The Definitive Glossary of HVAC Terms You Were Looking For | DIY Home Comfort

Welcome, DIY enthusiasts! Have you ever felt lost in the alphabet soup of HVAC terminology? You're not alone. That's why we've put together this comprehensive glossary of HVAC terms.

We're here to help you decode those puzzling acronyms and complex jargon like a pro. So grab your tool belt, put on your thinking cap, and let's dive into this comprehensive, user-friendly guide that will turn you from a novice into your home's very own HVAC expert.

Ready to bring more comfort to your home? Let's get started!

• Absolute Pressure: The total pressure within a system, including gauge and atmospheric pressure. It is sometimes denoted with an "a" at the end, such as BARa for bar absolute or PSIa for pounds per square inch absolute.
• Accumulator: A tank in a refrigeration system that stores liquid refrigerant coming from the evaporator and prevents it from flowing back into the compressor, which could cause damage.
• AC Fans: These are fans powered by alternating current (AC) motors used in various applications within heating, cooling, and ventilation systems to move air.
• Air Changes Per Hour (ACH): The rate at which the air in a space is replaced with ventilation per hour.
• ACRIB: Air Conditioning and Refrigeration Industry Board. An organization representing the interests of the air conditioning and refrigeration industry, providing guidance, regulation, and standards.
• Advanced Reciprocating Compressor: A more efficient type of compressor for refrigerant compression.
• Adiabatic Cooling: A process that reduces heat through a change in air pressure caused by volume expansion without the loss or gain of heat in the system.
  
• Air Conditioner: A device designed to control the temperature, humidity, and air quality within a defined space.
• Air-cooled System: An air conditioning system that uses air to remove heat from the refrigerant in the condenser, typically with the condenser located outside to expel waste heat to the atmosphere.
• Air Diffuser: An outlet or grille that directs airflow within a space, designed to distribute the air in a balanced manner.
• Air flow distribution: Refers to the way air is circulated through a space by the HVAC system.
• Air Handling Unit (AHU): A central unit that includes fan(s), heating and cooling elements, filter racks or chambers, dampers, humidifiers, and other equipment necessary for proper ventilation and air extraction.
• Air Purifier: Device that cleans the air in HVAC systems.
• AHRI: Air Conditioning, Heating, and Refrigeration Institute. A US based industry association that standardizes the testing and certification of HVAC equipment to ensure performance meets specified criteria.
• Alternating Current (AC): A type of electrical current in which the flow of electric charge periodically reverses direction. In HVAC systems, AC operates electric motors in equipment such as fans, compressors, and pumps.
• Ambient: Refers to the normal atmospheric conditions of temperature and pressure in the surrounding environment.
• Amps (Amperage): The measurement of the strength of an electrical current.
• Annual Fuel Utilization Efficiency (AFUE): A measure of how efficiently a furnace or boiler converts the energy in its fuel to heat over the course of a typical year.
• Application: The specific location or site where an air conditioning or cooling system or unit is to be installed or applied.
• ASHRAE: American Society of Heating, Refrigerating, and Air-Conditioning Engineers. A global society advancing human well-being through sustainable technology for the built environment.
• Auxiliary Heat: Supplemental heating that assists the primary system during high demand.
• Azeotrope or Azeotropic Mixture: A mixture of two or more compounds that behaves as a single substance because it has the same composition in both the liquid and vapor states. It cannot be separated into its components by simple distillation due to having no temperature glide except at specific points.
• Azeotropic Point: The specific temperature at which an azeotropic mixture boils, producing vapor with the same composition as the liquid.

• BEMS: Building Energy Management System. A computer-based system installed in buildings that controls and monitors the building's mechanical and electrical equipment, such as ventilation, lighting, power systems, fire systems, and security systems.
• Bioaerosols: Microscopic living organisms or fragments of living things suspended in the air.
• Blend: A refrigerant mixture made up of two or more refrigerants blended in a specific ratio, which can be separated by distillation. These blends may exhibit significant temperature glide.
• Blower Motor: An electric motor in an HVAC system that powers the fan to move air through the system.
• Breaker: An electrical switch designed to protect an electrical circuit from damage caused by an overload or short circuit.
• BREEAM: Building Research Establishment Environmental Assessment Methodology. The world's first and most widely used method of assessing, rating, and certifying the sustainability of buildings.
• British Thermal Unit (BTU): A unit of heat defined as the amount of heat required to raise the temperature of one pound of water by one degree Fahrenheit.
• Bubble Point: The temperature and pressure at which a refrigerant changes from a liquid to a gas.
• Breaker: An electrical switch designed to protect an electrical circuit from damage caused by an overload or short circuit.
• Boiler: A heating appliance that heats water for home heating or hot water.
  
• BTUh (British Thermal Units per Hour): A measure of the heat output per hour of heating equipment.
• Building Envelope: The elements of the outer shell of a building that maintain a dry, heated, or cooled indoor environment and facilitate its climate control.
• Burner Flame: The flame within a furnace where fuel is combusted to generate heat.

• CAE (Combined Annual Efficiency): Measures the amount of heat produced per dollar of fuel used for both home and water heating systems.
• Capacitor: An electrical component that stores and releases electrical energy, commonly used to start motors.
• Capacity: The ability of an HVAC system to heat or cool a given space.
• Capillary Tube: A small diameter tube that acts as a throttling device for refrigerant, best used in constant ambient conditions.
• Carbon Monoxide (CO): A poisonous gas produced by incomplete burning of carbon-based fuels.
• Carbon Monoxide Alarm: Detects carbon monoxide levels in the home.
• Carbon Usage Effectiveness (CUE): A metric indicating the ratio of total CO2 emissions from a data center to its IT equipment energy consumption.
• Cascade System: A refrigeration system with multiple refrigerant circuits in series, providing various levels of cooling.
• Cassette Unit: An air conditioning unit fitting into suspended ceilings, ideal for large or irregularly shaped rooms.
• Central Air Conditioning: A system that cools and dehumidifies air for an entire building or space using a central cooling unit and a system of ducts to distribute the conditioned air.
• Centrifugal Compressor: A type of compressor that uses a rotating wheel to compress gas using centrifugal force.
• Centrifugal Fan: A mechanical device for moving air or other gases in a direction at an angle to the incoming fluid.
• CFC: Chlorofluorocarbon, a compound with high ozone depletion potential, banned from use since 2000.
• CFM: Cubic Feet per Minute, a measure of airflow rate in an HVAC system.
• Change of State: The transition of a substance from solid to liquid or gas phases and vice versa.
• Chilled Beam System: A type of convection HVAC system that uses chilled water to cool the air. These systems are known for their energy efficiency and are used in various commercial buildings.
• Chilled Water System (CWS): A system that uses water as a secondary medium to transfer heat away from refrigerant in a chiller.
• Chiller: A machine that removes heat from a liquid via a vapor-compression or absorption refrigeration cycle.
• CIBSE: Chartered Institution of Building Services Engineers, a professional UK based association for building services engineers.
• CITB: Construction Industry Training Board, a UK organization providing training for the construction industry.
• Clean Room: A controlled environment with a low level of pollutants used in manufacturing or scientific research.
• Climate Control: See Precision Air Conditioning.
• Coefficient of Performance (COP): A ratio of heating or cooling provided to the energy consumed by the HVAC system.
• Coil: A heat transfer component within an HVAC system, typically a fin and tube heat exchanger.
• Cold Aisle: A data center design where server racks are aligned to create alternating rows of cold air intakes and hot air exhausts.
• Comfort Air Conditioning: Systems designed for human comfort, not necessarily for process operations or equipment cooling.
• Commercial Air Conditioning: HVAC systems for large buildings like hotels or hospitals.
• Commissioning: The process of ensuring that installed HVAC equipment operates correctly according to specifications.
• Compressor: A device that increases the pressure and temperature of the refrigerant in an HVAC system.
• Condensate: The liquid that forms when moist air is cooled below its dew point.
• Condensate Drain: A pipe that carries condensation from the air handler to the outside or a drain.
• Condenser: A component that condenses refrigerant gas into a liquid by removing heat.
• Condensing Unit: A self-contained unit consisting of a compressor and condenser used in refrigeration and air conditioning systems.
• Conduction: Heat transfer through direct contact between materials of different temperatures.
• Contactor: An electrical relay that controls the flow of electricity to components in an HVAC system.
• Constant Air Volume (CAV): An HVAC system that delivers a constant air flow rate with varying temperatures.
• Controller: A device that regulates the operation of different components within an HVAC system.
• Controls: Systems used to regulate the operation of HVAC equipment.
• Convection: Heat transfer by the movement of fluid or gas due to changes in density and thermal currents.
• Cooling Tower: A device that cools water by evaporative cooling, commonly used in industrial or large-scale air conditioning systems.
• CRAC: Computer Room Air Conditioning unit used for precise environmental control in data centers.
  
• CRAH: Computer Room Air Handling unit, part of the HVAC system specifically designed for server rooms.
• CWR: Chilled Water Return, the part of the system returning water back to the chiller after it has absorbed heat.
• Cycling: The process of an HVAC system turning on and off.

• Damper: A movable plate in ductwork that regulates airflow; it can be manual or automatic.
• Data Center: A facility that houses computer systems and associated components, such as telecommunications and storage systems, often requiring precise climate control.
• DC (Direct Current): Electrical current that flows in a single direction, used in various components of HVAC systems.
• Decibel (dB): A unit for measuring sound intensity.
• Degree-Day: A unit for estimating the energy demand for heating or cooling a building based on the difference between the outdoor temperature and a standard base temperature.
• Delta T (ΔT): This term represents the temperature difference between two points. In HVAC, it typically refers to the difference between the return air temperature and the supply air temperature. A correct ΔT indicates proper functioning of the HVAC system and is essential for efficient operation.
• Dehumidification: The process of removing moisture from the air to control humidity levels within a space.
  
• Dehumidifier: An appliance that reduces the level of humidity in the air, enhancing comfort and reducing the potential for mold growth.
• Density: A measure of how much mass is contained in a given volume of a substance, important in calculating system loads and refrigerant charges.
• Dew Point: The temperature at which air becomes saturated with moisture and water vapor begins to condense into dew, indicating the level of moisture in the air.
• Dew Point Control: In HVAC, controlling the dew point is crucial to prevent condensation, which can lead to mold growth and other moisture-related problems.
• Diffuser: An outlet that spreads air flow from an HVAC system evenly across a room.
• Direct Expansion Systems (DX): Air conditioning systems where the refrigerant directly expands in the evaporator, absorbing heat from the air.
• DOE (Department of Energy): The U.S. federal department responsible for energy policy and nuclear safety.
• Downflow: An air distribution method where air is pushed downward, typically used in systems with floor-mounted units.
• Downflow Furnace: A type of furnace where air enters from the top and heated air exits from the bottom.
• Drain Pan: A component that collects condensation from the HVAC system, preventing water damage and directing moisture away from the system.
• Drop-in: A refrigerant that can replace another without significant system modifications, often used when phasing out older refrigerants for newer, more environmentally friendly options.
• Dry Bulb Temperature: The temperature of air measured by a thermometer freely exposed to the air but shielded from radiation and moisture.
• Dry-Charged Unit: An air conditioner or heat pump charged with refrigerant at the installation site.
• Duct: Passageways used in HVAC systems to deliver and remove air to and from different areas within a building.
• Ductwork: The system of ducts used to transport air from the HVAC unit to various parts of the building.
• Dual Fuel Heat System: Combines a heat pump with a furnace for heating.

• EC Fan: Electronically Commutated fans use brushless DC motors with in-built electronics to convert AC to DC, offering up to 30% more efficiency and variable speed control.
• Economizer: A device that recycles energy or optimizes temperature differences to improve system efficiency and reduce energy consumption.
• EER (Energy Efficiency Ratio): A system efficiency measure at a given set of rating conditions, calculated by dividing the cooling capacity in kW by the power input in kW.
• Efficiency: A measurement of how effectively an HVAC system converts energy into heating and cooling.
• Electronic Air Cleaner (EAC): Reduces contaminants in indoor air.
• Electronic Expansion Valve (EEV): Utilizes a DC stepper motor to control refrigerant flow precisely, maintaining close control of evaporator superheat.
• Element: A component in a heater that converts electrical energy into heat.
• Emergency Heat: Backup heating system activated when primary heat source is insufficient, often used in extreme cold.
• Energy Star: An EPA program that certifies buildings and products like HVAC systems for energy efficiency.
• Energy Technology List (ETL) / ECA Scheme: A UK government program offering enhanced tax relief for investments in energy-saving equipment.
• Energy Efficiency Ratio 2 (EER2): A measure of the efficiency of air conditioners or heat pumps.
• Enthalpy: A thermodynamic property representing total internal energy plus total heat content, sometimes called total heat.
• Entropy: A measure of the rate at which an object absorbs heat.
• Energy Reuse Effectiveness (ERE): A ratio indicating the amount of energy emitted from a data center and reused elsewhere compared to the total energy consumed.
• Energy Reuse Factor (ERF): The ratio of energy reused within a facility to the total energy input to a data center.
• EPA (Environmental Protection Agency): The U.S. agency overseeing environmental protection and HVAC refrigerant regulations.
• ESEER: European Seasonal Energy Efficiency Ratio. Similar to EER but tailored for the European market, accounting for variable load performance across different seasons.
• Evaporator Coil: A component usually located indoors that allows the refrigerant to evaporate and absorb heat from the air passing over it.
• Evaporating Temperature: The temperature at which a refrigerant vaporizes inside an evaporator.
• Expansion Valve: A device that controls refrigerant flow into the evaporator, which can be thermostatically operated, a capillary tube, or a fixed orifice device.

• Fahrenheit: A temperature scale where water freezes at 32 degrees and boils at 212 degrees at standard atmospheric pressure.
• Fan: A device consisting of a motor and a blower wheel that creates airflow.
• Fan motor: A motor that powers a fan to move air in an HVAC system.
• Fan Coil Unit (FCU): A simple device consisting of a heating or cooling coil and fan. It is part of an HVAC system found in residential, commercial, and industrial buildings.
• Filter: A device in HVAC systems that prevents contaminants from entering the equipment must be maintained or replaced regularly. There is also a filter in the refrigeration system, known as a drier, which removes dirt and undesired particles.
• Filter Drier: A device used in the liquid line of a refrigeration system to absorb moisture and trap particulate matter.
• Fire Damper: A device installed in ductwork that is designed to automatically close upon detection of heat, to prevent the spread of fire inside the ductwork.
• Flooded: Describes a condition where there is excess refrigerant in the evaporator or condenser coils.
• Flue: A vent that removes the byproducts of combustion from a furnace.
• Flow Rate: The volume of fluid that passes a point in a system over a given period of time.
• Freon: A trade name for a group of CFC and HFC refrigerants used in cooling systems.
• Fresh Air Intake (FAI): An opening through which outdoor air is drawn into the system. This air may be used for combustion or as part of the ventilation process.
• Furnace: The major component in a home for heating, which facilitates the combustion of fuel and air to create heat and then circulates it through the home using a fan.
• Fuse: A safety device in an electrical circuit that breaks the circuit in the event of excess electrical charge by melting a metal strip.
• Fusible Link: A part of a fire damper that melts at a certain temperature, allowing the damper to close.

• Galvanized Steel: Steel coated with zinc to prevent corrosion, commonly used in ductwork construction.
• Garage Heaters: Units providing warmth for garages, often using natural or propane gas.
• Gas: In HVAC terms, it often refers to the refrigerant in its gaseous state.
  
• Gas Furnace: A furnace that uses natural gas as a fuel source to heat a space by burning the gas in a combustion chamber.
• Gauge Pressure: The pressure reading from a gauge zeroed at atmospheric pressure, allowing for negative readings when below atmospheric pressure.
  
• Gauges: Instruments used to measure pressure and sometimes temperature within HVAC systems.
• Generator: A device that converts mechanical energy to electrical energy, often used as a backup power source in HVAC systems.
• Geothermal Heat Pump: A system that transfers heat to or from the ground or a body of water, using the earth as a source or sink for heat.
• Glide: The range of boiling temperatures of the different components in a refrigerant blend, affecting the composition of the blend across temperatures.
  
• Glycol: A fluid used in HVAC systems for its antifreeze properties, which can be circulated through closed-loop systems to transfer heat.
• Glycol-Cooled System: An air conditioning system using a water/glycol solution as a condensing medium, with a glycol-cooled condenser typically located inside the air conditioner.
• Grains per Pound (GPP): A measurement unit for the amount of moisture in the air, indicating the weight of water vapor in a pound of dry air.
  
• Green Building: A building designed to reduce environmental impact, often incorporating energy-efficient HVAC systems and sustainable materials.
• Grid-Tie Inverter Solar System: Allows homeowners to use solar-generated electricity, with the utility grid as a backup power source.
• Grille: An opening or a protective barrier, usually louvered, installed at the end of ductwork.
  
• GWP (Global Warming Potential): A measure of how much heat a greenhouse gas traps in the atmosphere relative to carbon dioxide, with various refrigerants having different GWP values. 
  

• Hard start: A kit used to start a compressor that requires a higher starting voltage.
• HCFC: Hydrochlorofluorocarbon (e.g. R22), lower ozone-depleting, global warming gases.
• Heat: In HVAC terms, refers to the energy transferred between systems not in thermal equilibrium.
• Heat Capacity: The amount of heat needed to raise the temperature of a given mass by 1 degree Kelvin.
• Heat Exchanger: A piece of equipment built for efficient heat transfer between two physically separated fluids.
• Heat Gain: The increase in thermal energy within a space, resulting from sources such as solar radiation, equipment, lighting, and occupants.
• Heat Load: The total amount of heat energy that needs to be removed from a space to maintain a comfortable temperature, often measured in BTUs (British Thermal Units).
• Heat load calculation: The process of determining the amount of heat energy to be added or removed from a space by the HVAC system.
• Heat Loss: The transfer of heat from inside a building to the outside through elements such as walls, windows, roof, and floor. It is a critical factor in determining the heating requirements of a space.
• Heat Pump: A mechanical device that absorbs low-grade heat from a lower temperature location and upgrades and transfers the resultant higher temperature medium to heat a space.
• Heat Strip: A heat strip for a heat pump is an electrical heating element that provides supplemental heat during extremely cold temperatures when the heat pump alone is insufficient.
• Heat Transfer: The exchange of thermal energy between two interacting media passing through a heat exchanger, such as refrigerants, air, and water.
• Heating Seasonal Performance Factor (HSPF): A ratio that measures the efficiency of a heat pump's heating abilities over a typical heating season. It is calculated by taking the total heating required during the season, expressed in BTUs, divided by the total electricity consumed during the same period, expressed in watt-hours.
• Hemi-anechoic Chamber: A room designed to absorb reflections of sound or electromagnetic waves, with a solid floor to support heavy items like air conditioning units for sound level testing.
• HEPA (High-Efficiency Particulate Air): A type of filter that can trap a large number of very small particles.
• HFC: Hydrofluorocarbon (e.g., R134a), zero ODP, high global warming refrigerant gas.
• HFO: Hydrofluoroolefin (e.g., R1234ze), zero ODP, low global warming, 4th generation refrigerant gas.
• High-Performance Computing (HPC): Refers to aggregating computing power to solve large problems in science, engineering, or business.
• High Voltage: In HVAC, refers to voltages higher than 50 volts.
• Horizontal Flow: Describes an air handler or furnace positioned on its side
• Hot Aisle: A layout design in data centers where server racks are aligned in rows with hot air exhausts facing the aisle.
• Humidification: The process of adding moisture to the air within a space.
• HVAC: Stands for Heating, Ventilation, and Air Conditioning and is the technology of indoor and vehicular environmental comfort aimed at providing thermal comfort and acceptable indoor air quality.
• Hybrid Heat: A dual fuel system using both an electric heat pump and a furnace.
• Hydrocarbon Refrigerant (HC): Chemicals containing only carbon & hydrogen suitable for use as a refrigerant, such as propane (R290) and isobutane (R600a), with zero ODP and very low GWP.
• Humidifier: A device that adds moisture to the air.
• Humidistat: A device that measures humidity and controls a humidifier.
• Humidity: The presence of water vapor in the air.

• IAQ (Indoor Air Quality): Refers to the air quality within and around buildings and structures, especially concerning the health and comfort of occupants.
• Ignition: Elevating the temperature of a substance to the point of causing a combustive reaction.
• Industrial Refrigerator: A large-scale refrigeration system designed for industrial applications, such as food processing, pharmaceuticals, and extensive commercial facilities, where high-capacity cooling is required.
• Indoor Air Flow: The air movement inside a building, calculated in cubic feet per minute (CFM).
• Indoor Coil (Evaporator Coil): Absorbs heat from indoor air, usually located inside the air handler or attached to the furnace.
• Insulation: Material used to reduce heat loss or heat gain by providing a barrier between areas that are significantly different in temperature.
• Intermediate Fluid: A fluid used in a secondary loop of a refrigeration system, which is cooled or heated by the primary refrigerant and then used to cool or heat the space or substance of interest.
• Inverter Compressor: A type of compressor that uses an inverter to control the speed of the compressor motor to modulate cooling capacity.
• Interconnection Agreement: A connection or link between power systems that enables them to draw on each other's reserve capacity in time of need.
• ISO 9000: International standards for quality management and assurance.
• ISP (Internet Service Provider): An organization that provides services for accessing, using, or participating in the Internet. It is not directly related to HVAC but may be included in the context of smart HVAC systems that require internet connectivity.

• Joule: A unit of energy in the International System of Units (SI). It is equal to the energy transferred to an object when the force of one Newton acts on that object in the direction of the force's motion over a distance of one meter. In HVAC, it is a standard unit of energy measurement.
• Jacket Cooling: A method of cooling an engine or compressor in which a water jacket is used. Water circulates through the jacket, absorbing heat from the engine or compressor.
• J-T (Joule-Thomson) Effect: The temperature change of a gas or liquid when it is forced through a valve or porous plug while kept insulated so that no heat is exchanged with the environment. This principle is used in refrigeration and air conditioning systems.
• Jumper: A short length of conductor used to make a connection between terminals or around a break in a circuit.

• Kilowatt (kW): A unit of power equal to one thousand watts. In HVAC, it is often used to measure the power consumption of heating and cooling systems.
• Kilowatt-hour (kWh): A unit of energy equivalent to one kilowatt of power expended for one hour. It's the standard unit of energy for billing by electric utilities.
• K-Factor: In HVAC, the K-factor is a value that represents the thermal conductivity of a particular material. It's used to calculate the rate of heat transfer through a material.
• Kickstart: A device used to help start a compressor, especially under conditions of low voltage or high head pressures.
• Kink: A bend or fold in tubing or piping that restricts fluid flow. In HVAC systems, kinks in refrigerant lines can impede system performance and must be avoided during installation.
• Knockout: A pre-cut, removable section of an electrical box or HVAC equipment casing that allows for the passage of cables or ducts.
• Kyoto Protocol: An international agreement aimed at reducing greenhouse gas emissions. It has significant implications for HVAC systems, encouraging eco-friendly refrigerants and energy-efficient designs.

• Latent Cooling Capacity: The energy required to change the moisture content of the air during the air conditioning process, measured in kg/kg of dry air.
• Latent Heat: The heat required to convert a solid into a liquid or vapor, or a liquid into a vapor, without changing its temperature.
• Line Set: A pair of copper pipes connecting the outdoor unit (condenser) to the indoor unit (evaporator coil) in a split-system air conditioner or heat pump, including a liquid line and a suction line.
• Liquid Line: In HVAC, a pipe that carries refrigerant in its liquid state from the condenser to the evaporator.
• Load Calculation: The process of determining the heating or cooling needs of a space to ensure the HVAC system is appropriately sized, considering factors like area size, insulation, and occupancy.
• Low Pressure Stage: In a refrigerant system, it's the part from the outlet of the expansion device to the compressor suction inlet where the lowest temperature is achieved in multi-stage systems.
• Low Voltage: Electrical energy that runs at lower voltages than the main power supply, typically used for thermostats and controls in HVAC systems.
• Louver: An arrangement of parallel blades or slats designed to permit airflow and block direct light or rain, used in HVAC for ventilation and equipment protection.
• Low Boy: A type of furnace with lower height.
  

• Make Up Air Unit (MUA): An intake supply fan designed to replace air that has been exhausted from a building. It may be heated or cooled to minimize the impact on the building's climate control systems.
• Manufacturer Approved System: A system where the components, such as a condensing unit, furnace, or air handler, are approved by the manufacturer and matched according to AHRI standards. Installation of unmatched systems is strongly discouraged.
• Matched Systems: HVAC systems where the indoor and outdoor units are designed to work together for optimal efficiency and performance.
• Media: The filtering material in an air filter that captures dirt, dust, mildew, or bacteria.
• Megger: A brand of high-voltage test equipment used to measure insulation resistance.
• MERV Rating (Minimum Efficiency Reporting Value): A standard that rates the overall effectiveness of air filters. Higher MERV ratings correspond to finer filtration, meaning fewer dust particles and other airborne contaminants can pass through the filter.
• Metering Device: A component in an HVAC system that regulates the flow of refrigerant to the evaporator.
• Micron: A unit of measure equal to one millionth of a meter, used to describe the size of particles that an air filter can capture.
• Microchannel coil: A type of coil constructed from parallel flow aluminum tubes mechanically brazed to enhanced aluminum fins, offering improved heat transfer and a more compact, lighter, and corrosion-resistant design.
• Mini-Split: A type of ductless heating and cooling system that allows for individual temperature control in different rooms or spaces within a building.
• Mineral oil: A lubricant used with certain refrigerants, known for its low moisture absorption properties.
• Minimum Outside Air: The least amount of outdoor air that needs to be introduced into a building to maintain indoor air quality.
• Multipoise: A term used to describe a furnace that can be installed in different positions, such as upright or horizontal, to fit the space where it is being installed.
• Multi-meter: A portable instrument used to measure electrical voltage, current, and resistance.

• NATE Certification: North American Technician Excellence. Certification program for HVAC technicians.
• Natural Refrigerant: Non-fluorochemical refrigerants like ammonia (R717), carbon dioxide (R744), and hydrocarbons (e.g., propane R290) that exist naturally in the environment. They are low in direct Global Warming Potential (GWP) and Ozone Depletion Potential (ODP) potential.
• Nanotechnology: The manipulation and application of materials and devices at the nanoscale, with potential uses in HVAC equipment and materials.
• Near Azeotrope: Chemical product formed by combining compounds with vapor and liquid compositions nearly identical. It has a temperature glide of less than 2ºC.
• Negative pressure: A condition where the pressure inside an enclosed space is less than the pressure outside.
• Needle Valve: A type of valve with a slender, pointed stem used for precise control of refrigerant flow in HVAC systems.
• NEMA: National Electrical Manufacturers Association. An organization that sets standards for electrical equipment, including motors used in HVAC systems.
• Net Metering: A utility service that allows homeowners to receive credit for self-generated electricity.
• Nitrogen Purge: The process of using nitrogen gas to remove moisture and contaminants during HVAC system maintenance or repairs.
• Non-Condensable Gas: Gas in a refrigeration system that doesn't condense at the condenser's temperature and partial pressure, leading to higher head pressure, often due to air leakage.
• Non-Contact Voltage Tester: A tool used by HVAC technicians to detect the presence of voltage without direct electrical contact.
• Noise Criteria (NC): A standard rating system used to measure and describe indoor noise levels in HVAC systems and spaces.
• Non-programmable thermostat: A basic thermostat that does not allow for scheduled temperature changes.
• Normal Operating Range: The typical range of conditions within which an HVAC system operates effectively and efficiently.
• NOx (Nitrogen Oxides): Pollutants produced by combustion processes in HVAC systems, which can have adverse environmental and health effects.
• NSF: National Sanitation Foundation. An organization that develops and sets standards for sanitation, including those related to food service equipment and commercial refrigeration.
• NPLV (Normalized Part Load Value): A measure of chiller efficiency at part-load conditions, representing energy performance during typical operating scenarios.
• Nuisance Tripping: Unexpected and repeated tripping or shutting down of HVAC equipment due to various issues, such as electrical faults or sensor malfunctions.
• NVR (Negative Volumetric Ratio): The volume of a refrigerant vapor compared to the volume of the same mass of liquid refrigerant at a given pressure.
• Nut Driver: A hand tool used by HVAC technicians for tightening or loosening nuts and bolts.
• N+1 Redundancy: Ensures system availability in case of component failure, where components (N) have at least one independent backup component (+1).

• ODP (Ozone Depletion Potential): A measure of a substance's potential to deplete the ozone layer in the Earth's stratosphere, often associated with older refrigerants like CFCs and HCFCs.
• Off-Gassing: The release of volatile organic compounds (VOCs) or other chemicals from materials, products, or substances over time, which can affect indoor air quality.
• Off-Grid or Off-The-Grid System: An independent electricity-generating system, not connected to the utility grid.
• Oil Furnace: Converts heating oil into heat for homes.
• Ohm: The unit of electrical resistance, measured in ohms (Ω), which determines how easily electric current can flow through a conductor.
• Operating Cost: The expenses associated with running and maintaining HVAC equipment, including energy consumption and maintenance.
• Open Loop System: A type of geothermal heat pump system that uses groundwater as a heat source or sink.
• Outdoor Air: The air from the external environment, which is typically drawn into HVAC systems for ventilation and heating or cooling processes.
• Outdoor Coil: Also known as a condenser coil. It is a component of an air conditioning or heat pump system located in the outdoor unit. The outdoor coil facilitates the release of heat absorbed from indoor air during the cooling cycle into the outdoor air.
• Outdoor Unit: The outdoor component of a split HVAC system that contains the condenser coil, compressor, and other parts responsible for releasing heat into the external environment.
• Outside Air Damper: A device that controls outdoor air flow into a building's ventilation system, allowing for fresh air intake.
• Outside Air Temperature: The ambient temperature of the outdoor environment which can impact HVAC system operation and efficiency.
• Overload: A condition where an electrical circuit or component carries a current higher than its designed capacity can lead to overheating and damage.
• Overcurrent: An electrical condition where the current flowing through a circuit exceeds the rated current or capacity of the equipment or conductor, typically caused by short circuits or overload.
• Oversizing: The practice of installing HVAC equipment that is larger or has a higher capacity than what is required, which can result in inefficient operation and higher energy costs.

• Packaged Unit: A heating and cooling system with all components in one outdoor unit.
• Particles: Tiny substances in the air, including dust, dirt, bacteria, and viruses.
• Particulate Filter: A filter that removes particles from the air, such as dust and pollen.
• Pascals (Pa): The unit of measurement for pressure used in HVAC systems.
• Payback Period: The time it takes for the savings from an HVAC system to equal the initial investment cost.
• PEX Tubing: A type of flexible plastic piping used for water lines in HVAC systems, known for its heat resistance and durability.
• Pilot Light: A small, continuous flame that is used to ignite the gas burner in older HVAC systems.
• Piping: The conduits that carry refrigerant, water, or gas in and out of HVAC systems.
• Piston (Metering Device): A component in an HVAC system's compressor that helps regulate the flow of refrigerant.
• Pitch: The inclination angle of ductwork which facilitates proper drainage of condensate in certain parts of the system.
• Plenum: A central distribution box for circulating air in ductwork, whether it be for supply or return.
• Plenum Space: The area of a building that provides pathways for HVAC and sometimes wiring, typically above a drop ceiling or under a raised floor.
• Positive Pressure: A condition where the pressure inside an enclosed space is greater than the pressure outside.
• Pressure Balancing: The method of adjusting the HVAC system to ensure balanced air pressure across different areas.
  
• Pressure Drop: The reduction in pressure as air flows through ductwork or components in an HVAC system.
• Pressure Relief Valve: A safety device that relieves pressure in the system if it becomes too high.
• Pressure Switch: A safety device that monitors the pressure in the HVAC system and shuts it down if necessary.
• Preventive Maintenance: Routine maintenance to keep an HVAC system running efficiently and to prevent breakdowns.
• Programmable Thermostat: An electronic thermostat that adjusts the system's operations based on programmed settings.
• Propeller Fan: A fan used in HVAC systems for moving air, with blades similar to a propeller.
• PSI (Pound per Square Inch): A unit of pressure measurement used in HVAC to gauge system pressure levels.
• Psychrometrics: The study of the physical and thermodynamic properties of gas-vapor mixtures.
• PTAC (Packaged Terminal Air Conditioner): A self-contained heating and air conditioning system often found in hotels.
• Purge Cycle: The process of removing impure refrigerant or air from a part of the HVAC system.
• Purge Unit: A device that eliminates non-condensable gases from a refrigeration system.
• PURON: A brand name for the refrigerant R-410A, used by Carrier and Bryant.
• PVC (Polyvinyl Chloride): A type of plastic used for condensate drain lines in HVAC systems.

• Quadrant Damper: A type of damper characterized by a quadrant-shaped mechanism that allows for the adjustment of the damper blade position in order to regulate airflow.
• Quick Connect/Disconnect: Refers to fittings used on HVAC equipment that can be connected or disconnected without the use of tools, often used for gauges and hoses in refrigerant lines.
• Q-Value (also called Q-Factor): In HVAC, it can refer to the thermal quality of a material or system, essentially a measure of heat flow resistance. It's also used in acoustics to describe how underdamped an oscillator or resonator is, and may pertain to HVAC in terms of noise and vibration control.
• Quenching: The process of cooling a hot material rapidly, such as in the manufacturing process of certain HVAC components. This can also refer to rapid cooling within refrigeration cycles.

• R-Value: Measure of thermal resistance for insulation materials.
• Radiant Barrier: Insulation that reflects radiant heat.
• Radiant Ceiling Panels: Ceiling-mounted heating devices.
• Radiant Floor: A heating system installed under the floor.
• Radiant Heating: Heating method using infrared radiation to heat surfaces.
• R1234ze (HFO Refrigerant): Eco-friendly refrigerant with very low GWP.
• R134a (HFC Refrigerant): Non-ozone-depleting refrigerant commonly used in various cooling applications.
• R22 (HCFC Refrigerant): An ozone-depleting refrigerant being phased out.
• R407C (HFC Refrigerant): A blend of refrigerants serving as a replacement for R22.
• R410A (HFC Refrigerant): A high-efficiency refrigerant replacing R22 in newer systems.
• Rack: In HVAC terms, could refer to structures holding HVAC components, more commonly a computing term.
• Reclaim: Processing and upgrading refrigerant for reuse.
• Recovery: Removing refrigerant from a system for recycling or disposal.
• Reciprocating Compressor: Compressor that uses pistons for compressing refrigerant.
• Recycle: Process of cleaning recovered refrigerant before reuse.
• Refrigerant: Fluid used for heat transfer within a refrigeration system.
• Refrigerant Charge: The specific amount of refrigerant in a system.
• Refrigeration Cycle (Vapour Compression Cycle): The cycle of refrigerant in a closed system that facilitates cooling.
• Refrigerant Lines: Pipes that carry refrigerant to and from system components.
• Register: An adjustable air outlet for conditioned air distribution.
• Relative Humidity (%RH): Ratio of current moisture in the air to the maximum possible at the same temperature.
• Relay: An electrical device, typically incorporating an electromagnet, that is activated by a current or signal in one circuit to open or close another circuit.
• Remote Electronic Expansion Valve (REEV): An expansion valve that can be controlled remotely.
• Remote Room Sensor: Assesses average conditions for the HVAC system.
• Reversing Valve: Valve in heat pumps to reverse refrigerant flow when switching between heating and cooling.
  
• Return Air: Used air returned to an HVAC system for reconditioning.
  
• Return Ducts: Ducts that carry return air back to the HVAC system.
• Return vent: The ductwork component that allows air to return to the furnace or air handler.
• Retrofit: Updating or modifying systems for better performance or efficiency.
• RCI (HI) and RCI (LO): Metrics for measuring the efficiency of server rack cooling in data centers.
• Riello Burner: A high-efficiency burner for oil furnaces.
• R-number: The ASHRAE-assigned number for recognized refrigerants.
• Rotary Compressor: A compressor using rotors instead of pistons.
• Rotary Screw Compressor: A compressor using interlocking screws.
• Rooftop Unit (RTU): Air-handling unit typically found on commercial building roofs.
• Return Air Plenum: Space where return air is collected.
• Return Side: Part of the HVAC system for returning air to be reconditioned.
  

• Saturation: The point at which a liquid can no longer dissolve in a solution or a vapor in a gas, relative to temperature and pressure.
• Scroll Compressor: A type of compressor used in HVAC systems that operates with two spiral-shaped scrolls to compress the refrigerant.
• SEER (Seasonal Energy Efficiency Ratio): A measure of the cooling efficiency of an air conditioner or heat pump over a typical cooling season.
• SEER 2 (Seasonal Energy Efficiency Ratio 2): Measures the efficiency of central air conditioners and heat pump systems.
• Sensible Heat: The heat that causes a change in temperature of a substance but does not contribute to a change in phase.
• Sensible Heat Ratio (SHR): The ratio of sensible heat to total heat in a HVAC system's process air stream.
• Sequencer: A device in electric heaters that controls the order in which heating elements turn on and off.
• Setback Thermostat: A thermostat that can change the temperature setting automatically at different times of the day.
• SHGC (Solar Heat Gain Coefficient): A measurement of how well a window blocks heat from sunlight.
• Short to Ground: A fault condition where an electrical circuit unintentionally connects to the ground or earth.
• Sling Psychrometer: A tool used to measure relative humidity by using the difference in temperatures between a wet bulb thermometer and a dry bulb thermometer.
• Solenoid: An electromechanical device that creates a magnetic field when electric current is passed through it, often used to control valves.
• Smart Recovery and Setback Programming: Manages system efficiency and comfort.
• Smoke Damper: A damper used in ductwork that automatically closes to prevent the spread of smoke in the event of a fire.
• Soffit Vent: Vent located under the eaves of a roof that allows fresh air to enter into an attic space.
• Soft Start: A method for reducing the load and torque in the powertrain and electrical current surge of the motor during startup.
• Sound Attenuator: A device designed to reduce noise in duct systems.
  
• Space Heater: A self-contained appliance that provides heat to a single room or space.
• Split System: An HVAC system where the condensing unit is placed outside the building and the evaporator coil is inside, often connected to a furnace or air handler.
• Splitter Damper: A damper used in a branch duct to adjust airflow.
• Spill Air: Air that is deliberately exhausted to balance air pressures within a building.
• Sprung: A term sometimes used to describe a duct that has expanded or "sprung" out of its intended shape, often due to pressure or damage.
• Stack Effect: The movement of air into and out of buildings driven by indoor-to-outdoor temperature differences.
• Stale Air: Air that lacks freshness because it has been sitting idle or has been recirculated too long.
• Start Capacitor: A device that provides the extra energy needed to start the compressor or a motor.
• State: The physical form of a substance, such as solid, liquid, or gas.
• Starved: An HVAC condition where the system has an insufficient amount of refrigerant, affecting its cooling or heating capacity.
• Static Pressure: The pressure exerted by a fluid at rest in a ductwork system.
  
• Steam Humidifier: A device that adds moisture to the air by heating water to produce steam.
• Stratification: The layering of air due to different air temperatures, where warmer air rises and cooler air settles at the bottom.
• Sub-base: The mounting base for a thermostat.
• Subcooling: The process of lowering the temperature of liquid refrigerant below its condensation point at a given pressure.
• Suction Line: The pipe that carries refrigerant vapor from the evaporator to the compressor.
• Supply Air: The conditioned air that is delivered to the living space from the HVAC system.
• Supply Duct: The ductwork that carries conditioned air from HVAC equipment to the living or working space.
• Supply Plenum: The component of the HVAC system that receives air from the supply fan and distributes it to the conditioned space via the supply duct system.
• Supply Side: The portion of the HVAC system that involves the delivery of conditioned air.
• Superheat: The temperature of vaporized refrigerant above its boiling point at a given pressure.
• Swamp Cooler: A type of evaporative cooler that cools air by passing it over water-saturated pads.
• System Balancing: Adjusting the HVAC system to provide the right amount of air to each room or space.
• Soffit Vent: Vent located under the eaves of a roof that allows fresh air to enter into an attic space.
  

• Temperature Differential: The difference in temperature between two points, such as the supply air and the return air in a heating or cooling system.
• Temperature Setback: A method of reducing heating and cooling energy consumption by adjusting the thermostat to a lower temperature during periods of unoccupancy or sleep.
• Thermal Break: A material of low thermal conductivity placed in an assembly to reduce or prevent the flow of thermal energy between conductive materials.
• Thermal Bridging: A term for the movement of heat across an object that is more conductive than the materials around it, undermining the insulating properties.
• Thermal Comfort: A state of mind that expresses satisfaction with the surrounding environment, often achieved when the HVAC system properly regulates indoor temperature, humidity, and airflow.
• Thermal Energy Storage: A technology that stores thermal energy for later use, enhancing energy efficiency and reliability.
• Thermal Envelope: The part of a building that serves as the barrier between the heated or cooled interior spaces and the unconditioned outdoor environment or unheated spaces.
• Thermal Expansion Valve (TXV): A precision device used to meter the flow of refrigerant to the evaporator while controlling the amount of superheat at the outlet of the evaporator.
• Thermal Load: The amount of heating or cooling energy needed to maintain indoor comfort conditions.
• Thermal Zone: An area or space in a building that has its temperature controlled independently from other areas or spaces.
• Thermistor: A type of resistor whose resistance varies significantly with temperature, used as a temperature sensor.
• Thermocouple: A sensor used to measure temperature, consisting of two different conductors that produce a voltage when there is a difference in temperature between them.
• Thermostat: A device that regulates the temperature of a space by activating heating or cooling systems to maintain the desired temperature.
• Thermodynamics: The branch of physical science that deals with the relations between heat and other forms of energy such as work.
• Ton of Cooling: A unit of power used to describe the cooling capacity of an HVAC system, equal to the heat required to melt one ton (2000 pounds) of ice in a 24-hour period, which is 12,000 BTU/h.
• Tonnage: A term describing the cooling capacity of an air conditioning system. One ton is the ability to cool 12,000 BTUs in one hour.
• Total Heat (Enthalpy): The sum of both sensible and latent heat contained in a substance or air.
• Transformer: An electrical device that transfers electrical energy between two or more circuits through electromagnetic induction, often used to step down voltage for control systems.
• Transducer: A device that converts one form of energy to another, commonly used in HVAC systems to convert pressure measurements into an electrical signal.
• Turndown Ratio: The ratio of the maximum fuel input rate to the minimum fuel input rate of a modulating or staging heating appliance.
• Two-Stage Furnace: A furnace with two heat output levels, high for cold winter days and low for milder days, to provide greater comfort and efficiency.

• U-Value: The rate of transfer of heat through a structure divided by the difference in temperature across the structure. It is expressed in watts per square meter per degree Celsius (W/m²°C).
• UL (Underwriters Laboratories): An independent organization that writes testing standards and tests products for safety, including HVAC equipment.
• Ultraviolet (UV) Lights or Lamps: Used in HVAC to disinfect air, improving air quality by preventing the growth of microbes.
• Unconditioned Space: An area or space in a building that is not heated or cooled and where temperature is not intentionally controlled.
• Unitary System: A self-contained HVAC system with all components in a single unit or a paired indoor-outdoor system.
• Universal Refrigerant Certification: Allows individuals to work with and handle various types of refrigerants, required in many regions for HVAC professionals.
• Upflow Furnace: A furnace that intakes air from the bottom and releases heated air from the top, often installed in basements.
• USGBC: U.S. Green Building Council. Promotes sustainable building practices and is known for LEED certification.
• Upflow Unit: A type of air conditioning system that discharges air into the conditioned space via a top-mounted discharge plenum or through an overhead duct system.
• UPS (Uninterruptible Power Supply): An electrical apparatus that provides emergency power when the main power source fails, which can be critical for maintaining HVAC system operations during power outages.
  

• Vacuum Test: A test to check the integrity of a refrigeration system by drawing a vacuum to ensure no leaks before charging it with refrigerant.
• Vapor Barrier: Material designed to prevent the transmission of water vapor through building envelopes to protect against moisture-related issues.
• Vapor Compression Cycle: A refrigeration cycle that uses a compressor to increase the temperature and pressure of the refrigerant vapor, facilitating its condensation and heat release.
• Vapor Pressure: The pressure exerted by a vapor in thermodynamic equilibrium with its liquid or solid form at a given temperature.
• Vapor: The gaseous state of a substance that is below its critical temperature and can exist in equilibrium with its liquid phase.
• Vapor Barrier or Seal: A barrier to prevent air, moisture, and contaminants from migrating through walls, floors, and ceilings, often used around sensitive equipment or insulation.
• Variable-Speed Technology: For efficient operation of HVAC systems.
• VAV (Variable Air Volume): A system that regulates the volume of air delivered to a space to maintain comfort conditions.
• Vent: An opening designed to convey air, gases, or vapors from one area to another, often out of a building or space.
• Vent Pipe: A pipe that allows the passage of gases from plumbing systems to the exterior to prevent pressure buildup.
• Vent Stack: A vertical vent pipe that expels sewage gases from the drainage system of a building to the outdoors.
• Ventilation: The mechanical or natural process of supplying and removing air from a space for temperature control and air quality.
• Ventilation Rate: The rate at which outside air replaces indoor air within a space, usually measured in cubic feet per minute (CFM).
• Ventilator: A device that introduces fresh air into a space while expelling stale air to maintain air quality.
• Ventilator, Energy Recovery (ERV): An appliance that transfers heat and moisture between incoming and outgoing air streams to improve efficiency.
• Ventilator, Heat Recovery (HRV): A ventilator that recovers heat from exhausted air to preheat incoming fresh air, thereby reducing energy consumption.
• VFD (Variable Frequency Drive): An electronic controller that adjusts the speed of an electric motor, commonly used for fan and pump motors in HVAC systems.
• VRF (Variable Refrigerant Flow): A system that can provide simultaneous heating and cooling to different zones in a building by varying the flow of refrigerant.
• Volume Damper: A damper installed within the ductwork that can be adjusted to control air volume flow through the duct.

• Water-Cooled System: An air conditioning system that uses water to remove heat from the refrigerant in the condenser.
• Watt: The unit of power in the International System of Units (SI) equivalent to one joule per second. In HVAC, it's used to measure the power consumption of devices.
• Wet Bulb Temperature: The lowest temperature that can be obtained by evaporating water into the air at a constant pressure> This is important for assessing cooling systems' performance.
• Wet Bulb Thermometer: A type of thermometer that has a wetted wick over its bulb and is used alongside a dry bulb thermometer to measure humidity.
• Wheel Heat Exchanger: Also known as a rotary heat exchanger or energy recovery wheel, it transfers heat and moisture between two separate airstreams.
• Window Unit: A self-contained air conditioning unit that is installed in a window or through a wall, providing cooling (and sometimes heating) to the room in which it is installed.
• Working Fluid: A pressurized gas or liquid in a refrigeration cycle that absorbs and rejects heat as it circulates through the system.
  

• Xenon Lamp Weathering Test: This is a test that uses a xenon arc lamp to simulate the effects of sunlight on materials.

• Zero Energy Building (ZEB): A building with a net energy consumption of zero over a typical year. It produces as much energy on-site as it uses, often through a combination of energy efficiency and renewable energy.
• ZEK 014-4-08/11 Test: A standard test for the outgassing behavior of materials used in vehicle interiors, which can also be relevant for HVAC components in transportation systems.
• Zinc Coating (Galvanizing): A process where a protective zinc coating is applied to steel or iron to prevent rusting. In HVAC, ducts and other components may be galvanized for protection.
• Zone: In HVAC, a zone is an area or space in a building that has its own temperature control.
• Zone Valve: A device used to control the flow of water or steam in a hydronic heating or cooling system for different zones in the structure.
• Zoning System: A zoning system for HVAC uses devices such as dampers in the ductwork to selectively control and redirect air to different parts of a building, providing customized temperature zones.