Below are some commonly used terms in the heating, ventilation and air conditioning industry. These may be useful when dealing with contractors or assessing a solution to your problem.


      Absolute Zero - The lowest temperature theoretically attainable on the Kelvin scale (approximately 273.16° C).

      Air Conditioner - Assembly of equipment for the simultaneous control of air temperature, relative humidity, purity, and motion.

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

      Air Handling Unit (AHU) - In HVAC refers to equipment that includes a blower or fan, heating and/or cooling coils, and related equipment such as controls, condensate drain pans, and air filters. Does not include ductwork, registers or grilles, or boilers and chillers.

      AFUE (Annual Fuel Utilization Efficiency) - 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-Source - Air is being used as the heat source or heat sink for a heat pump.

      Ambient Temperature - The temperature, usually of the air, that surrounds operating equipment.

      Blower - In HVAC the device in an air conditioner that distributes the filtered air from the return duct over the coil/heat exchanger. This circulated air is cooled/heated and then sent through the supply duct, past dampers, and through supply diffusers to the living/working space.

      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.

      Carbon Dioxide (CO2) - A colorless, odorless noncombustible gas with the formula CO2 that is present in the atmosphere. It is formed by the combustion of carbon and carbon compounds (such as fossil fuels and biomass), by respiration, which is a slow combustion in animals and plants, and by the gradual oxidation of organic matter in the soil.

      Carbon Monoxide (CO) - A colorless, odorless but poisonous combustible gas with the formula CO. Carbon monoxide is produced in the incomplete combustion of carbon and carbon compounds such as fossil fuels (i.e. coal, petroleum) and their products (e.g. liquefied petroleum gas, gasoline), and biomass.

      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.

      Central Heating System - In HVAC a system where heat is supplied to areas of a building from a single appliance through a network of ducts or pipes.


      Combustion - The process of burning; the oxidation of a material by applying heat, which unites oxygen with a material or fuel.

       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.

      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.

      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.

      Dehumidifier -A device for reducing the level of humidity in a room or home.


      Direct Water Heater - A type of water heater in which heated water is stored within the tank. Hot water is released from the top of the tank when a hot water faucet is turned. This water is replaced with cold water that flows into the tank and down to just above the bottom plate under which are the burners.

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

      Duct(s) - The round or rectangular tube(s), generally constructed of sheet metal, fiberglass board, or a flexible plastic-and-wire composite, located within a wall, floor, and ceiling that distributes heated or cooled air in buildings.

      EER - Energy Efficiency Ratio 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.

      Enthalpy - 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.

      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.

      Exhaust Ventilation - HVAC term for mechanical removal of air from a portion of a building (e.g., piece of equipment, room, or general area).


       Flue - The structure (in a residential heating appliance, industrial furnace, or power plant) into which combustion gases flow and are contained until they are emitted to the atmosphere.

       Forced Air System or Furnace - HVAC term for a type of heating system in which heated air is blown by a fan through air channels or ducts to rooms.

       Free Delivery - There are no ducts and the unit may be installed in the field without ducts if needed.

       Freon - A registered trademark for a cholorfluorocarbon (CFC) gas that is highly stable and that has been historically used as a refrigerant.

      Ground-Source - The ground or soil below the frost line is being used as the heat source or heat sink for a heat pump.

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

      HVAC - Heating, ventilation, and air conditioning.

      Heat Pump - An air conditioner capable of heating by refrigeration. It may or may not include a capability for cooling. Outside air or water is used as a heat source or heat sink, depending upon whether the system is heating or cooling.

      Heating Capacity - A measure of the ability of a unit to add heat to an enclosed space.

      HSPF - Heating Seasonal Performance Factor 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.

      Humidity - A measure of the moisture content of air; may be expressed as absolute, mixing ratio, saturation deficit, relative, or specific.

      Insulation - Any material that slows down the transfer of heat.

      Ionizer - A device that removes airborne particles from breathable air. Negative ions are produced and give up their negative charge to the particles. These new negative particles are then attracted to the positive particles surrounding them. This accumulation process continues until the particles become heavy enough to fall to the ground.

      (K) Factor - The insulating value of any material. Also known as conductivity.

      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.

      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.

      Negative Pressure - Condition that exists when less air is supplied to a space than is exhausted from the space, so the air pressure within that space is less than that in surrounding areas. Under this condition, if an opening exists, air will flow from surrounding areas into the negatively pressurized space.

      Outdoor Air Supply - HVAC term for air brought into a building from the outdoors (often through the ventilation system) that has not been previously circulated through the system. Also known as "Make-Up Air".


      Pressure, Static - In flowing air, the total pressure minus velocity pressure. The portion of the pressure that pushes equally in all directions.

      Propane - A hydrocarbon gas, C3H8, occurring in crude oil, natural gas, and refinery cracking gas. It is used as a fuel, a solvent, and a refrigerant. Propane liquefies under pressure and is the major component of liquefied petroleum gas (LPG).


      Radiant Floor - A type of radiant heating system where the building floor contains channels or tubes through which hot fluids such as air or water are circulated. The whole floor is evenly heated. Thus, the room heats from the bottom up.   Radiant floor heating eliminates the draft and dust problems associated with forced air heating systems.

      Radiant Heating System - HVAC term for a heating system where heat is supplied (radiated) into a room by means of heated surfaces, such as electric resistance elements, hot water (hydronic) radiators, etc.

      Refrigerant - The compound (working fluid) used in air conditioners, heat pumps, and refrigerators to transfer heat into or out of an interior space. This fluid boils at a very low temperature enabling it to evaporate and absorb heat.

      Refrigeration - The process of the absorption of heat from one location and its transfer to another for rejection or recuperation.

      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 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.

      Single Package - A central air conditioner which combines both condenser and air handling capabilities in a single packaged unit.

      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.

      Supercooled Liquid - Liquid refrigerant cooled below its saturation point.

      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.

      Therm - A unit of heat containing 100,000 British thermal units(BTU).

      Ton (Air Conditioning) - A unit of air cooling capacity; 12,000 Btu per hour.

      Ton - The unit of measurement for air conditioning system capacity. One ton of air conditioning removes 12,000 Btu's of heat energy per hour from a home. Central air conditioners are sized in tons. Residential units usually range from 1 to 5 tons.

      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.

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

      Year-Round - Air Conditioner which uses gas or oil for heating.

      Zone - In HVAC an area within the interior space of a building, such as an individual room(s), to be cooled, heated, or ventilated. A zone has its own thermostat to control the flow of conditioned air into the space.

      Zoning - The combining of rooms in a structure according to similar heating and cooling patterns. Zoning requires using more than one thermostat to control heating, cooling, and ventilation equipment.