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Storage tank which receives liquid refrigerant from evaporator and prevents it from flowing into suction line before vaporizing.  


Air Changes Per Hour is the number of times that air in a building is completely replaced with outdoor air in one hour.

Activated Carbon

Specially processed carbon used as a filter drier; commonly used to clean air.

Adiabatic Compression

Compressing refrigerant gas without removing or adding heat.



Annual Fuel Utilization Efficiency is the rating that reflects the efficiency of a gas furnace in converting fuel to energy.

Air Change

The amount of air required to completely replace the air in a room or building; not to be confused with recirculated air.

Air Coil

Coil on some types of heat pumps used either as an evaporator or condenser.

Air Diffuser

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

Air Distribution

The transportation of a specified air flow, generally by means of ductwork.


The distribution or movement of air.

Air Handler

Fan-blower, filter and housing parts of a system.  

Air Infiltration


Leakage of air into rooms through cracks, windows, doors, and other openings.

Air Terminal Device


Grille or air distribution outlet designed to direct airflow into desired patterns.



The process of adjusting the flow of air in duct systems, or water flow in hot-water heating systems.

Balance Point

The lowest outdoor temperature at which the refrigeration cycle of a heat pump will supply the heating requirements without the aid of a supplementary heat source.  An air-handling device for moving air in a distribution system.


Fan or air handling device for moving air in a distribution system.

Blower Door


A large powerful variable-speed fan mounted in a doorway that blows air into (pressurizes) or sucks air out of (depressurizes) a building.  It’s used to test for air leakage.


A piece of duct used to connect ducts with registers.


British Thermal Unit which measures the quantity of heat required to raise the temperature of one pound of water one degree Fahrenheit.


How many BTUs are used per hour.



Cubic Feet per Minute is a standard measurement of airflow that indicates how many cubic feet of air pass by a stationary point in one minute.


Amount of refrigerant placed in a refrigerating unit.



Pump of a refrigerating mechanism, which draws a low pressure on cooling side of refrigerant cycle and squeezes or compresses the gas into the high pressure or condensing side of the cycle.

Condenser Coil

[See Outdoor Coil]

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.  


Coefficient Of Performance compares the heating capacity of a heat pump to the amount of electricity required to operate the heat pump in the heating mode.  COPs vary with the outside temperature: as the temperature falls, the COP falls also, since the heat pump is less efficient at lower temperatures.  


A device that is located in ductwork to adjust airflow.  This movable plate opens and closes to control airflow.

Decibel (db)

Describes the relative loudness of a sound.

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 Gas-Fired Heater

The burner fires directly in the air stream being heated, rather than through a heat exchanger.  100% of available BTUs are delivered to the heated space because no flue or heat exchanger is required.  This results in no wasted energy.

Downflow Furnace

A furnace that pulls in cool return air from the top and blows 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.  


Hollow pipes or channels that carry air from the air handler to the air vents.


The rating on heating & cooling equipment, 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.



Energy Recovery Ventilator is a device that preheats incoming outside air during the winter and pre-cools incoming air during the summer to reduce the impact of heating and or cooling the indoor air.

Emergency Heat

The back up electric heat built into a heat pump system.  The same as an auxiliary heater, except it is used exclusively as the heat source when the heat pump needs repair.


Uncontrolled air leakage out of a building.

Evaporator Coil

[See Indoor Coil]





The air flow leaving the treated space by one of the following methods:
Extraction:  air is discharged into the atmosphere 
Relief:  air is allowed to escape from the treated space if the pressure in that space rises above a specified level
Recirculation:  air is returned to the air treatment system
Transfer:  air passes from the treated space to another treated space

Fan Coil


An indoor component of a heat pump system used in place of a furnace, to provide additional heating on cold days when the heat pump does not provide adequate heating.


A device for removing dust particles from the air or unwanted elements from liquids.

Fire Dampers


Components that are installed in an air distribution system between two fire separating compartments and are designed to prevent propagation of fire and or smoke.

Flow Hood

A diagnostic tool used to measure air flow through ducts, supply registers and return grilles.  

Forced Air

A type of heating system that uses a blower motor to move air through the furnace and into the ductwork.


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

Grille An air terminal device with multiple passages for the air.

Coverings for the ducts where they open to the conditioned space.  Also referred to as Registers.

Heat Exchanger

A device that enables furnaces to transfer heat from combustion safely into breathable air.

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

A heating and air conditioning unit that heats or cools by moving heat.  A Heat Pump is a reverse cycle air conditioner.  It uses a compression cycle system to supply or remove heat to a temperature controlled space.

Heat Source A body of air or liquid from which heat is collected.  

Heat Recovery Ventilator is a device that brings fresh, outside air into a building while simultaneously exhausting stale indoor air outside.  In the process of doing this, an HRV removes heat from the exhaust air and transfers it to the incoming air, pre-heating it.  


Heating Seasonal Performance Factor, similar to SEER but it measures the amount of heating your heat pump delivers per every dollar spent on electricity.  


A device that adds moisture to warm air being circulated or directed into 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 refers to the indoor comfort industry.  

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.  Also referred to as the evaporator coil.

Indoor Unit

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


Process by which the primary air sets into motion an air volume, called secondary air, in the room.


Airflow inward into a space through walls, leaks around doors and windows or through the building materials used in the structure.

Kilowatt (kW)

A kilowatt equals 1,000 Watts.  A kilowatt hour (kWh) is the amount of kilowatts of electricity used in one hour of operation of any equipment.

Kilowatt Hour (kWh)

A kilowatt hour (kWh) is the amount of kilowatts of electricity used in one hour of operation of any equipment.  

Latent Cooling Load

The net amount of moisture added to the inside air by plants, people, cooking, infiltration, and any other moisture source. 

Latent Heat

Heat, that when added or removed, causes a change in state – but no change in temperature.  

Load Estimate


A series of studies performed to determine the heating or cooling requirements of your building.  The analysis uses information such as the square footage, window and door areas, insulation quality, and local climate to determine the heating and cooling capacity needed by your furnace, heat pump, or air conditioner.  


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. 

Outdoor 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.  Also referred to as the condenser coil.  

Outdoor Unit

Air conditioner or heat pump.  

Package Unit or System

A self-contained unit or system that has the Air Handler & Condenser in the same unit.


Air flow passage made of duct board, metal, drywall or wood.  Joins supply and return ducts with HVAC equipment.  

Pressure Balancing

The process of neutralizing pressure differences within a building.  

Primary Air Flow Rate


Volume of air entering a supply air terminal device within a time unit.


Processing or returning used refrigerant to the manufacturer or processor for disposal or reuse.



Substance used in refrigerating mechanism.  It absorbs heat in evaporator by change of state from a liquid to a gas, and releases its heat in a condenser as the substance returns from the gaseous state back to a liquid state.

Refrigerant Lines

Two copper lines that connect the Condenser or Outdoor Coil to the Evaporator or Indoor Coil.  


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


[See Grilles]  


The ductwork that carries air from the building to the air handler.

Return Air

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


Seasonal Energy Efficiency Ratio is the amount of cooling your air conditioner or heat pump delivers per every dollar spent on electricity.

Sensible Cooling Load

The heat gain of the building due to conduction, solar radiation, infiltration, appliances, people, and pets.  


Sensible Heat

Heat, that when added or removed, causes a change in temperature but not in state.

Setback Thermostat

An electronic thermostat with a built-in memory that can be programmed for different temperature settings at different times of the day.

Set Point

The temperature to which a thermostat is set to result in a desired heated space temperature.

Single Package


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

Sound Attenuators

Components which are inserted into the air distribution system and designed to reduce airborne noise which is propagated along the ducts.

Split System

Refrigeration or air conditioning installation, which places condensing unit outside or away from evaporator.  



The ductwork that carries air from the air handler to the rooms in the building.  


Supply Air


The air flow entering the treated space.



Another measurement of heat.  One therm equals 100,000 BTUH.



A temperature sensitive switch for controlling the operation of a heater or furnace.  


A cooling unit of measure.  Heat pumps and air conditioners are generally sized in tons.  Typical sizes for single-family residences are between two and five tons.  Each ton equals 12,000 BTUH.

Upflow Furnace

A furnace that pulls cool return air in from the bottom and expels warm air out the top into the ductwork.



Components inserted into air ducts or devices which permit modification of the air resistance of the system and consequently a complete shut-off of the air flow.

Watt (W)

A unit of electricity.


A system in which living areas or groups of rooms are divided into separate spaces and each space’s heating and air conditioning is controlled independently.