HVAC Glossary

A2L Refrigerant

An A2L refrigerant is one rated by ASHRAE Standard 34 as lower toxicity (A) and lower flammability (2L), meaning it can burn but has a maximum burning velocity of 10 cm/s or less, which makes ignition hard and flames slow.

AFUE

AFUE (annual fuel utilization efficiency) is the percentage of the fuel a furnace or boiler burns over a typical year that ends up as heat delivered from the appliance, with the rest lost up the vent; a 95% AFUE furnace turns 95 of every 100 units of fuel energy into heat.

Air Changes per Hour

Air changes per hour (ACH) is the number of times the full volume of air in a building is replaced with outside air in one hour, calculated as airflow in cubic feet per minute times 60, divided by the building's volume in cubic feet.

Air Conditioning

Air conditioning is the process of cooling indoor air by pulling heat and moisture out of it and moving that heat outdoors, most often with a refrigerant loop driven by a compressor.

Air Filter

An HVAC air filter is the replaceable screen of pleated or fiber media in the return air path of a furnace, heat pump or central AC that traps dust and particles before the air reaches the blower and coil.

Air Handler

An air handler is the indoor cabinet of a ducted HVAC system that holds the blower, the indoor coil and the filter rack, and pushes conditioned air through the ducts.

Balance Point Temperature

A balance point temperature is the outdoor temperature at which a heat pump's heating output exactly equals the home's heat loss, so that below it the heat pump alone cannot keep up and backup heat must help.

Blower Motor

A blower motor is the electric motor inside a furnace or air handler that spins the fan wheel pushing air across the heat exchanger or coil and through the home's ducts.

Boiler

A boiler is a heating appliance that heats water, or turns it into steam, and circulates it through pipes to radiators, baseboard convectors or tubing in the floor, which release the heat into each room.

BTU (British Thermal Unit)

A BTU, or British thermal unit, is a unit of heat energy: the amount of heat needed to raise the temperature of one pound of water by 1°F, about the energy released by burning one kitchen match.

Building Automation System

A building automation system (BAS) is a centralized network of sensors, controllers, actuators and software that monitors and controls a building’s heating, ventilation and air conditioning, and often its lighting and other systems, following schedules, setpoints and control sequences.

Central Air Conditioning

Central air conditioning is a whole-house cooling system that chills air at one indoor coil and sends it to every room through a network of supply and return ducts.

CFM (Cubic Feet per Minute)

CFM, or cubic feet per minute, is the unit HVAC uses for airflow: the volume of air, in cubic feet, that a blower, duct, register or fan moves each minute.

Coefficient of Performance (COP)

The coefficient of performance (COP) is the ratio of useful heat a heat pump delivers to the energy it uses, both in the same units, at one set of operating conditions; a COP of 3 means three units of heat for every unit of electricity.

Cold-Climate Heat Pump

A cold-climate heat pump is an air-source heat pump built and certified to keep delivering useful heat efficiently in very cold weather; under ENERGY STAR Version 6.2 it must reach a COP of at least 1.75 at 5°F and keep at least 70% of its 47°F heating capacity at 5°F.

Compressor

An HVAC compressor is the motor-driven pump in an air conditioner or heat pump that squeezes low-pressure refrigerant vapor into hot, high-pressure gas, which keeps refrigerant moving through the cooling or heating loop.

Condensate Drain Line

A condensate drain line is the small pipe, usually 3/4-inch PVC, that carries water dripping off an air conditioner's indoor coil, or from a high-efficiency furnace, out of the equipment to a drain or outdoors.

Condensate Overflow Switch

A condensate overflow switch is a safety device, usually a float, that senses water rising in an air conditioner's drain pan or line and cuts power to the cooling system before the water can overflow.

Condensation Temperature

Condensation temperature is the temperature at which refrigerant vapor turns back into liquid inside the condenser, fixed by the high-side (head) pressure, and is also called the saturated condensing temperature.

Condenser Unit

A condenser unit is the outdoor box of a split air conditioner that holds the compressor, the condenser coil and a fan, and releases the heat pulled out of your home into the outside air.

Cooling Degree Days

Cooling degree days (CDD) are a measure of how much cooling a place needs, found by subtracting 65°F from each day's average outdoor temperature and adding up the results over a month, season or year.

Dehumidification

Dehumidification is the process of removing water vapor from indoor air, usually by passing it over a cold coil so the moisture condenses and drains away.

Demand-Controlled Ventilation

Demand-controlled ventilation (DCV) is a control strategy that varies the amount of outdoor air an HVAC system brings into a space based on how many people are actually there, usually measured with carbon dioxide (CO₂) or occupancy sensors, instead of supplying the full design amount all the time.

Differential Temperature (Delta T)

Differential temperature (delta T) is the difference between the temperature of the air entering a furnace, heat pump or AC (return air) and the air leaving it (supply air), measured in degrees Fahrenheit.

Dirty Condenser Coil

A dirty condenser coil is the outdoor coil of an air conditioner or heat pump coated with dirt, grass clippings, pollen, lint or leaves, which blocks airflow and keeps the system from shedding heat outdoors.

Dual-Fuel Heat Pump

A dual-fuel heat pump is a heating system that pairs an electric air-source heat pump with a gas, propane or oil furnace, using the heat pump in mild weather and switching to the furnace below a set outdoor temperature, so each runs when it is cheaper or better able to meet the load.

Duct Balancing

Duct balancing is the process of measuring the airflow at each supply register and adjusting dampers in the branch ducts until every room receives close to the airflow its duct design calls for.

Duct Insulation

Duct insulation is a layer of material, usually fiberglass with a vapor-retarder jacket, wrapped around or built into HVAC ducts to slow heat flow through the duct walls and stop condensation where ducts pass through unconditioned space.

Duct Leakage

Duct leakage is the air that escapes from, or is pulled into, a heating and cooling duct system through gaps, holes and loose joints instead of moving between the equipment and the rooms.

Ductless Mini-Split

A ductless mini-split is a heating and cooling system that links one outdoor unit to one or more indoor heads mounted in the rooms, joined by refrigerant lines, power and a drain through a small hole in the wall, with no ducts.

EER2

EER2 is the energy efficiency ratio of an air conditioner or heat pump at one hot, steady condition: the cooling it delivers in BTU per hour divided by the electric power it draws in watts, with 95°F outside, tested under the federal Appendix M1 procedure in force since 2023.

Energy Efficiency Ratio: see EER2

Energy Recovery Ventilator (ERV)

An energy recovery ventilator (ERV) is a balanced ventilation unit that exchanges stale indoor air for fresh outdoor air through a core that transfers both heat and water vapor between the two streams, so incoming air arrives closer to indoor temperature and humidity than it would through a plain fan.

ENERGY STAR HVAC Rating

An ENERGY STAR HVAC rating is the U.S.

Evaporation Temperature

Evaporation temperature is the temperature at which refrigerant boils inside the evaporator coil, fixed by the suction pressure, and is also called the saturated suction temperature.

Evaporator Coil

An evaporator coil is the indoor coil of a central air conditioner or heat pump where cold refrigerant boils and absorbs heat from the air blown across it, cooling and drying your home's air.

Expansion Valve

An expansion valve is the metering device in an air conditioner or heat pump that lowers the pressure of liquid refrigerant before it enters the evaporator coil and controls how much refrigerant flows in.

Fresh Air Intake

A fresh air intake is the opening, duct and hood that bring outdoor air into a home on purpose, for whole-house ventilation through a fan, ventilator or the HVAC return.

Frozen Evaporator Coil

A frozen evaporator coil is an indoor air conditioner or heat pump coil that has dropped below 32°F and grown a layer of ice, which blocks airflow and stops the system from cooling.

Furnace

A furnace is a heating appliance that warms air, by burning natural gas, propane or oil or by running electric elements, and then uses a blower to push that warm air through ducts to the rooms of a home.

Furnace Flame Sensor

A furnace flame sensor is a thin metal rod in the burner flame that lets the furnace control confirm the burners are actually lit; if it does not detect flame within seconds, the control shuts off the gas.

Furnace Limit Switch

A furnace limit switch is a temperature safety switch that shuts off a furnace's burners when the heat exchanger area gets too hot, usually because too little air is moving across it.

Geothermal Heat Pump

A geothermal heat pump, or ground-source heat pump, is a heating and cooling system that moves heat between a building and the ground through fluid-filled pipes buried in the earth or set in water, using the ground’s steady temperature instead of outdoor air as its heat source and heat sink.

Heat Pump

A heat pump is an electric system that heats and cools a home by moving heat rather than making it, pulling heat from outdoor air, the ground or water in winter and pushing indoor heat outside in summer.

Heat Pump Defrost Cycle

A heat pump defrost cycle is a short, automatic reversal of the system that sends warm refrigerant to the outdoor coil to melt frost that builds up while the heat pump is heating in cold, damp weather.

Heat Recovery Ventilator (HRV)

A heat recovery ventilator (HRV) is a balanced ventilation unit that exhausts stale indoor air and brings in an equal flow of fresh outdoor air, passing both streams through a heat exchanger so the outgoing air warms or cools the incoming air without mixing them; it transfers heat (sensible energy) only, not moisture.

Heating and Cooling Capacity

Heating and cooling capacity is the rate at which an HVAC system can add heat to or remove heat from a home, stated in BTU per hour at defined test conditions, such as 95°F outdoors for cooling and 47°F and 17°F for heat pump heating.

Heating and Cooling System: see HVAC

Heating Degree Days

Heating degree days (HDD) are a measure of how much heating a place needs, found by subtracting each day's average outdoor temperature from 65°F and adding up the results over a month, season or year.

Heating Seasonal Performance Factor: see HSPF2

HEPA Filter

A HEPA filter (high-efficiency particulate air filter) is a pleated mechanical filter that, by definition, removes at least 99.97% of airborne particles 0.3 microns in size, the size that is hardest to catch.

HSPF2

HSPF2 is the heating seasonal performance factor of an air-source heat pump: the total heat it delivers over a model heating season, in BTUs, divided by the electricity it uses, in watt-hours, measured under the federal Appendix M1 test required since 1 January 2023.

Humidification

Humidification is the process of adding water vapor to indoor air, usually in winter, to raise relative humidity when heating has made the air dry.

HVAC

HVAC is short for heating, ventilation and air conditioning, the group of equipment that heats, cools, filters and moves air through a building to keep it comfortable and healthy.

HVAC Commissioning

HVAC commissioning is the set of measured tests a technician performs when a heating or cooling system is started up, such as airflow, static pressure, refrigerant charge and combustion, to confirm the installed system runs as designed, with the results recorded in a commissioning report.

HVAC Commissioning Report: see HVAC Commissioning

HVAC Ductwork

HVAC ductwork is the network of supply and return ducts, plenums, fittings and registers that carries air from a furnace, air handler or heat pump to the rooms of a building and back again.

HVAC Energy Audit

An HVAC energy audit, usually called a home energy audit or assessment, is an inspection and set of measured tests, such as a blower door test and duct leakage test, that shows where a building loses energy through its envelope, ducts and heating and cooling equipment, and ranks the fixes worth making.

HVAC Error Code

An HVAC error code is a numbered fault message, shown as blinking LED flashes or on a display, that a furnace, heat pump or AC control board uses to report what stopped or limited the system.

HVAC Installation

HVAC installation is the work of sizing, placing, connecting and starting up heating, ventilation and air conditioning equipment in a building, including the ducts or piping, refrigerant lines, wiring, venting, drains and controls, so the system runs safely and delivers its rated capacity and efficiency.

HVAC Load Calculation: see Manual J Load Calculation

HVAC Maintenance

HVAC maintenance is the routine, scheduled care of heating, cooling and ventilation equipment, such as changing filters, cleaning coils, clearing drains, tightening electrical connections and checking refrigerant and combustion, done to keep the system safe, efficient and reliable and to catch problems before they cause a breakdown.

HVAC Replacement

HVAC replacement is the removal of an existing furnace, air conditioner, heat pump or boiler, or a whole system, and the installation of new equipment in its place, usually because the old system has failed, costs too much to repair or run, or no longer meets the home’s needs.

HVAC Retrofit

An HVAC retrofit is an upgrade to an existing heating, cooling or ventilation system, such as sealing ducts, adding a variable-speed blower, zoning, new controls, ventilation or an add-on heat pump, that improves comfort, efficiency or air quality while keeping much of the original system in place.

HVAC Tonnage

HVAC tonnage is the cooling capacity of an air conditioner or heat pump measured in tons, where one ton equals 12,000 BTU of heat removed per hour; a 3-ton system is rated to remove about 36,000 BTU each hour.

HVAC Tune-Up

An HVAC tune-up is a single scheduled visit in which a technician cleans, inspects, tests and adjusts a heating or cooling system before its busy season, typically a cooling check in spring and a heating check in fall, and records what was found.

HVAC Warranty

An HVAC warranty is a written promise from an equipment maker, an installer or a third party to repair or replace heating and cooling parts, or pay for labor, if they fail from defects within a set period and under stated conditions.

Indoor Air Quality

Indoor air quality (IAQ) is the condition of the air inside and around a building, especially how it affects the health and comfort of the people in it.

Inverter-Driven HVAC System: see Variable-Speed Compressor

Latent Heat

Latent heat is the energy absorbed or released when water changes between vapor and liquid without a change in temperature; in HVAC, it is the moisture part of the cooling load.

Manual D Duct Design

Manual D duct design is the ACCA standard method for sizing a home's supply and return ducts so the blower can deliver each room's required airflow within its available static pressure, without noisy air speeds.

Manual J Load Calculation

A Manual J load calculation is the ACCA standard method for working out how many BTU per hour a home loses in winter and gains in summer at local design temperatures, so heating and cooling equipment can be sized to the house.

Manual S Equipment Selection

Manual S equipment selection is the ACCA standard procedure for choosing heating and cooling equipment whose real capacity, at your home's design conditions, matches the loads from a Manual J calculation within set size limits.

MERV Rating

A MERV rating (Minimum Efficiency Reporting Value) is a number from 1 to 16 that shows how well an HVAC air filter captures particles between 0.3 and 10 microns, measured under ASHRAE Standard 52.2.

Packaged HVAC Unit

A packaged HVAC unit is a heating and cooling system built into one outdoor cabinet, holding the compressor, both coils, the blower and often a gas burner, that connects to the home's ducts through a wall or roof.

Preventive Maintenance: see HVAC Maintenance

R-32 Refrigerant

R-32, or difluoromethane (HFC-32), is a single-compound hydrofluorocarbon refrigerant with a GWP of 675 on EPA's table, used in many new air conditioners, heat pumps and mini-splits as a lower-GWP alternative to R-410A.

R-410A Refrigerant

R-410A is a hydrofluorocarbon (HFC) refrigerant blend, sold under names such as Puron, that was the standard refrigerant in US home air conditioners and heat pumps from about 2010 until new systems moved to lower-GWP refrigerants in 2025.

R-454B Refrigerant

R-454B is a refrigerant blend of about 69% R-32 and 31% R-1234yf by weight that replaces R-410A in many new US air conditioners and heat pumps, with a global warming potential of 465 on EPA's table and a mildly flammable A2L safety class.

Refrigerant

Refrigerant is the working fluid in an air conditioner, heat pump or refrigerator that absorbs heat by boiling at low pressure and releases it by condensing at high pressure as it circulates in a sealed loop.

Refrigerant Leak

A refrigerant leak is any escape of refrigerant from the sealed loop of an air conditioner or heat pump, through a crack, a worn joint, a valve core or corrosion, which lowers cooling and heating capacity over time.

Refrigerant Line Set

A refrigerant line set is the pair of copper tubes that connects the outdoor unit of a split air conditioner or heat pump to the indoor coil: a larger insulated suction line and a smaller liquid line.

Refrigerant Recovery

Refrigerant recovery is the removal of refrigerant from an air conditioner, heat pump or refrigeration system into an external storage cylinder, using certified recovery equipment, so it is not released to the air when the system is opened for service, repair or disposal.

Refrigeration Cycle

The refrigeration cycle is the closed loop in which a refrigerant absorbs heat by boiling at low pressure in an evaporator, is compressed, releases that heat by condensing at high pressure in a condenser, and is expanded back to low pressure to repeat.

Relative Humidity

Relative humidity (RH) is the amount of water vapor in the air compared with the most the air could hold at that same temperature, shown as a percentage.

Return Air Vent

A return air vent is the grille that pulls room air back into the duct system and the blower, so it can be filtered, heated or cooled, and sent out again through the supply vents.

SEER: see SEER2

SEER2

SEER2 is the seasonal energy efficiency ratio of a central air conditioner or heat pump: the cooling it delivers over a typical season, in BTUs, divided by the electricity it uses, in watt-hours, measured under the federal Appendix M1 test that took effect on 1 January 2023.

Sensible Heat

Sensible heat is heat that changes the temperature of air or another material without changing its state, the part of a heating or cooling load you can measure with a thermometer.

Short Cycling

Short cycling is when a heating or cooling system turns on and shuts off again after only a few minutes, many times an hour, instead of running full, steady cycles.

Split-System HVAC

A split-system HVAC is a heating and cooling setup divided into two cabinets: an outdoor unit with the compressor and condenser coil, and an indoor unit with the evaporator coil and blower, joined by copper refrigerant lines.

Static Pressure

Static pressure in HVAC is the resistance to airflow that a blower must push against in the ducts, filter, coil and grilles, measured in inches of water column; total external static pressure (TESP) adds the supply and return readings.

Subcooling

Subcooling is the number of degrees by which liquid refrigerant leaving the condenser is cooler than its condensing (saturation) temperature at the measured liquid-line pressure, found as saturated liquid temperature minus liquid line temperature.

Superheat

Superheat is the number of degrees by which refrigerant vapor in the suction line is warmer than its boiling (saturation) temperature at the measured suction pressure, found as suction line temperature minus saturated suction temperature.

Supply Vent

A supply vent is the register, grille or diffuser at the end of a supply duct where heated or cooled air from the HVAC system enters a room, usually with louvers or a damper to aim and adjust the airflow.

Thermostat

A thermostat is the wall control that senses room temperature and switches your heating and cooling equipment on and off to hold the temperature you set.

Tons of Cooling: see HVAC Tonnage

Variable-Speed Compressor

A variable-speed compressor is an air conditioner or heat pump compressor driven by an inverter, an electronic drive that changes motor speed, so the system can run continuously across a wide range of capacity instead of switching between full power and off.

Ventilation Rate

Ventilation rate is the amount of outdoor air a ventilation system brings into a home, or stale air it exhausts, usually measured in cubic feet per minute (cfm).

Zoning System

An HVAC zoning system is a set of motorized dampers, a control panel and two or more thermostats that divides one ducted system into separate areas, opening and closing dampers so each zone gets air only when its thermostat calls.