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Heating and Air Conditioning Glossary

Below are some commonly used Heating and Air Conditioning terminologies 

Air Change:

An Air change refers to  replacing air in a room, building or assigned space.. Air changes in a confined space are important for a variety of reasons, mainly though, we need fresh air to live. Without sufficient fresh air exchange, moisture can be trapped, molds can feed, and other allergens, excessive gases and toxic chemicals or residues can accumulate.

Air Conditioner:

An Air Conditioner often referred to as an AC is mechanical equipment designed to absorb heat from an area and reject it to an other area. Cooling is typically accomplished by using a refrigeration cycle and heating can be accomplished with the refrigeration cycle, gas furnace, electric furnace or oil furnace.

Air Cooled:

Air cooling is a method of dissipating heat. It works by making the object to be cooled have a larger surface area or have an increased flow of air over its surface, or both. An example of the former is to add fins to the surface of the object, either by making them integral or by attaching them tightly to the object’s surface to ensure efficient heat transfer. In the case of the latter it is done by using a fan blowing air into or onto the object one wants to cool. In many cases the addition of fins adds to the total surface area making a heatsink that makes for greater efficiency in cooling.

Air Diffuser:

Air diffusers have one of the most important functions in air management; to control the pattern of the airflow into an assigned space.

Air Flow:

The movement of air through an Air Conditioning System and the through space to be conditioned.

Air Handler:

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


Annual Fuel Utilization EfficiencyA rating that denotes the efficiency of gas heating equipment. It is the percentage of heat efficiently transferred from fuel being combusted. A higher rating indicates more efficient equipment. 


Air Conditioning And Refrigeration Institute which is a non-profit, voluntary organization comprised of heating, air conditioning and refrigeration manufacturers. ARI publishes standards for testing and rating heat pumps and air conditioners to provide a standardized measure of comparison for capacity and efficiency.

Auto Changeover:

A control or thermostat that provides for automatic switching of an air conditioning system from heating to cooling.


An air handling device for moving air through a HVAC system.


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 Cooling and Heating, this can be expressed in BTU’s.

Carbon Monoxide:

Carbon Monoxide (CO) is a colorless, odorless gas produced by burning any fuel. CO is poisonous and deadly. Some of the symptoms of CO poisoning are similar to those of the flu: headaches, fatigue, shortness of breath, nausea and dizziness. For safety Carbon Monoxide Alarms should always be installed whenever a gas fired appliance is used or installed.

Central Air Conditioning 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 radially after energy is added to the vapor within the impeller.


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 quantity of refrigerant required for proper system operation.

Chilled Water System:

A type of air conditioning system that has no refrigerant in the unit itself. The refrigerant is contained in a chiller, which is located remotely. The chiller cools water, which is piped to the air conditioner to cool the space.

Comfort Air Conditioning:

Comfort air conditioning systems are designed for the comfort of people, not the protection of computer-based electrical systems. Unlike people, computers generate dry (sensible) heat, but not humidity. Only about 60-70% of a comfort system’s total capacity is dedicated to the removal of sensible heat, while 30-40% is for dehumidification. With a large percentage of their total capacity devoted to the removal of moisture, comfort systems can lower room humidity far below acceptable standards. A larger comfort system is required to obtain the same sensible capacity as a precision cooling system.


In refrigeration usually an electrically powered  pump used to pump refrigerant from a condensing unit through an evaporator coil.  


A device that transfers and rejects heat from a refrigeration system. Two common types of condensers are air-cooled condensers and water-cooled condensers. Most residential systems have an air-cooled condenser.

Condensing Unit:

Part of a refrigeration system which pumps refrigerant through the system. The outdoor portion of a split system that contains the compressor and condenser coil. The condensing unit typically also contains various controls required for its operation.

Cooling Capacity:

A measure of the ability of a unit to remove and transfer heat. Cooling capacity is commonly measured in BTU’s or Tons. One ton equals 12,000 BTU’s.

Cooling Load:

The cooling load calculation method was first introduced in the 1979 ASHRAE Cooling and Heating Load Manual. This method is regarded as a reasonably accurate approximation of the total heat gains through a building envelope for the purposes of sizing HVAC equipment.


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

Defrost Cycle:

A process used in heat pump units for removing ice or frost buildup from the outdoor coil.


The reduction of water vapor in air by cooling the air below the dew point, causing moisture to condense on the evaporator coil.

Direct Vent:

Pulling outside air in for combustion and  directly venting combustion gases to the outside.


Refers to a type of HVAC system that discharges air downward.

Downflow Furnace:

A furnace that  returns air from the top and supplies air through the bottom. 


A  conduit made of sheet metal, fiberglass board, flexible hose, or other suitable material used for transfering air to and from an air handling unit.


A Direct Expansion system typically uses a metering device such as a thermostatic or electronic expansion valve.


electronically commutated motors (ECMs, EC motors) are electric motors powered by direct-current (DC) electricity and having electronic commutation systems, rather than mechanical commutators and brushes. The current-to-torque and frequency-to-speed relationships of BLDC motors are linear.


In Ventilating systems it is a device that controls a damper to allow outside air to enter the return air system when temperatures are adequate for reducing operational costs. In a chiller it is a refrigeration device for increasing system efficiency.

Electronic Air Cleaner:

An electronic device that filters out large particles and contaminants in indoor air. It then electronically pulls out tiny particles that have been magnetized, such as viruses and bacteria, drawing them to a collector plate.

Emergency Heat:

Can be supplemental or auxiliary heat used for second stage heat for a heat pump system.

Energy Efficiency Ratio:

EER, 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.


A heat transfer device, it absorbs heat from the surrounding air or liquid and moves it by means of a refrigerant to be rejected elsewhere. It is also known as an evap coil, cooling coil, blower coil, chilling unit or indoor coil. For air cooling or heating purposes it is usually a series or network of tubes with heat sink fins attached to the surface, resembling a radiator.


A device used to remove dust, particles, and contaminents from air for the purposes of providing clean and healthy air and to protect the HVAC equipment. Filters vary greatly in particle arrestance; 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.


That part of a HVAC system which uses gas, oil, electricity or other fuels for combustion and generation of heat.

Glycol-Cooled System:

A type of air conditioning system that uses  a water/glycol solution for transferring heat. 

Ground Water-Source:

Water from a well to be used for heat transfer, as a heat source or for heat rejection.

Heat Exchanger:

A device for the transfer of heat energy from the source to the conveying medium.

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 Rejection:

The three most common methods of heat rejection are air cooled, water cooled and glycol cooled.

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).

Horizontal Furnace:

A furnace that lies on its side,  returning air from one side and supplying air to the other.


Heating Seasonal Performance Factor, is 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.


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.


Heating, Ventilation, and Air Conditioning.

Indoor Unit:

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

Indoor Coil:

See Evaporator.

Induced Draft Furnace:

A furnaces that  draws combusted gases and heat through the heat exchanger through use of a draft inducer motor and assembly.


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


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

Latent Cooling Capacity:

An A/C system’s capability to remove moisture from the air.

Latent Heat:

The heat energy needed to change the state of a substance (i.e.: from a liquid to a gas) but not it’s temperature.

Matched System:

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

Microprocessor Controls:

An electronic control system that operates and monitors an air conditioning system. Microprocessor controls are commonly used on modern precision air conditioning systems to maintain precise control of temperature and humidity and to monitor the units operation.

Natural-Draft Furnace:

A furnace in which the natural flow of air from around the furnace provides the air to support combustion. It also depends on the pressure created by the heat in the flue gases to force them out through the vent system.

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  in commercial and residential applications, the package unit is installed either beside or on top of a building or home.

Packaged Unit:

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

Precision Air Conditioning:

Precision A/C systems are primarily designed for cooling electronic equipment. These systems maintain precise temperature and humidity for protecting mission-critical applications from even the slightest increase in temperature.

Reciprocating Compressor:

A reciprocating compressor or piston compressor is a positive-displacement compressor that uses pistons driven by a crankshaft to deliver gases at high pressure.


A refrigerant is a substance used in a heat cycle usually including, for enhanced efficiency, a reversible phase change from a liquid to a gas. Traditionally, fluorocarbons, especially chlorofluorocarbons, were used as refrigerants, but they are being phased out because of their ozone depletion effects. Other common refrigerants used in various applications are ammonia, sulfur dioxide, and non-halogenated hydrocarbons such as methane.

Refrigerant Line set:

Usually two insulated copper tubes that connect an outdoor condensing unit to an indoor evaporator coil.


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

Return Air:

Air drawn into a HVAC system for conditioning and supply to the space to be conditioned.

Reversing Valve:

A device in a heat pump that reverses the flow of refrigerant in a system to be used for cooling or heating.

Screw Compressor:

A rotary screw compressor uses two meshing helical screws, known as rotors, to compress the gas. In a dry running rotary screw compressor, timing gears ensure that the male and female rotors maintain precise alignment. In an oil-flooded rotary screw compressor, lubricating oil bridges the space between the rotors, both providing a hydraulic seal and transferring mechanical energy between the driving and driven rotor. Gas enters at the suction side and moves through the threads as the screws rotate. The meshing rotors force the gas through the compressor, and the gas exits at the end of the screws

Scroll Compressor:

A scroll compressor is a device for compressing refrigerant. It is used in air conditioning equipment, a scroll compressor uses two interleaving scrolls to pump, compress or pressurize refrigerants.


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.

Sensible Cooling Capacity:

An A/C system’s capability to remove heat from the air.

Sensible Heat:

Heat energy that causes a rise or fall in the temperature of a gas, liquid or solid when added or removed from that material. Sensible heat changes the temperature by changing the speed at which the molecules move.


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

Single Package:

A heating and air conditioning system that has all components completely encased in one unit.

Split System:

A refrigeration or HVAC system consisting of two or more major components. The system usually consists of a condensing unit containing a condenser coil, compressor, and controls installed outside the building and a air handling unit containing a blower assembly, evaporator coil, and controls installed within the building.

Supercooled Liquid:

Liquid refrigerant cooled below its saturation point.


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.


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 is usually a second stage of a heating system used to aid in conditioning a space when outside ambient temperatures are low. The suplemental heating can be acheived by using electrical resistance heat strips, gas-fired heating, or oil-fired heating.


A physical property of matter that quantitatively expresses the common notions of hot and cold. Objects of low temperature are cold, while various degrees of higher temperatures are referred to as warm or hot.


A temperature control device used to control a refrigeration or heating system.


The unit of measure used in refrigeration to describe the cooling capacity of a system. One ton of capacity 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.


Designation of the type of air flow that an air conditioning system is designed for. Typically this system uses a top-mounted discharge plenum and / or an overhead duct system.

Upflow Furnace:

A furnace that supplies air through the top and returns air from the bottom.

Vacuum Pump:

An electrically powered pump used to remove air, moisture, contamination and non-condensables from a refrigeration system.


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.


A device used to move air through or within a structure. A ventilator can remove stale indoor air and make it up with fresh incoming air.

Water Cooled System:

A type of air conditioning system that uses freon as a refrigerant and water to reject heat from the system. 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 a cooling tower or other suitable source.

Water Source:

Water being used as a medium for heat transfer. Sources include: wells, lakes, ponds,rivers, swimming pools, and city water supply.

Zone System:

A control and actuator system used in Air Conditioning for dividing and controlling the air flow and temperature of different areas.  


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.

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