specified indoor design condition.

Heat Pump:

An air conditioner that contains a valve that allows it to alternate between heating and cooling.

Heat Source:

A body of air or liquid from which heat is collected. With any heat pumps, the air outside the home is used as the heat source during 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):

means the total heating output of a heat pump in British Thermal Units during its normal usage period for heating divided by the total electrical energy input in watt-hours during the same period.


Heating, Ventilation, and Air Conditioning.


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


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


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




Indoor Unit:

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




Kilowatt (kW):

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.





An abbreviation for a screen type - Liquid Crystal Display.

Load Calculation:

A mathematical design tool used to determine the heat gain and heat loss in a building so that properly sized air conditioning and heating equipment may be installed.




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 that is located outside the home and functions as a heat transfer point for collecting heat from and dispelling heat to the outside air.




Package System:

A piece of air conditioning and heating equipment where all components are located in one cabinet. Used occasionally in residential applications, the package unit is installed either beside or on top of the home.

Packaged Unit:

A self-contained heating and/or air conditioning system.





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.

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.

Reversing Valve:

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




SEER (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, as measured in watt-hours. This ratio is determined using specified federal test procedures. The higher the SEER, the more efficient the unit. The more efficient the unit, the lower the operating cost.

Split System:

A central air conditioner consisting of two or more major components. The system usually consists of a compressor-containing unit and condenser, installed outside the building and a non-compressor - containing air handling unit installed within the building. This is the most common type of system installed in a home.


Creating a drop in temperature by removing sensible heat from a refrigerant liquid.

Superheated Vapor:

Refrigerant vapor heated beyond its saturation point.


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

Supplementary Heat:

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






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.


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 (2000 lbs.) of ice in a 24 hour period. One ton of cooling is equal to 12,000 Btu/hr.





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.




Vacuum Pump:

A pump used to remove air and moisture from a refrigeration system at a pressure below atmospheric pressure.

Vapor Barrier:

A moisture-impervious layer applied to the surfaces enclosing a humid space to prevent moisture travel to a point where it may condense due to lower temperature.

Vapor Seal:

A vapor seal is an essential part of preventing moisture infiltration into or migration out of a critical space, such as a data processing center or other room that contains sensitive electronic instrumentation. Essentially, a vapor seal is a barrier that prevents air, moisture, and contaminants from migrating through tiny cracks or pores in the walls, floor, and ceiling into the critical space. 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.





A unit of power that equals one joule per second. Named after James Watt.




Zone System:

A method of dividing a home into zones and enabling you to control the amount of comfort provided to each.


The practice of providing independent heating and/or cooling to different areas in a structure. Zoning typically utilizes a system controller, zoning dampers controlled by a thermostat in each zone, and a bypass damper to regulate static pressure in the supply duct.