Here you will find definitions of general Heating, Ventilation and Air Conditioning (HVAC) terms used throughout this site.

-------------------------[A]-------------------------

Air Conditioner:
Assembly of equipment for the simultaneous control of air temperature, relative humidity, and the comfort of the living space. Your air conditioning system may consist of central air or a window mounted system

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 or office 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 (DOE) test procedures.

Auxiliary Heat:
Suplemental heat provided by electric resistance heating coils built into the heat pump system, or in some cases, a gas furnace, to help maintain living space comfort at peak heating loads. During very cold conditions the heat pump may have trouble keeping the living space comfortable. Auxiliary heat will automatically engage to make up the heating shortfall.

-------------------------[B]-------------------------

Backup Heat:
See Emergency Heat

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 may be, but is not always, needed to maintain indoor comfort.

Blower:
An air handling device for moving air in a distribution system. This is more commonly referd to as the indoor fan.

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.

BTU/hr (BTUH):
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.

-------------------------[C]-------------------------

Capacity:
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.

Carbon Monoxide:
Carbon Monoxide (CO) is a colorless, odorless gas produced by burning any fuel. CO is poisonous. The symptoms of CO poisoning are similar to those of the flu: headaches, fatigue, shortness of breath, nausea and dizziness. The Consumer Product Safety Commission (CPSC) recommends a yearly, professional inspection of your gas burning appliances.

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.

Charge:
Amount of refrigerant in a system.

Coefficient of Performance (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.

Compressor:
The pump that moves the refrigerant from the indoor coil to the outdoor coil and back to the indoor coil again. The compressor is often called "the heart of the system" because it circulates the refrigerant through the refrigerant loop.

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 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, liquifies it in the condenser and returns it to the refrigerant control. The outdoor portion of a split system air conditioner or heat pump contains the compressor and outdoor coil.

Cooling Capacity:
A measure of the ability of a unit to remove heat from an enclosed space.

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

-------------------------[D]-------------------------

Damper:
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 or an air source heat pump during the heating season.

Dehumidification:
The reduction of water vapor in air by cooling the air below the dew point; at which time the water vapor condenses on the indoor coil and is drained away.

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

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

Duct:
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.

Ductwork:
Pipes or channels that carry air throughout a building.

-------------------------[E]-------------------------

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 (Backup Heat):
The back-up heat built into a heat pump system. In cases where the compressor has failed, switching the thermostat to emergency heat will remove the compressor from operation and heat will be provided by electric resistance heating coils or in some cases, a gas furnace.

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 Air Conditioning and Refrigeration Institute-specified test conditions.

Evaporator:
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.

-------------------------[F]-------------------------

Fahrenheit (Represented as degrees "F"):
The scale of temperature measurement most commonly used in the United States of America.

Filter:
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.

Freon:
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.

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

-------------------------[G]-------------------------

Ground Water-Source:
Water from an underground well is being used as the heat source or heat sink for a heat pump.

-------------------------[H]-------------------------

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.


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 an air-source heat pump, the air outside the home is used as the heat source during the heating cycle. Geo-thermal heat pumps use either ground water from a well or from a buried loop as the heat source.

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, pulling in return air from one side and expelling warm air from the other.

HVAC:
Heating, Ventilation, and Air Conditioning.

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

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

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

-------------------------[I]-------------------------

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.

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

Insulation:
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.

-------------------------[K]-------------------------

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.

-------------------------[L]-------------------------

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.

-------------------------[M]-------------------------

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.

-------------------------[O]-------------------------

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.

-------------------------[P]-------------------------

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.

-------------------------[R]-------------------------

Refrigerant:
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.

Register:
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.

-------------------------[S]-------------------------

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.

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

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.

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

Superheated Vapor:
Refrigerant vapor heated beyond its saturation point.

Superheating:
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.

-------------------------[T]-------------------------

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

Thermostat:
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.

Tonnage:
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.

-------------------------[U]-------------------------

Upflow:
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.

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

-------------------------[V]-------------------------

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.

-------------------------[W]-------------------------

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 water may be wells or underground loops buried for that purpose

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

-------------------------[Z]-------------------------

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

Zoning:
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.