HVAC glossary

113 heating and cooling terms in plain English – the words a technician may use on the phone or on a quote.

A2L refrigerant: A safety class for refrigerants with lower toxicity and mild flammability, such as R-454B and R-32, used in many new home air conditioners and heat pumps. Equipment and installation practices are designed for them.
ACCA: Air Conditioning Contractors of America, the trade group that publishes the Manual J, Manual S and Manual D design standards.
AFUE: Annual Fuel Utilization Efficiency: the share of a furnace or boiler's fuel that becomes heat for the home. A 95% AFUE furnace turns 95 cents of every fuel dollar into heat; the rest leaves through the vent.
AHRI certificate: A certificate from the Air-Conditioning, Heating, and Refrigeration Institute confirming the efficiency rating of a specific matched combination of outdoor unit, indoor coil and furnace or air handler. Ask for its reference number on any quote.
Air changes per hour (ACH): How many times the air in a home is replaced in an hour. Used to describe air leakage, often measured with a blower door test.
Air handler: The indoor unit that holds the blower and the cooling coil and pushes conditioned air through the ducts. It is paired with a heat pump or an outdoor air conditioner.
Air purifier: A device that removes particles or other pollutants from indoor air, either portable or built into the duct system.
Ambient temperature: The temperature of the surrounding air, such as the outdoor air around a condenser.
Auxiliary heat: Backup heat, often electric heat strips, that switches on when a heat pump cannot keep up in very cold weather or is defrosting.
Balance point: The outdoor temperature below which a heat pump can no longer meet the home's heating need on its own and backup heat begins to help.
Baseboard heater: A low heater along the base of a wall, either electric or fed with hot water from a boiler.
Blower door test: A test that uses a powerful fan in a door frame to measure how leaky a house is and find where air escapes.
Blower motor: The motor that drives the fan moving air across the coil or heat exchanger and out through the ducts.
Boiler: Equipment that heats water or makes steam for radiators, baseboards or in-floor tubing. A combi boiler also supplies hot water for taps.
BTU: British thermal unit: a measure of heat. Heating and cooling capacity is given in BTU per hour; 12,000 BTU per hour equals one ton of cooling.
Btu/h: British thermal units per hour, the usual unit for the heating or cooling capacity of equipment.
C-wire: The common wire that provides continuous power to a thermostat. Many smart thermostats need one.
Capacitor: A small electrical part that gives motors and compressors the jolt they need to start. A failed capacitor is a common reason an outdoor unit hums but does not run.
Carbon monoxide (CO): A colorless, odorless poisonous gas produced when fuel burns incompletely. Fuel-burning equipment must vent it safely, and every home with such equipment needs working CO alarms.
CFM: Cubic feet per minute, the measure of how much air a fan or duct moves.
Charge (refrigerant): The amount of refrigerant in a system. Too much or too little cuts efficiency and can damage the compressor.
Chiller: Large equipment that cools water for air conditioning in commercial buildings.
Cold-climate heat pump: A heat pump designed to keep producing useful heat at very low outdoor temperatures.
Combustion air: The outside air a fuel-burning furnace or boiler needs to burn fuel safely.
Compressor: The pump in an air conditioner or heat pump that moves refrigerant around the system. It is the most expensive part to replace.
Condensate drain: The line that carries away the water an air conditioner pulls out of the air. When it clogs, water can back up and leak or shut the system off.
Condenser: The outdoor part of an air conditioner or heat pump, with a fan and coil, that releases heat from the house to the outside air.
Condensing furnace: A high-efficiency gas furnace, usually 90% AFUE or more, that recovers extra heat from its exhaust and vents through plastic pipe.
Contactor: An electrical switch in the outdoor unit that turns the compressor and fan on when the thermostat calls. A worn contactor is a common repair.
COP: Coefficient of performance: heat delivered divided by energy used at a given temperature. A COP of 3 means three units of heat for each unit of electricity.
Defrost cycle: A short period when a heat pump melts frost off its outdoor coil in cold weather. Steam from the unit and a brief switch to backup heat are normal.
Dehumidifier: Equipment that removes moisture from the air, either on its own or built into the HVAC system.
Dew point: The temperature at which moisture in the air condenses. Air conditioners remove humidity by cooling air below its dew point at the coil.
Diffuser: A vent cover designed to spread supply air across a room.
Dual fuel: A system that pairs an electric heat pump with a gas furnace and uses whichever is cheaper or more effective at the current outdoor temperature.
Duct: A passage, usually sheet metal or flexible tube, that carries heated or cooled air to rooms (supply) and back to the system (return).
Duct leakage test: A test that pressurizes the duct system to measure how much air escapes through leaks.
Duct sealing: Closing leaks in ductwork with mastic or approved tape, often in attics and crawlspaces, so conditioned air reaches the rooms.
Ductless: A heating and cooling system that delivers air directly from indoor units in the rooms, without ductwork. Usually a mini-split.
ECM motor: Electronically commutated motor: an efficient, variable-speed motor used in many blowers and fans.
EER: Energy Efficiency Ratio: cooling output divided by power used at one fixed set of conditions. Higher is better.
EER2: The updated version of EER, measured under the test procedure used since 2023.
Electric furnace: A furnace that heats air with electric resistance elements instead of burning fuel.
Emergency heat: A heat pump thermostat setting that runs only the backup heat. Meant for when the heat pump has failed.
ENERGY STAR: A U.S. EPA program that labels equipment more efficient than the federal minimum. Some rebates require it.
EPA Section 608: The federal certification technicians need to buy and handle refrigerant. It applies in every state, on top of any state or city license.
ERV and HRV: Energy and heat recovery ventilators bring in fresh air while recovering most of the heat or coolness from the air going out.
Evaporator coil: The indoor coil where refrigerant absorbs heat from the air, cooling it. It is also where an iced-up system freezes.
Expansion valve: A device that controls how much refrigerant flows into the indoor coil. A TXV (thermostatic expansion valve) adjusts automatically.
Fan coil: An indoor unit with a fan and a coil, used with heat pumps or with hot or chilled water.
Flame sensor: A small rod that tells a gas furnace its burner is lit. A dirty sensor is a common reason a furnace lights and then shuts off.
Flue: The pipe or chimney that carries combustion gases from a furnace, boiler or water heater outside.
Furnace: Equipment that heats air by burning gas or oil, or with electric elements, and blows it through the ducts.
Gas valve: The valve on a furnace or boiler that controls gas flow to the burners.
Geothermal heat pump: A heat pump that exchanges heat with the ground through buried pipes. It costs more to install but uses less energy to run.
Grille: The cover over a return-air opening.
Heat exchanger: The metal chamber in a furnace where hot combustion gases warm the air without mixing with it. A cracked heat exchanger can leak carbon monoxide and is a serious safety problem.
Heat load: The amount of heat a home gains (cooling load) or loses (heating load) that the system must handle. Found with a load calculation.
Heat pump: A system that moves heat instead of making it: it heats a home in winter and cools it in summer. Modern cold-climate models keep working well below freezing.
Heat strips: Electric resistance heaters in an air handler that provide backup or emergency heat for a heat pump.
HEPA filter: High-efficiency particulate air filter. Very fine filtration, usually in portable purifiers or special systems because of its airflow resistance.
HSPF: Heating Seasonal Performance Factor: the heating efficiency rating for a heat pump. Higher is better. Newer systems are rated HSPF2.
HSPF2: The updated version of HSPF, used for heat pumps made from 2023 onward.
Humidifier: Equipment that adds moisture to dry winter air, either portable or attached to the furnace.
Hydronic heating: Heating with hot water or steam sent through pipes to radiators, baseboards or in-floor tubing, usually from a boiler.
Igniter: The part that lights the burner in a modern gas furnace, either a hot surface igniter or a spark. A weak igniter can stop a furnace starting.
Inducer motor: A small fan in a modern furnace that pulls combustion gases through the heat exchanger and out the vent.
Inverter: Electronics that let a compressor or fan run at variable speeds, matching output to demand.
Latent heat: Heat involved in removing moisture from air. An air conditioner handles both latent (humidity) and sensible (temperature) heat.
Limit switch: A safety switch that shuts a furnace burner off if it overheats.
Line set: The pair of copper pipes that carry refrigerant between the indoor and outdoor units.
Load calculation: A measurement of how much heating and cooling a home needs, based on its size, insulation, windows and climate. Often called a Manual J. It should come before sizing new equipment.
Manual D: The ACCA method for designing duct systems so each room gets the right airflow.
Manual J: The industry method for a load calculation, published by the Air Conditioning Contractors of America (ACCA).
Manual S: The ACCA method for choosing equipment whose real capacity matches the calculated load.
Mechanical permit: A permit from the local building department for installing or replacing heating and cooling equipment. Usually followed by an inspection.
MERV: Minimum Efficiency Reporting Value: a 1 to 16 rating of how well an air filter catches particles. A higher MERV filter catches more but can restrict airflow if the system was not built for it.
Mini-split: A ductless heat pump or air conditioner with an outdoor unit linked to one or more wall-mounted indoor units, used for additions, older homes without ducts and single rooms.
Packaged unit: A system with all its main parts in one outdoor cabinet, often on a roof or slab, connected to the ducts.
Pilot light: A small flame that stays lit to ignite the burners in older gas furnaces and boilers.
Plenum: The box at the top or bottom of the furnace or air handler where the main supply or return ducts connect.
Pressure switch: A safety switch that confirms the inducer motor is venting combustion gases before a furnace lights.
PTAC: Packaged terminal air conditioner: the through-the-wall units common in hotels and apartments.
R-22: An older refrigerant, also called Freon, no longer produced in the U.S. since 2020. Systems that use it rely on recycled supply.
R-32: A lower-impact A2L refrigerant used in some newer air conditioners and heat pumps.
R-410A: The refrigerant used in most home air conditioners and heat pumps installed in recent decades. Its production is being phased down, but existing systems can still be serviced.
R-454B: A lower-impact A2L refrigerant used in many new home air conditioners and heat pumps.
Radiant heating: Heat delivered through warm floors, walls or panels, often by hot-water tubing under the floor.
Refrigerant: The fluid that carries heat in an air conditioner or heat pump. R-22 was phased out in 2020; newer systems use lower-impact refrigerants such as R-454B or R-32.
Register: A supply vent cover with louvers you can open or close.
Return air: The air drawn back from the rooms to the system through return vents to be heated or cooled again.
Reversing valve: The valve that switches a heat pump between heating and cooling.
Rollout switch: A furnace safety switch that shuts off the gas if flames roll out of the burner area.
Rooftop unit (RTU): A packaged heating and cooling unit installed on a roof, common on commercial buildings.
SEER and SEER2: Seasonal Energy Efficiency Ratio: the cooling efficiency rating for an air conditioner or heat pump. Since 2023, new systems are rated SEER2, measured under a tougher test than the older SEER. Higher is better.
Sensible heat: Heat that changes air temperature, as opposed to latent heat, which changes humidity.
Short cycling: When a system turns on and off every few minutes without finishing a cycle. It wastes energy and wears equipment, and it usually points to sizing, airflow or a control problem.
Smart thermostat: A Wi-Fi thermostat with schedules, remote control and often learning features. Many need a C-wire for steady power.
Split system: A system with an outdoor unit (condenser or heat pump) and an indoor unit (furnace or air handler) linked by refrigerant lines.
Static pressure: The resistance air meets moving through the filter, coil and ducts. High static pressure strains the blower and cuts airflow.
Subcooling and superheat: Temperature measurements technicians use to check a system has the right refrigerant charge.
Supply air: Conditioned air delivered to rooms through supply vents.
Temperature split: The difference between the air temperature entering the return and leaving the supply vents. A quick health check for cooling.
Thermocouple: A safety sensor in older furnaces that confirms the pilot light is lit before gas flows.
Thermostat: The control that tells the system when to run. Smart thermostats add schedules and remote control.
Ton: A unit of cooling capacity equal to 12,000 BTU per hour. It has nothing to do with the weight of the equipment. Most homes use 2 to 5 tons.
Two-stage and variable-speed: Equipment that can run at more than one output. Variable-speed equipment adjusts smoothly, which usually means steadier temperatures and quieter running than a single-stage system.
UV lights (HVAC): Ultraviolet lamps installed near the indoor coil to limit mold and bacteria growth on it.
Ventilation: Bringing outdoor air into a home and exhausting stale air, naturally or with fans, ERVs or HRVs.
VRF system: Variable refrigerant flow: a commercial heat pump system that serves many zones from shared outdoor units.
Wet bulb temperature: A temperature reading that reflects humidity; used by technicians to check cooling performance and charge.
Zone damper: A motorized plate in a duct that opens or closes to control airflow to one zone.
Zoning: Dividing a home into areas, each with its own thermostat and dampers in the ducts, so each area can be heated or cooled separately.

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