Dictionary of Termsdictionary4

“Understand Your HVAC”






































Absolute Humidity
The weight of water vapor in a given amount of air space. Absolute humidity is measured by grains per cubic foot.

Absorption Cycle
Absorption chillers differ from mechanical vapor compression chillers in that they utilize a thermal or chemical process to produce the refrigeration effect necessary to provide chilled water. There is no mechanical compression of the refrigerant taking place within the machine as occurs within more traditional vapor compression type chillers. Most commercial absorption chillers utilize lithium bromide (a salt) and water as the fluid pair.

The metering (flow control) device inside a Carrier centrifugal chiller. Its unique design always feeds the cooler with liquid refrigerant, which has a much greater cooling capability than a gaseous refrigerant.

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

Air Cooled
Uses a fan to discharge heat from the condenser coil to the outdoors.

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

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

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.

Annual Fuel Utilization Efficiency(AFUE)
A rating that denotes the efficiency of gas heating equipment. It is the amount of heating your equipment delivers for every dollar spent on fuel. A higher rating indicates more efficient equipment. This rating is calculated in accordance with the Department of Energy test procedures.

Air-Conditioning and Refrigeration Institute 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 you with a standardized measure of comparison. So, ARI ensures a level of quality within the industry.

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

Auto Changeover
A control package that provides for automatic switching from a primary air conditioning system to a backup in the event of a failure of the primary system.




British Thermal Unit(BTU)
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.

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.

Two metals with different rates of expansion fastened together. When heated or cooled they will warp and can be made to open or close a switch or valve.

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

British Thermal Unit(BTU)
The temperature at which the addition of any heat will begin a change of state from a liquid to a vapor.

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.

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.
headaches, fatigue, shortness of breath, nausea and dizziness. The Consumer Product Safety Commission (CPSC) recommends a yearly, professional inspection.

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 radially after energy is added to the vapor within the impeller.

Centigrade (Measure of Temperature)
A temperature scale with the freezing point of water 00 and the boiling point 1000 at sea level.

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.

Amount of refrigerant in a system.

Chilled Water System
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.

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 reduction of volume of a vapor or gas by mechanical means.

Compression Ratio
The ratio determined by dividing the discharge pressure, in PSI, by the suction pressure in PSI.

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

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 (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 Anticipator
A resistance heater (usually not adjustable) in parallel with the cooling circuit. It is ‘on’ when the current is ‘off”, adding heat to shorten the off cycle.

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.

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




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
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.

Department of Energy (DOE)
A federal agency that sets industry efficiency standards and monitors the use of various energy sources.

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.

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.

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

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.




In a chiller with a two-stage centrifugal compressor, the discharge from the first stage impeller and the inlet to the second stage impeller are at a pressure level approximately half way between the cooler pressure and condenser pressure. With this arrangement, an economizer may be used. This is a shell within which refrigerant liquid from the condenser drops down to the interstage pressure, flashing off some of the refrigerant which is drawn directly into the second stage impeller. This reduces the amount of refrigerant which has to be compressed by the first stage impeller, improving the refrigeration cycle efficiency.

A rating on comfort equipment is similar to the miles per gallon rating on your car. The higher the rating number, 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. It then electronically pulls out tiny particles that have been magnetized, such as viruses and bacteria, drawing them to a collector plate.

Emergency Heat (Supplemental or Auxillary Heat)
The back-up heat built into a heat pump system.

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.

Heat content or total heat, including both sensible and latent heat. The amount of heat contained in a refrigerant at any given temperature with reference to -40°F.

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 takes heat and moisture out of indoor air as liquid refrigerant evaporates.




The insulating value of any material. Also known as conductivity.

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 arrestance; the higher the MERV rating, the better the filter.

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

Free Delivery
There are no ducts and the unit may be installed in the field without ducts if needed. Ground-Source – The ground or soil below the frost line is being used as the heat source or heat sink for a heat pump.

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 an environmental system which converts gas, oil, electricity or other fuel into heat for distribution within a structure.




In an absorption cycle, the vessel in which the lithium bromide solution is reconcentrated by boiling off the previously absorbed water.

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. Water/glycol is piped to the unit from a dry-cooler or other suitable source. 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.




heating, ventilation, and air conditioning) is a major sub-discipline of mechanical engineering. The goal of HVAC design is to balance indoor environmental comfort with other factors such as installation cost, ease of maintenance, and energy efficiency. The discipline of HVAC includes a large number of specialized terms and acronyms, many of which are summarized in this glossary.

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 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.

Horizontal Furnace
A furnace that lies on its side, pulling in return air from one side and expelling warm air from the other.

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.

Indoor Coil
Refrigerant containing portion of a fan coil unit similar to a car radiator typically made of several rows of copper tubing with aluminum fins.

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

Any material that slows down the transfer of heat.

Integrally Controlled Motor (ICM)
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.

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.



Kilowatt (kw)Insulation
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.1,000 watts. A common unit of electrical consumption measured by the total energy created by one kilowatt in one hour.




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.

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.




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
A control system that uses computer logic to operate and monitor 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.

National Fire Protection Association.




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.

Payback Analysis
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.

Precision Air Conditioning
Precision A/C systems are primarily designed for cooling electronic equipment, rather than people. These pre-packaged systems offer excellent reliability and typically have a high ratio of sensible-to-total cooling capacity and a high CFM/ton ratio.

Purge Device
A device which removes air and water vapor from the refrigerant inside a chiller. A purge is a necessity in negative pressure designs, but is not necessary in positive pressure designs where air and water vapor are kept out of the system by the internal pressure.




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

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

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.




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.

Sensible Cooling Capacity
An A/C system’s capability to remove heat from the air. (lower the temperature)

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 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 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.

Storage Tank
A steel shell in which the refrigerant charge for a chiller may be temporarily stored while the chiller is serviced.

Super-Cooled 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.

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




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

Highly sophisticated programmable thermostat that senses the outdoor temperature, indoor air temperature, and indoor relative humidity. A microprocessor communicates with the heating and cooling equipment to determine the most efficient way to achieve perfect comfort. When used with a variable speed blower motor, this cite can reduce humidity even when cooling isn’t necessary (as when the indoor temperature is 70 degrees but relative humidity is high, eg, when it’s raining).

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.

Up-Flow Furnace
A furnace that pulls return air in from the bottom and expels warm air from the top.




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.

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 ventilator 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 a cooling tower or other suitable source.

Water Source
Water is being used as the heat source or heat sink for a heat pump. Sources of underground water are wells and sources of surface water are lakes, large ponds, and rivers.

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



Year RoundWatt
Air Conditioner which uses gas or oil for heating.



Zone Systems
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.

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