HVAC Glossary

One hundred common HVAC terms, alphabetized for quick reference while you read bids and contractor proposals. Every term has a permanent link you can bookmark or share.

Search glossary

ACCA: Air Conditioning Contractors of America; publishes Manual J, D, and S standards.
AFUE: Annual Fuel Utilization Efficiency; measures furnace heating efficiency as a percentage.
AHRI: Air-Conditioning, Heating, and Refrigeration Institute; certifies equipment performance ratings.
Air balance: Adjusting dampers and fan speed so each room receives intended airflow.
Air handler: Indoor unit that moves conditioned air through ductwork.
Ambient temperature: Outdoor air temperature surrounding HVAC equipment.
ASHRAE: Professional society publishing HVAC design and comfort standards.
Blower motor: Fan motor moving air through the furnace or air handler.
Boiler: Heats water circulated through radiators or hydronic coils.
BTU: British Thermal Unit; heat needed to raise one pound of water one degree Fahrenheit.
BTU/h: BTUs per hour; common capacity measurement for heating and cooling.
Capacitor: Electrical component helping start and run compressor and fan motors.
Cased coil: Evaporator coil installed inside a sheet metal cabinet.
CFM: Cubic feet per minute of airflow through ducts or equipment.
Charge: Correct amount of refrigerant in a sealed refrigeration system.
Charge check: Verifying refrigerant level using pressures, temperatures, and superheat/subcooling.
Chiller: Central plant that produces chilled water for large-building cooling.
Combustion air: Air supplied to furnaces for safe, complete fuel burning.
Compressor: Pump that pressurizes refrigerant in the cooling cycle.
Condensate drain: Pipe removing moisture collected on the cooling coil.
Condenser: Outdoor coil where refrigerant releases heat to outside air.
Contactor: Heavy-duty electrical switch that energizes compressor circuits.
Cooling tower: Heat rejection device used with water-cooled chillers.
Damper: Plate or valve regulating airflow inside ductwork.
Defrost cycle: Heat pump mode that melts ice buildup on the outdoor coil.
Dehumidification: Removing moisture from indoor air during cooling operation.
Delta T: Temperature difference across a coil, room, or heat exchanger.
Desuperheater: Accessory capturing waste heat for domestic hot water.
Duct sealing: Closing duct leaks to improve efficiency, comfort, and IAQ.
Ductwork: Channels distributing conditioned air throughout a building.
Economizer: Uses cool outdoor air for free cooling when conditions allow.
EER: Energy Efficiency Ratio at a fixed outdoor temperature.
EER2: Updated EER test procedure reflecting real-world conditions.
Energy Star: EPA label identifying high-efficiency HVAC equipment.
Enthalpy: Total heat content of air including moisture energy.
ERV: Energy recovery ventilator exchanging heat and moisture between air streams.
Evaporator coil: Indoor coil where refrigerant absorbs heat from home air.
Expansion valve: Device metering refrigerant into the evaporator coil.
Fan coil: Unit with a coil and fan, often in hydronic systems.
Filter drier: Removes moisture and debris from refrigerant lines.
Filter grille: Return-air opening that houses an air filter.
Fire damper: Safety damper that closes to slow fire spread in ducts.
Flame sensor: Proves burner ignition before a gas furnace continues heating.
Flue: Vent carrying combustion gases safely outdoors.
Freon: Legacy trade name often used generically for refrigerants.
Furnace: Heats air using gas, oil, propane, or electric resistance.
Gas valve: Controls fuel flow to furnace burners.
Gauge port: Service access point for refrigerant pressure readings.
Ground loop: Buried piping exchanging heat for geothermal systems.
Hard start kit: Capacitor accessory helping aging compressors start.
Head pressure: High-side refrigerant pressure at the condenser.
Heat exchanger: Transfers furnace combustion heat to supply air safely.
Heat pump: Moves heat in or out of a space instead of only generating it.
Heat strip: Electric resistance backup heat element in air handlers.
HRV: Heat recovery ventilator exchanging heat without moisture transfer.
HSPF: Heating Seasonal Performance Factor for heat pump heating efficiency.
Humidifier: Adds moisture to dry winter indoor air.
HVAC: Heating, ventilation, and air conditioning.
IAQ: Indoor air quality related to comfort, odors, and health.
Ignitor: Device that lights gas furnace burners electronically.
Inducer motor: Draft fan that vents combustion gases from furnaces.
Inverter: Variable-speed drive technology for compressors and fans.
Latent heat: Energy absorbed or released during moisture phase changes.
Latent load: Cooling work required to remove humidity from air.
Limit switch: Safety control shutting burners off on overheating.
Line set: Copper tubing connecting indoor and outdoor units.
Liquid line: High-pressure refrigerant line from condenser to metering device.
Load calculation: Manual J analysis sizing equipment to home characteristics.
Low ambient kit: Accessory allowing cooling operation in cooler outdoor weather.
Make-up air: Outdoor air brought in to replace exhausted building air.
Manual D: ACCA standard for residential duct system design.
Manual J: ACCA standard for residential heating and cooling load calculation.
Manual S: ACCA standard for selecting equipment based on load results.
Matched system: Indoor and outdoor components certified to operate together.
MERV: Minimum Efficiency Reporting Value rating filter particle capture.
Mini-split: Ductless system with one or more wall-mounted indoor units.
Modulating furnace: Furnace adjusting heat output gradually rather than on/off only.
NATE: North American Technician Excellence certification for HVAC techs.
Negative pressure: Building pressure lower than outdoors, pulling air inward.
OEM: Original equipment manufacturer parts made for specific models.
Orifice: Fixed bore device metering refrigerant in some systems.
Overcharge: Excess refrigerant that can reduce efficiency and damage compressors.
P-trap: Drain trap preventing sewer gases from entering the air stream.
Package unit: Self-contained outdoor cabinet with heating and cooling components.
Plenum: Air distribution box attached to a furnace or air handler.
Positive pressure: Building pressure higher than outdoors, pushing air outward.
Programmable thermostat: Thermostat that schedules automatic temperature setbacks.
PSI: Pounds per square inch; common refrigerant pressure unit.
Psychrometrics: Study of air properties involving temperature and humidity.
R-32: Lower global-warming-potential refrigerant used in newer equipment.
R-410A: Common non-ozone-depleting refrigerant blend in residential systems.
R-value: Insulation resistance to heat flow; higher is better.
Radiant heat: Infrared heating from floors, panels, or ceilings.
Reciprocating compressor: Piston-based refrigerant pump design.
Reclaim: Processing used refrigerant for reuse under EPA rules.
Recovery: Removing refrigerant from a system before service or disposal.
Reheat: Re-warming overcooled dehumidified air for comfort.
Relay: Low-voltage controlled electrical switch.
Return air: Air drawn back to the system for reconditioning.
Return drop: Vertical duct returning air to the equipment.
Reversing valve: Switches heat pump operation between heating and cooling.
Scroll compressor: Spiral compressor design common in modern equipment.
SEER: Seasonal Energy Efficiency Ratio for cooling efficiency.
SEER2: Updated SEER test standard with more realistic conditions.
Sensible heat: Heat that changes air temperature without phase change.
Sensible load: Cooling required to lower dry-bulb air temperature.
Setback: Scheduled thermostat change reducing demand during unoccupied hours.
Short cycling: Frequent on/off operation from oversizing or control faults.
Single stage: Equipment operating at one full output level only.
Smart thermostat: Wi-Fi connected control with scheduling and remote access.
Sound blanket: Noise insulation wrapped around compressors.
Split system: Separate indoor and outdoor equipment designed as a pair.
Static pressure: Duct resistance to airflow; key diagnostic measurement.
Subcooling: Temperature drop below refrigerant saturation on the liquid line.
Suction line: Low-pressure refrigerant line returning gas to the compressor.
Superheat: Temperature rise above refrigerant saturation on the vapor line.
Supply air: Conditioned air delivered from equipment into rooms.
Swamp cooler: Evaporative cooler using water evaporation to cool air.
Thermostat: Control device regulating temperature setpoints.
Ton: 12,000 BTU per hour of cooling capacity.
Tonnage: Total system cooling capacity expressed in tons.
Two stage: Equipment with high and low capacity operating modes.
TXV: Thermostatic expansion valve metering refrigerant by temperature.
Undercharge: Insufficient refrigerant causing poor capacity and coil freezing.
UV light: Air handler accessory reducing microbial growth on coils.
Vapor line: Another name for the suction refrigerant line.
Variable speed: Motor or compressor adjusting output to match demand.
VAV: Variable air volume system modulating airflow in commercial buildings.
Ventilation: Controlled exchange of outdoor and indoor air.
VFD: Variable frequency drive controlling fan or pump motor speed.
Wet bulb: Temperature reading that accounts for evaporative cooling effect.
Zone damper: Motorized damper controlling airflow to a specific zone.
Zone panel: Control panel operating multiple zone dampers and thermostats.
Zoning: Independent temperature control in different areas of a home.