TERMINOLOGY

A-Coil
- A heat exchanger containing two diagonal coils that are connected together in a manner that resembles the letter "A".

AC
- Abbreviation for alternating current, an electric current that reverses its direction many times a second.

AC or DC
- Abbreviation for equipment capable of operating on alternating or direct current.

ACCA
- Air Conditioning Contractors of America.

Acoustical
- Of or pertaining to sound.

AFUE
- Annual Fuel Utilization Efficiency. A measure of a gas furnace's efficiency in converting fuel to energy. The higher the rating, the more efficient the unit. The unit is more efficient when the rating is higher.

AGA
- Abbreviation for American Gas Association, Inc.

Air Cleaner
- A piece of machinery that removes unwanted particles from moving air.

Air Conditioner
- A system for controlling the humidity, ventilation, and temperature in a building or vehicle.

Air flow Volume
- The amount of air the system circulates through your home, indicated in cubic feet per minute (cfm). Correct airflow depends on the indoor unit, the ductwork, the outdoor unit, and even whether the filters are clean.

Air handler
- The portion of the central air conditioning or heat pump system that moves cooled or heated air throughout a home's ductwork. In some systems a furnace will handle this function.

ARI
- Air Conditioning & Refrigeration Institute.

ASHRAE
- American Society of Heating, Refrigeration and Air Conditioning Engineer

BTU
- British thermal unit. The standard of measurement used to gauge the amount of heat required to raise the temperature of one pound of water by one degree (Fahrenheit).

BTUh
- British thermal units per hour. 12,000 BTUh equals one ton of cooling.

Burner
- A instrument that uses fuel to support combustion.

Burner (sealed combustion)
- A burner that acquires all air for combustion from outside the heated space.

Burner Orifice
- The opening through which gas flows to the air/gas mixing chamber of the burner.

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

Celsius
- The metric temperature scale in which water freezes at zero degrees and boils at 100 degrees, designated by the symbol "C". To convert to Fahrenheit, multiply a Celsius temperature by 9, divide by 5 and add 32 (25 x 9 equals 225, divided by 5 equals 45, plus 32 equals 77 degrees Fahrenheit).

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

Charge
- Adding refrigerant to a system. Refrigerant is contained in a sealed system or in the sensing bulb to a thermostatic expansion valve.

Compressor
- 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.

Condensate
- Vapor that liquefies due to the lowering of its temperature to the saturation point.

Condenser coil (or outdoor 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.

Condenser Fan
- The fan that distributes air over the air-cooled condenser.

Contactor
- A switch that can repeatedly cycle, making and breaking an electrical circuit. When ample current flows through the A-coil which is built into the contactor, the resulting magnetic field causes the contacts to be pulled in or closed.

Crankcase Heater
- This is the electric resistance heater installed on compressor crankcases to keep the crankcase oil at a temperature higher than the coldest part of the system to prevent migration. Many newer cooling systems do not require crankcase heaters; however heat pumps do require crankcase heaters. Crankcase heaters are used to overcome the problem of migration and condensation of refrigerant in the crankcases of compressors used in air conditioning and heat pump systems.

CSA
- Canadian Standards Association.

Damper
- Found in ductwork, this movable plate opens and closes to control airflow. Dampers can be used to balance airflow in a duct system. They are also used in zoning to regulate airflow to certain rooms.
DC
- Direct current electricity. This type of electricity (as opposed to Alternating Current, or AC) flows in one direction only, without reversing polarity.
Defrost
- The process of removing ice or frost buildup from the outdoor coil during the heating season.
Degree-day
- A computation that measures the amount of heating or cooling needed for a building. A degree-day is equal to 65 degrees Fahrenheit minus the mean outdoor temperature.
Dehumidifier
- An air cooler that removes moisture from the air. This unit reduces water vapor in air by cooling the air below the dew point, removing water vapor from the air by chemical means, refrigeration, etc.
Diffuser
- A grille over an air supply duct having vanes to distribute the discharging air in a defined pattern or direction.
DOE
- Department of Energy
Downflow Furnace
- A furnace that pulls in return air from the top and discharges warm air at the bottom.
Drain Pan
- Also referred to as a condensate pan. This is a pan used to catch and collect condensate (in residential systems vapor is liquefied on the indoor coil, collected in the drain pan and removed through a drain line).
Dry Bulb Temperature
- Heat intensity, measured by a dry bulb thermometer.
Dry Bulb Thermometer
- A device that measures air temperature independently of humidity.
Ductwork
- A pipe or conduit through which air is supplied. Ducts are typically made of metal, fiberboard, or a flexible material. In a home comfort system, the size and application of ductwork is critical to performance and is as principal as the equipment.
DX
- Direct expansion. A system in which heat is passed on by the direct expansion of refrigerant.

EER
- Energy Efficiency Ratio (steady state). 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.
EPA
- Environmental Protection Agency. ENERGY STAR is a joint program of the U.S. Environmental Protection Agency and the U.S. Department of Energy helping us all save money and protect the environment through energy efficient products and practices.
Evaporator Coil (or Indoor Coil)
- The other half of an air conditioning system, this network of tubes filled with refrigerant which is located inside the home within the indoor unit, take heat and moisture out of indoor air as liquid refrigerant evaporates.
Expansion Valve
- A refrigerant-metering valve with a pressure or temperature-controlled orifice.

Fahrenheit
- The temperature scale on which water freezes at 32 degrees and boils at 212 degrees; designated by the letter F. To convert Fahrenheit to Celsius, subtract 32 from the Fahrenheit number, multiply by 5 and divide by 9 (77 32 equals 45, times 5 equals 225, divided by 9 equals 25 degrees Celsius). This is the most commonly used scale of temperature measurement in the United States of America.
Fan
- Any device that creates air currents.
Filter
- Any device used to remove dust and other impurities from air for the purposes of reducing the load on the respiratory system and to protect the HVAC equipment through a straining process. Filters vary greatly in particle arrestant; the higher the MERV rating (Minimum Efficiency Reporting Value- a number from 1 to 16 that is relative to an air filter’s efficiency), the better the filter.
Flue
- Any vent or duct, pipe, or chimney for carrying exhaust gases from a fireplace, furnace, water heater, boiler, or generator to the outdoors.
Furnace
- That part of an environmental system which converts gas, oil, electricity, or other fuel into heat for distribution within a structure.
Fuse
- A fuse is a type of overcurrent protection device. Its essential component is a metal wire or strip that melts when too much current flows, which breaks the circuit in which it is connected, thus protecting the circuit's other components from damage due to excessive current.

GAMA
- Gas Appliance Manufacturers Association.
Gas Furnace Heat Exchanger
- Found in the furnace, the heat exchanger transfers heat to the surrounding air, which is then transported throughout your home.

Heat Exchanger
- An area, box, or coil where heat flows from the warmer to the colder fluid or surface. A device for the transfer of heat energy from the source to the conveying medium.
Heat Gain
- The amount of heat introduced to a space from all heat producing sources by appliances, solar energy, occupant respiration and lighting.
Heat Loss
- The rate of heat transfer from a building interior to the outdoors.
Heat Pump
- An automated compression cycle refrigeration system that can be switched to either heat or cool the controlled space.
Heat Transfer
- The movement of heat energy from one area to another. The means for such movement are convection, conduction, and radiation. Heat will flow naturally from a warmer to a cooler space or material.
Heating Coil
- Any coil that serves as a heat source.
Hertz
- A measure of the number of cycles or wavelengths of electrical energy per second; U.S. electricity supply has a standard frequency of 60 hertz.
HSPF
- Heating Seasonal Performance Factor. This rating is used in measuring the heating efficiency of a heat pump by taking in account the variations in temperature that can occur within a season and is the average number of BTU of heat dispatched for every watt-hour of electricity used by the heat pump over a heating season. The higher the number, the more efficient the heat pump system.
Humidifiers
- A machine that adds water vapor to the air to increase humidity.
Humidistat
- A humidity sensing device designed to regulate humidity input by reacting to changes in the moisture content of the air.
Humidity
- The measure of the moisture content of air. Air conditioners can remove moisture for added comfort.
Humidity, Absolute
- The ratio of the mass of water vapor to the volume occupied by a mixture of water vapor and dry air. It is measured in grams of water vapor per cubic meter of air.
Humidity, Relative
- A measure of the percent of moisture actually in the air compared with what would be in it if it were fully saturated at that temperature. When the air is fully saturated, its relative humidity is 100 percent.
HVAC
- Heating, Ventilating and Air Conditioning.

Ignition
- The lighting of a gaseous mixture to the temperature at which combustion takes place.

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

Latent Heat
- A type of heat, which when added to or taken from a substance, does not change the temperature of the substance yet enables the heat energy to change its state.

Media
- The material in a filter that traps and holds the impurities. These HVAC filter media include fiberglass filter media, polyester filter media, dog hair filter media, blue/green filter media, charcoal filter media, and others.

NEC
- National Energy Council / National Electric Code.
NEMA
- National Electrical Manufacturing Association.

OEM
- Original equipment manufacturer.

Package Unit
- A heating and cooling system contained in one outdoor cabinet. This package unit is typically installed on the roof, besides, or sometimes in the attic of a home.
PSI
- Pounds per square inch.
PSIA
- Pounds per square inch, absolute.
PSIG
- Pounds per square inch gauge.
PVC
- Polyvinyl chloride; a type of plastic. In recent years, PVC has been replacing traditional building materials such as wood, concrete and clay in many areas.

Reciprocating Compressor
- A compressor that uses pistons driven by a crankshaft to deliver gases at high pressure.
Refrigerant
- A chemical substance that produces a refrigerating effect while expanding and vaporizing.
Refrigerant Charge
- The required amount of refrigerant in a system.

SEER
- Seasonal Energy Efficiency Ratio. A measure of seasonal or annual efficiency of a central air conditioner or air conditioning heat pump. It takes into account the variations in temperature that can occur within a season and is the average number of BTU (British thermal units) of cooling delivered for every watt-hour of electricity used by the heat pump over a cooling season. 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 rating is 10.
Self-Contained System
- A refrigerating system that can be moved without disconnecting any refrigerant lines; also known as a package unit.
Sensible Heat
- That heat which, when added to or taken away from a substance, causes a change in temperature
Sensor
- Any device that responds to a change in the conditions being measured, permitting the condition to be controlled.
Setpoint
- The temperature or pressure at which a controller is set for desired comfort level.
Split System
- The combination of an outdoor unit (air conditioner or heat pump) with an indoor unit (furnace or air handler). Split systems must be paired for optimal efficiency. This is the most common type of system installed in a home.

Thermostat
- A series of sensors and relays that monitor and control the functions of a heating and cooling system by turning the device on or off when a specified temperature is reached.
Thermostatic Expansion Valve
- A thermostatic expansion valve (TXV) is 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. Also called a thermal expansion valve.
Ton
- A unit of measurement used for determining 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.
Two-stage cooling
- The air conditioner/heat pump has a compressor with two degrees of operation: high for hot summer days and low for cooler days. While the low setting is adequate to meet household-cooling demands 80% of the time, a two-stage unit runs for longer periods and produces more balanced temperatures.
Two-stage heating
- The furnace has two stages of heat output: high for cold winter days and low for milder days. Since the low setting is adequate to meet household-cooling demands 80% of the time, a two-stage unit runs for extended periods and delivers more uniform heat distribution.

U-Factor
- The factor amounting to the resistance of heat flow through various building materials.
UL
- Underwriters Laboratories.
Upflow Furnace
- A furnace in which air is drawn in through the bottom or sides and expels warm air out the top.

Vacuum
- A pressure below atmospheric pressure. 30 inches Mercury (periodic symbol "Hg") is a perfect vacuum.
Volt
- A unit of measure of electrical force given to the electrons in an electric circuit. A single volt is the electromotive force that will cause a steady current of one ampere to surge through a resistance of one ohm. Abbreviated by the symbol "v".
Voltage
- The force that pushes electrical current along wires and cables. Term voltage used to indicate the potential difference in a circuit, voltage is also known as the pressure which causes current to flow.

Watt
- The unit of electrical power equal to the flow of one amp at a potential difference of one volt. A single watt is equivalent to the work done at a rate of 1 joule (the SI unit of energy measuring heat, electricity and mechanical work) per second.
Wet Bulb Thermometer
- A thermometer whose bulb is covered with a piece of water soaked cloth. It is a type of temperature measurement that reflects the physical properties of a system with a mixture of a gas and a vapor, usually air and water vapor.