HVAC Terminology

Use the terms below to learn more about your HVAC products

A-COIL--A heat exchanger (evaporator coil) consisting of two diagonal coils that are joined together in a manner that looks like the letter “A”.

ACCA---Air-Conditioning Contractors of America.

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.

AHRI---Air Conditioning, Heating & Refrigeration Institute

AIR CLEANER (Also Air Filtration System)--- Removes airborne allergens from your home.

AIR CONDITIONER---Any device that can change the temperature, humidity or general quality of the air. More specifically, an air conditioner makes your home cooler, by drawing heat energy out of the house and transferring that heat to the outdoors

AIR FLOW VOLUME---The amount of air the system circulates through your home, expressed in cubic feet per minute (cfm). Proper air flow depends on the outdoor unit, the indoor unit, the ductwork and even whether the filters are clean.

AIR HANDLER--An air moving and/or mixing unit. Residential air handlers include a blower, a coil, an expansion device, a heater rack and a filter. Heaters for air handlers are sold as accessories. In some models, heaters are factory installed. The portion of your heating and cooling system that forces air through your home’s ductwork.

ASHRAE-American Society of Heating, Refrigeration and Air Conditioning Engineers- ASHRAE is an international voluntary organization for people involved in heating, ventilation, air conditioning, or refrigeration (HVAC&R).

The society promotes the general sciences of HVAC&R. The main Society headquarters is located in Atlanta, Georgia with local chapters located across the United States and throughout the world.

ASHRAE publishes a four-volume Handbook that serves as a resource for members, with one of the four volumes updated each year. ASHRAE also publishes a well recognized series of standards and guidelines relating to HVAC Systems. ASHRAE Standards are often referenced in other model building codes and in the US Green Building Council’s Leadership in Energy Design (LEED) green building rating system.

BTU---British thermal unit; the amount of heat required to raise or lower the temperature of one pound of water one degree Fahrenheit. The heat extracted from your home by an air conditioner is measured in BTUs.

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

BURNER---A device that uses fuel to support combustion.

BURNER (SEALED COMBUSTION)--- A burner that obtains 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.

CADR (Clean Air Delivery Rate) ---provides the number of cubic feet of clean air that a unit delivers each minute. Efficiency of the system times the air flow of the system.

CAPACITY---The output or producing ability of cooling or heating systems. Cooling and heating capacities are referred to in British thermal units (BTUs) per hour.

CERTIFIED MATCHED SYSTEM---The Air Conditioning, Heating and Refrigeration Institute (AHRI) puts heating and cooling equipment through rigorous certification processes to ensure systems deliver the promised performance at certain test conditions.

CFM---Abbreviation for cubic feet per minute, a standard measurement of airflow. A typical system requires 400 cfm of air flow per ton of air conditioning.

CHARGE---To add refrigerant to a system. This is refrigerant contained in a sealed system or in the sensing bulb to a thermostatic expansion valve.

COMPRESSOR---This is the heart of an air conditioning or heat pump system. It is part of the outdoor unit and pumps refrigerant in order to meet the cooling requirements of the system.

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

CONDENSER COIL (OR OUTDOOR COIL)--- Part of the outdoor portion of a heating or cooling system, which releases or collects heat from the outside air.

CONDENSER FAN---The fan that circulates air over the air-cooled condenser.

CONTACTOR---A switch that can repeatedly cycle, making and breaking an electrical circuit. When sufficient current flows through the A-coil 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 boil off liquid refrigerant that may have combined with compressor oil. Many newer cooling systems do not require crankcase heaters, however heat pumps do require crankcase heaters.

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.

DB (DECIBELS)--- A unit to measure sound.

DEFROST---To melt frost (as in from an air conditioner or heat pump coil).

DEGREE-DAY---A degree-day is a computation that gauges 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.

DIFFUSER----A grille over an air supply duct having vanes to distribute the discharging air in a specific pattern or direction.

DOE---Department of Energy

DOWNFLOW FURNACE---A furnace that intakes air at its top and discharges air at its bottom.

DRAIN PAN---This is 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---An instrument that measures air temperature independently of humidity.

DUAL COMPRESSOR’S---An outdoor unit featuring two compressors, one for every day cooling and a second larger one for extreme temperature days. Provides superior durability designed for cost-saving efficiency and low sound levels.

DUCTWORK---A pipe or conduit through which air is delivered. 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 important as the equipment.

DX---Direct expansion; a system in which heat is transferred by the direct expansion of refrigerant.

EER---Energy Efficiency Ratio (steady state)

EAC---Electronically filters out magnetized airborne particles from contaminants in indoor air, sending them to a collector plate.

ENERGY STAR---Energy Star is a government-backed labeling program that helps people and organizations save money and reduce greenhouse gas emissions by identifying factories, office equipment, home appliances and electronics that have superior energy efficiency.

EPA--- Environmental Protection Agency

ERV--- Energy-efficient system to exchange stale indoor air for an equal amount of fresher outdoor air.

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)

FAN--- Any device that creates air currents.

FILTER--- Any device that removes impurities through a straining process.

FLUE--- Any vent or passageway that carries the products of combustion from a furnace.

FURNACE--- That part of the heating system in which the combustion of fossil fuel and transfer of heat occurs.

FUSE--- A metal strip in an electrical circuit that melts and breaks the circuit when excessive current flows through it. The fuse is designed to break in order to save more expensive electrical components.

GAMA--- Gas Appliance Manufacturers Association

GAS FURNACE HEAT EXCHANGER--- Located in the furnace, the heat exchanger transfers heat to the surrounding air, which is then pumped throughout your home.

HEAT EXCHANGER--- An area, box or coil where heat flows from the warmer to the colder fluid or surface. This is the main component of the furnace that transfers heat into your home.

HEAT GAIN--- Heat added to the conditioned space by infiltration, solar radiation, occupant respiration and lighting.

HEAT LOSS--- The rate of heat transfer from a heated space to the outdoors.

HEAT PUMP---A mechanical-compression cycle refrigeration system that can be reversed to either heat or cool the controlled space.

HEAT TRANSFER--- The movement of heat energy from one point to another. The means for such movement are conduction, convection, and radiation.

HEATING COIL---Any coil that serves as a heat source.

HEPA FILTER--- Filter that traps airborne particles by forcing air through very dense mesh.

HERTZ---In alternating current (AC electricity), the number of cycles per second.

HORIZONTAL FLOW--- Often used in attics/crawl spaces, this furnace takes air from its side, heats it and sends warm air out the other side.

HSPF---Heating Seasonal Performance Factor. This rating is used in measuring the heating efficiency of a heat pump. The higher the number, the more efficient the heat pump system.

HUMIDIFIER---A machine that adds water vapor to the air to increase humidity.

HUMIDISTAT--- A humidity-sensing control that cycles the humidifier on and off.

HUMIDITY---The presence of water vapor in the air

HUMIDITY, absolute---Weight of water vapor per cubic foot of dry air, expressed as grains of moisture per cubic foot

HUMIDITY, relative---The amount of moisture in the air expressed as a percentage of the maximum amount that the air is capable of holding at a specific temperature

HVAC---Abbreviation for Heating, Ventilating and Air Conditioning

HYBRID HEAT (also hybrid system)--- Provides energy-efficient comfort for moderate heating conditions. Can use either gas or electricity

HYBRID SYSTEM (hybrid heat)--- This type of system provides energy-efficient comfort for moderate heating conditions. Depending on the weather and your comfort needs, it can use either gas or electricity, with an electric heat pump as the outdoor component and a gas furnace the indoor component

IGNITION---The lighting of a fuel to make it burn

INDOOR COIL (evaporator coil)--- Part of an indoor air unit, it functions with the air conditioner or heat pump outdoors to cool and condition
indoor air that flows over it by drawing out heat and moisture

INDOOR/OUTDOOR SYSTEM (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 matched for optimum efficiency.

INVERTER COMPRESSOR---The air conditioner and heat pump utilize inverter technology compressors to convert the electrical current from AC to DC. In doing so, the systems almost infinitely vary the compressor and fan speed motors. The air conditioner and heat pump can modulate anywhere from 40% to 100% of capacity, instantly adjusting to exactly offset the demands on the home. Because these units do not cycle on and off like a traditional system, they save a considerable amount of energy and reduce noise levels.


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

LOAD CALCULATION---Assessed by a dealer, the calculation factors in a number of criteria in your home (square footage, number of windows, year-round weather concerns) and determines what size components you should include as part of your total comfort system.

MATCHED SYSTEM---System includes multiple components designed and engineered to work seamlessly together to enhance overall performance, reliability and efficiency.

MEDIA---The material in a filter that traps and holds the impurities.

NATE---North American Technician Excellence—certification acknowledging a dealer/technician as one of the most knowledgeable and experienced at installing and servicing high-performance, precision-engineered heating and cooling systems.

NEC---National Energy Council

OEM---Original Equipment Manufacturer

OPERATING COST---Cost of running your home comfort system, based on energy usage

ORIFICE---An opening or hole; an inlet or outlet

OUTDOOR COIL (Condenser Coil)--- In a heat pump, it takes in heat to warm your home. In an air conditioner, it dissipates heat from the refrigerant, changing the refrigerant from vapor to liquid to cool your home.

PACKAGED SYSTEM OR UNIT---A single cabinet, typically installed outside, that houses both heating and cooling components

PAYBACK ANALYSIS---Number of years required before your monthly energy savings offset the original HVAC investment.

PSI---pounds per square per inch

PSIA---per square inch, absolute

PSIG---Pounds per square inch gauge.

PVC---Polyvinyl chloride; a type of plastic

RECIPROCATING COMPRESSOR---A compressor whose piston or pistons move back and forth in the cylinders

RECLAIMING---Returning used refrigerant to the manufacturer for disposal/reuse

RECYCLING---Removing, cleaning and reusing refrigerant

REFRIGERANT---A chemical that produces a refrigerating effect while expanding and vaporizing. Most residential air conditioning systems contain R-22 refrigerant. R-22 is regulated under the Montreal Protocol and in the United States by the Environmental Protection Agency. R-22 is scheduled to be in production until the year 2020.

REFRIGERANT CHARGE---The required amount of refrigerant in a system

REFRIGERANT LINES---Two copper lines connecting the outdoor condenser to the indoor evaporator coil

SEER---Seasonal Energy Efficiency Ratio; a measure of cooling efficiency for air conditioners and heat pumps. The higher the SEER rating the more energy efficient the unit is.

SEET---Seasonal Extreme Environmental Test; Trane facility that simulates series of rigorous tests to ensure the quality/reliability of outdoor units

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 rise or fall in temperature

SENSOR---Any device that reacts 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 with the expectation that this will be a nominal value depending on the range of the controller.

SPINE™ FIN COIL---All-aluminum outdoor coil that features the patented Spine Fin™ design. It provides greater heat-exchanging capabilities (meaning higher efficiencies) and is more resistant to corrosion than a traditional copper/aluminum coil.

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 matched for optimum efficiency.

THERMOSTAT---A thermostat consists of a series of sensors and relays that monitor and control the functions of a heating and cooling system.

THERMOSTATIC EXPANSION VALVE---A refrigerant metering device that maintains a constant evaporator temperature by monitoring suction vapor superheat; also called a thermal expansion valve.

TON---A unit of measurement used for determining cooling capacity. One ton is the equivalent of 12,000 BTUs per hour.

TWO STAGE COMPRESSOR---Compressor composed of a high and low stage to increase efficiency and comfort while reducing operation noise.

TWO STAGE HEATING / TWO STAGE COOLING---Two-stage heating and cooling is considered to be more efficient, because it operates at low speed most of the time. However, on days when more heating or air conditioning is required, it switches to the next stage for maximum comfort.

U-FACTOR---The factor representing resistance to heat flow of various building materials

UL---Underwriters Laboratories

UPFLOW FURNACE---A furnace in which air is drawn in through the sides or bottom and discharged out the top

VACUUM---A furnace in which air is drawn in through the sides or bottom and discharged out the top

VARIABLE SPEED MOTOR---The fan motor inside Trane’s variable-speed air handlers is designed to vary its speed based on your home’s heating and air conditioning requirements. Working in conjunction with your thermostat, it keeps the appropriate-temperature air (e.g. warm air on cold days) circulating throughout your home, reducing temperature variances in your home. It also provides greater air circulation and filtration, better temperature distribution, humidity control, higher efficiency and quiet performance.

VOLT---The unit of measure used to describe a difference in electrical potential, abbreviated by the symbol “v.”

WATT---The unit of electrical power equal to the flow of one amp at a potential difference of one volt.

WET BULB THERMOMETER---A thermometer whose bulb is covered with a piece of water-soaked cloth. The lowering of temperature that results from the evaporation of water around the bulb indicates the air’s relative humidity

ZONING---A method of dividing a home into different comfort zones so each zone can be independently controlled depending on use and need; an air conditioning system capable of maintaining varying conditions for various rooms or zones.

ZONING SYSTEM---A method of dividing a home into different comfort zones so each zone can be independently controlled depending on use and need; an air conditioning system capable of maintaining varying conditions for various rooms or zones