Dehumidification
The process of removing moisture from air to achieve a target relative humidity level within a conditioned space. Typically achieved by cooling air below its dew point, then reheating to the desired supply temperature.
The complete A–Z reference for heating, ventilation, air conditioning, and refrigeration (HVACR) terminology. Search, explore, understand.
In this industry, precision matters, and so does language. Whether you’re troubleshooting a low suction issue, discussing SHR with a consultant, or explaining airflow problems to a client, knowing your HVAC terminology is part of the job.
This HVAC glossary brings together over 200 essential HVAC terms and definitions used daily by contractors, technicians, and engineers. From core refrigeration cycle principles to air conditioner terms, efficiency metrics, and even common HVAC slang terms heard on-site, this glossary of HVAC terms is built as a practical reference you can rely on in the field, during commissioning, or when training new techs.
The process of removing moisture from air to achieve a target relative humidity level within a conditioned space. Typically achieved by cooling air below its dew point, then reheating to the desired supply temperature.
The process of adding moisture to air within a space to maintain a target relative humidity level. Types include steam (isothermal), ultrasonic, and evaporative (adiabatic) humidifiers. Critical in data centers, healthcare, and printing environments.
The amount of water vapor present in the air. Relative humidity (RH%) expresses this as a percentage of the maximum moisture air can hold at that temperature. Typical HVAC comfort targets are 40–60% RH.
Indoor Air Quality. A measure of the healthfulness of air inside a building, including CO₂ levels, particulates, humidity, temperature, VOCs, and pathogen load. Critical for occupant health, comfort, and productivity.
A standardized rating system (ASHRAE 52.2) that measures an air filter's ability to capture airborne particles across a range of sizes. Scale runs from 1 (minimal filtration) to 20 (HEPA-equivalent). Residential filters are typically MERV 1–8; commercial HVAC systems commonly use MERV 8–13; hospitals and cleanrooms may use MERV 14–16 or higher. Higher MERV ratings improve air quality but increase airflow resistance, which can raise static pressure and reduce system efficiency if the equipment is not sized accordingly.
The blend of recirculated return air and fresh outdoor air at the inlet of an air handling unit. The ratio is controlled by mixing dampers and influenced by ventilation requirements and economizer operation.
Fresh air drawn from the outdoor environment into an air handling system for ventilation. Supply rates are governed by building regulations and standards (e.g., ASHRAE 62.1, CIBSE guides) based on occupancy type and room volume.
The ratio of water vapor present in air to the maximum it could hold at the same temperature, expressed as a percentage. 100% RH means fully saturated air. Typical comfort: 40–60% RH. Data centers commonly target 40–55% RH.
Air Conditioning Contractors of America (ACCA) is a U.S. trade association for HVAC contractors that provides standards, certification, training programs, and HVAC trade shows.
The Air-Conditioning, Heating, and Refrigeration Institute (AHRI) develops global performance standards, testing, and certification for HVAC and refrigeration equipment. For those starting an HVAC business, the AHRI is a solid resource for understanding best practices in the HVAC industry.
The American Society of Heating, Refrigerating, and Air-Conditioning Engineers (ASHRAE) is a global professional organization that sets industry standards, guidelines, and research for HVAC&R.
ASHRAE Standards 62.1 and 62.2 define ventilation requirements for commercial and residential buildings to maintain indoor air quality.
The U.S. Department of Energy provides energy efficiency programs, including Energy Star, as well as technical guidance and regulations for HVAC equipment.
Energy Star is a U.S. EPA/DOE program that labels energy-efficient HVAC equipment and buildings.
A mandatory U.S. certification for HVAC technicians handling refrigerants. It also serves as a baseline qualification when recruiting HVAC technicians who can legally and safely perform jobsite work.
The Green Grid is an industry group providing data center efficiency and sustainability metrics, such as PUE, WUE, and ERE.
The International Ground Source Heat Pump Association specializes in geothermal heat pump design, installation, and training.
The Kyoto Agreement is an international agreement to reduce greenhouse gas emissions, signed in Kyoto, Japan. It established reduction targets and provided the regulatory framework that underpins progressive restrictions on HFC refrigerants in HVAC equipment.
Leadership in Energy and Environmental Design is a U.S. program recognizing sustainable building practices, including efficient HVAC systems.
The ACCA standard for residential duct system design. Provides a systematic method for sizing supply and return ducts, selecting fittings, and calculating static pressure to ensure adequate airflow to every room. Proper Manual D design prevents common field problems, including excessive static pressure, noise, uneven comfort, and reduced equipment life. Works in conjunction with Manual J (loads) and Manual S (equipment selection).
The ACCA (Air Conditioning Contractors of America) standard method for calculating residential heating and cooling loads. Named for its original manual format, it accounts for building envelope, infiltration, internal gains, occupancy, and climate data to determine the correct equipment size in BTUs. Required by many building codes for new construction and replacement equipment permits.
The ACCA standard for selecting residential HVAC equipment based on the results of a Manual J load calculation. Ensures that chosen equipment matches actual building loads rather than simply the nearest available size. Covers selection criteria for cooling capacity, sensible heat ratio, airflow, and heating output under specific design conditions. Companion standard to Manual J and Manual D.
The Montreal Protocol on Substances That Deplete the Ozone Layer is an international treaty enforcing the phase-out of ozone-depleting substances such as CFCs and HCFCs.
Underwriters Laboratories provides safety certification and testing for HVAC equipment and components.
A data center hot/cold aisle layout strategy. Server rack fronts face each other across a cold aisle where cool supply air is delivered. Rack backs exhaust into a hot aisle, enabling efficient heat capture and return to cooling units.
Computer Room Air Conditioning unit. A precision air conditioning unit specifically designed for cooling IT equipment, maintaining tight temperature and humidity tolerances within data centers and server rooms.
Computer Room Air Handler. Like a CRAC unit, but uses chilled water (rather than a direct refrigerant circuit) as the cooling medium. Suited to larger, centralized data center cooling architectures.
A facility housing computer systems, networking equipment, and storage. Requires precise environmental control of temperature, humidity, and air quality to ensure reliability and uptime of critical IT infrastructure.
The aggregation of computing power to deliver performance far beyond a standard workstation, used for large-scale problems in science, engineering, or business. HPC environments generate very high heat densities requiring specialist cooling strategies.
In a data center hot/cold aisle layout, server rack backs exhaust hot air into the hot aisle. Containment systems isolate this hot air from the cold supply air, improving cooling efficiency and reducing recirculation.
Physical panels or barriers that separate hot and cold aisles, improving cooling efficiency and preventing air recirculation.
An organization providing services for accessing, using, or participating in the Internet. ISPs often operate or co-locate equipment in data centers, making data center cooling design directly relevant to their infrastructure.
A resilience strategy where N components required to run the system are supported by at least one independent backup (+1). Ensures continued operation if any single component fails. Common specification for critical cooling infrastructure in data centers.
A standardized metal frame used to house servers, network switches, storage devices, and other electronic equipment in a data center. Standard rack width is 19 inches; height is measured in "U" units (1U = 44.45mm).
A metric measuring the percentage of racks where the inlet air temperature is within ASHRAE recommended or allowable ranges. RCI(hi) tracks racks at the upper limit, while RCI(lo) tracks the lower limit. Reveals how uniformly the data center is being cooled.
Cooling device mounted on the back of server racks to remove heat at the point of generation.
A computer system that provides services or resources to other computers across a network. Servers generate significant heat relative to their physical size, making data center cooling design a critical engineering challenge.
Uninterruptible Power Supply. An electrical apparatus that provides emergency power to a load, such as IT equipment or critical HVAC controls, when the main power supply fails. Essential in data centers for maintaining uptime during power outages.
Common U.S. measure of heating or cooling capacity. Often used in residential and commercial specs.
A data center metric indicating the ratio of total CO₂ emissions generated by the facility to the total energy consumed by IT equipment. Lower CUE values indicate better carbon efficiency.
Cubic feet per minute is an imperial measurement of air volume flow rate. Not commonly used in the UK or Europe, where m³/s or l/s are standard, but encountered on equipment specified to North American standards.
Coefficient of Performance. The ratio of useful refrigerating or heating effects produced to the energy input (work) required. A higher COP indicates a more efficient system. Modern chillers typically achieve COP values of 3–6.
A mechanical or control strategy that reduces energy consumption by exploiting favourable ambient conditions. In cooling, this may mean using outdoor air directly or cooling a water circuit when ambient temperatures are low enough, reducing or eliminating compressor use.
Energy Efficiency Ratio. An HVAC efficiency rating calculated as cooling capacity (BTU/hr) divided by power input (W). Higher EER = better efficiency at that operating point.
A data center sustainability metric: the ratio of energy reused outside the data center (waste heat for building heating) to total energy consumed. A lower ERE indicates that more energy is being productively reused elsewhere.
The ratio of data center energy reused elsewhere in the facility to the total energy brought into the data center control volume, including IT, cooling, power, and lighting. Complements the ERE metric.
A mode of operation where outdoor ambient conditions are cold enough to cool a water or glycol circuit without running the refrigeration compressor. Systems with free cooling capability can significantly reduce compressor run hours in cooler climates.
The seasonal heating efficiency of heat pumps is expressed as BTU output per watt-hour of energy input.
A part-load efficiency rating for commercial air conditioners, reflecting real-world operation in the U.S. climate.
The SI unit of energy. In HVAC, energy is expressed in joules (J), kilojoules (kJ), or megajoules (MJ). Power is energy per unit time: 1 W = 1 J/s. Heat content (enthalpy) is typically expressed in kJ/kg in psychrometric calculations.
The SI unit of power is equal to 1,000 watts. Used throughout HVAC to express cooling capacity, heating capacity, and power consumption. A standard office split system might be 2–7 kW; a large chiller plant thousands of kW.
The SI unit of energy is power consumed over time. HVAC energy consumption is reported in kWh or MWh. 1 kWh = 3,600 kJ. Not to be confused with kW (power, not energy): a 5 kW unit running for 2 hours consumes 10 kWh.
The portion of total cooling capacity used to reduce the moisture content of the air (dehumidification). A high sensible-to-latent ratio is desirable in IT cooling, where the load is almost entirely sensible.
Power Usage Effectiveness. The dominant data center energy efficiency metric: total facility energy divided by IT equipment energy. A PUE of 1.0 is theoretically perfect (all energy serves IT). Best-in-class data centers achieve PUE values of 1.1–1.2.
The seasonal cooling efficiency of air conditioners or heat pumps. Expressed as BTU/hr per watt, common in U.S. residential/commercial units.
The portion of total cooling capacity that reduces air temperature without changing moisture content. Expressed in kW. In IT cooling, a high sensible heat ratio is desirable as server equipment generates heat but almost no moisture.
The ratio of sensible cooling capacity to total (sensible + latent) cooling capacity. Precision air conditioning units have high SHR (~0.9+) suited to IT loads; comfort units have lower SHR to handle occupant moisture loads.
A unit of cooling capacity equal to 12,000 BTU/hr (or approximately 3.517 kW). Historically defined as the cooling rate needed to freeze one US short ton of water in 24 hours. Residential systems are typically 1.5 to 5 tons; commercial and industrial systems can range from tens to thousands of tons. Often used interchangeably with "ton of refrigeration."
A data center sustainability metric developed by The Green Grid, assessing water used on-site for operations, including humidification and evaporative cooling. Lower WUE indicates less water consumption per unit of IT energy consumed.
The SI derived unit of power is defined as one joule per second. Named after Scottish engineer James Watt (1736–1819). In HVAC, watts express the rate of energy consumption or heat transfer. Larger values are kilowatts (kW) or megawatts (MW).
A building that produces as much energy as it consumes annually, often incorporating high-efficiency HVAC and renewable systems.
Gauge pressure plus atmospheric pressure. It represents the total pressure relative to a perfect vacuum, unlike gauge pressure, which is relative to atmospheric pressure.
A storage vessel that receives liquid refrigerant from the evaporator and prevents it from flowing back into the compressor, protecting the compressor from liquid slugging.
A fan driven by an alternating current (AC) motor. Used to provide air movement in heating, cooling, and ventilation systems. Common in HVAC equipment where variable speed is not required.
The process of adjusting airflow rates across a building's duct system to ensure each zone or room receives the correct volume of conditioned air. Typically performed by a certified technician after initial installation, major renovations, or complaints about uneven temperatures. Involves measuring and tuning supply, return, and exhaust airflows at each register or diffuser.
An appliance, system, or mechanism designed to control temperature, humidity, and air quality within a defined space. Can refer to a simple wall-mounted split unit or an entire central plant system.
A central unit consisting of fans, heating and cooling elements, filter racks, dampers, humidifiers, and other components to deliver conditioned and ventilated air throughout a building or space.
An air conditioning system that uses ambient air as the transfer medium to reject heat from the refrigerant in the condenser. The condenser is typically located outdoors.
An air distribution outlet or grille designed to direct airflow and promote balanced distribution of conditioned air throughout a space. Shape and blade pattern affect air throw, spread, and coverage area.
The location or site where an air conditioning or cooling system is to be installed. Application characteristics (such as heat load type, environment, and duty cycle) determine the most appropriate equipment selection.
A Building Energy Management System (BEMS) is a computer-based control platform installed in buildings to monitor and control mechanical and electrical systems — including HVAC, lighting, power, fire, and security — to optimize energy use and operational costs.
A fixed metering device consisting of a small-diameter tube that restricts refrigerant flow into the evaporator. Simple and inexpensive, but only effective in applications with stable ambient conditions and consistent load.
A refrigeration arrangement using two or more separate refrigerant circuits working in series. Used to achieve very low temperatures, where a single refrigerant circuit would be impractical.
A ceiling-mounted air conditioning unit designed to sit flush within a suspended ceiling. Ideal for large open-plan areas or irregularly shaped rooms, this typically distributes air evenly in four directions.
A dynamic compressor that uses a high-speed rotating impeller to impart velocity to refrigerant gas, then converts velocity to pressure. Suited to large-capacity chillers due to high efficiency at full load.
An air conditioning system in which a chiller cools water (or a glycol solution), which is then circulated to remote air handling units or fan coils. Allows the refrigerant circuit to remain contained within the chiller mechanical room.
A machine that removes heat from a liquid (typically water or glycol) via a vapor-compression or absorption refrigeration cycle. The cooled liquid is then distributed to cool air or process equipment throughout a facility.
A controlled environment where airborne particulates, microbes, and chemical contaminants are strictly limited through filtration, pressurization, and defined air change rates. Typically used in manufacturing, pharma, or research.
See Precision Air Conditioning (PAC). Refers to systems providing tight temperature (±1°C) and humidity (±5% RH) control for sensitive environments such as data centers, laboratories, and server rooms.
A heat exchanger assembly of fins and tubes mounted inside an air handler or ductwork. Refrigerant or water flows through the tubes while air passes over the fins, enabling heat transfer between the two media.
Air conditioning systems designed primarily for the comfort of human occupants or livestock, rather than for precise process control. Contrasts with precision or close-control systems used in IT environments.
Air conditioning for large commercial buildings such as hotels, hospitals, and offices, providing ventilation and space conditioning across multiple zones. Encompasses packaged rooftop units through to full central plant systems.
The process of verifying that an installed HVAC system operates as designed. Includes setting up controls, calibrating sensors, balancing airflow and water flow, and confirming performance against the project specification.
The heart of any refrigeration system. It raises the pressure and temperature of refrigerant vapor, enabling it to condense at a higher temperature and reject heat to the environment. Common types include scroll, screw, reciprocating, and centrifugal. Understanding core components goes hand in hand with knowing the essential HVAC tools used to service and troubleshoot them.
A heat exchanger that removes the superheat from compressed refrigerant gas and condenses it into high-pressure liquid, rejecting heat to either outdoor air (air-cooled) or water (water-cooled).
A packaged assembly comprising a compressor, motor, and condenser heat exchanger. Supplied as a single unit to be matched with a remote evaporator in a split refrigeration or air conditioning system.
An HVAC system type where the supply air volume flow rate remains fixed. Temperature control is achieved by varying the supply air temperature. Simpler than VAV but less energy-efficient at part load.
Systems and devices that regulate the operation of HVAC equipment. A sensing device compares actual conditions (temperature, humidity) to setpoint targets and signals actuators to maintain the desired state.
A heat rejection device that uses evaporative cooling to remove heat from water circulated through a chiller or condenser. Water-cooled systems with cooling towers typically achieve lower condensing temperatures and higher efficiency than air-cooled systems.
Chilled Water Return. The pipe carries water back to the chiller after it has absorbed heat from the conditioned space or process load. CWR temperature is typically higher than CWS temperature.
Chilled Water Supply. The pipe delivers cold water from the chiller to the cooling coils in air handlers or fan coil units. Typically maintained at 6–12°C depending on system design.
A modulating device, typically a series of louvres or blades, used inside ductwork or air handling equipment to control the volume of airflow. Can be manual or motorized.
An air conditioning unit that discharges supply air downward, typically beneath a raised floor, for distribution to IT equipment through perforated floor tiles. Common in data centers.
A heat rejection device that cools water or glycol by passing it through finned coils over which outdoor air is blown. Unlike cooling towers, dry coolers use no water evaporation. They are suitable for water-scarce environments and free cooling applications.
A conduit used in HVAC systems to distribute supply air, return air, or exhaust air throughout a building. Made from galvanized steel, flexible insulated fabric, or fiberglass board, and sized to balance airflow and pressure.
The complete network of metal, fiberglass, or flexible channels used to distribute conditioned air from the air handling unit to occupied spaces and to return it. Proper duct design, including sizing, sealing, and insulation, is critical to system efficiency, static pressure management, and indoor air quality. Often the most overlooked factor in residential and commercial HVAC performance.
Safety device installed in HVAC ductwork to detect smoke and trigger alarms or shutdowns.
Electronically Commutated Fan. Uses a brushless DC motor with integrated electronics that convert AC supply to DC internally. Typically more efficient than standard AC fan motors, with the benefit of variable speed control via an analogue or digital signal.
A mechanical or control device that modulates outdoor air intake to reduce mechanical cooling when ambient conditions are favorable.
An electronically controlled expansion device that uses a DC stepper motor to precisely regulate refrigerant flow into the evaporator. Provides tighter superheat control than a thermostatic expansion valve, improving efficiency and system stability.
The indoor heat exchanger coil in a DX system. Refrigerant evaporates within the coil as it absorbs heat from the passing air, cooling and often dehumidifying the airstream before delivery to the conditioned space.
A metering device that regulates the flow of liquid refrigerant from the high-pressure liquid line into the lower-pressure evaporator. Types include thermostatic (TEV), electronic (EEV), and fixed-orifice (capillary tube).
A terminal unit consisting of a coil (heating and/or cooling), a fan, control valve(s), and a local controller. Typically connected to a central chilled and/or hot water system. Widely used in hotels, offices, and apartment buildings.
A device containing a desiccant (moisture-absorbing material) installed in the refrigeration liquid line. Removes moisture and small particulates that could cause acid formation, valve blockages, or compressor damage.
A damper installed in ductwork to prevent the spread of smoke or fire between building zones.
The floor area or space occupied by an air conditioning unit when installed. An important consideration in mechanical room design, data center layouts, and commercial applications where space is at a premium.
A forced-air heating unit that generates heat by burning natural gas, propane, or oil — or via electric resistance elements — and distributes it through ductwork using a blower. This is the most common heating system in North American homes. Key specs include AFUE (Annual Fuel Utilization Efficiency), BTU output, and airflow capacity. Modern high-efficiency furnaces can achieve AFUE ratings of 95% or higher.
An air conditioning system using a water/glycol solution as the condensing medium. The glycol-cooled condenser is located inside the unit; the glycol mixture is piped to a dry cooler or similar external heat rejector and prevents freezing in winter operation.
A device that transfers heat between two physically separated fluids without them mixing. In HVAC, types include refrigerant-to-air coils, plate heat exchangers (water-to-water), and shell-and-tube units (refrigerant-to-water).
A refrigeration cycle operating in reverse: extracting heat from a low-temperature source (outdoor air, ground, or water) and delivering it at a higher temperature to heat a space or water. Types include air-to-air, air-to-water, water-to-water, and ground-to-water.
A test room designed to completely absorb sound reflections, with a solid floor to support heavy equipment such as air conditioning units. Used to measure acoustic performance under controlled conditions.
Stands for Heating, Ventilation, and Air Conditioning. The broad term for systems controlling the indoor environment of buildings, including temperature, humidity, air quality, and fresh air supply. Sometimes expanded to HVAC&R to include Refrigeration.
A compressor fitted with a variable frequency drive (VFD) to control motor speed and modulate capacity continuously as demand varies. Inverter compressors deliver significant energy savings over fixed-speed compressors at part load.
A Variable Frequency Drive that controls the speed of an electric motor by varying the frequency and voltage of its power supply. Used on compressors, fans, and pumps to precisely match output to actual demand, delivering major energy savings at part load.
The pipe carries high-pressure liquid refrigerant from the condenser (or receiver) to the expansion device. Liquid line components typically include a filter-drier, sight glass, and solenoid valve.
A dedicated supply fan system that replaces air exhausted from a building. The MUA may heat or cool incoming outside air to temper it, reducing its impact on the main HVAC system. It is not designed to handle the full space load.
A high-efficiency heat exchanger using flat aluminium tubes with multiple small internal channels, brazed to corrugated aluminium fins. Derived from automotive technology. Offers a lower refrigerant charge, better heat transfer, and improved corrosion resistance compared to traditional round-tube coils.
A device installed in the discharge line to separate compressor oil from the refrigerant before it reaches the condenser. Reduces oil circulation through the system and ensures adequate oil return to the compressor.
A self-contained, wall-mounted unit typically used in hotels, apartments, and offices. Provides heating and cooling without ductwork.
A self-contained air conditioning or air handling unit with all components — heating, cooling, fans, and controls — pre-assembled in a single enclosure. Typically installed outdoors on a rooftop or ground pad, with ductwork connecting to the conditioned space.
Air conditioning systems specifically designed for IT environments, providing tight control of temperature (±1°C) and relative humidity (±5% RH), high sensible heat ratios, and 24/7 reliability. Also known as CRAC units.
Tests performed on a refrigeration circuit before evacuation and charging to verify the mechanical integrity of all components and joints. Typically carried out using dry nitrogen to a specified test pressure, checking for leaks.
A thermostat that can automatically adjust temperatures according to user-defined schedules to save energy.
The process of evacuating air and moisture from a refrigeration system before charging it with refrigerant.
An EEV installed remotely from the main unit controller, with its own dedicated control panel. Used in purpose-designed evaporators where running all control wiring back to a central controller is impractical.
The modification of an existing refrigeration system to improve performance, such as converting to a lower-GWP refrigerant, adding EC fans or VFDs, or upgrading controls. Distinct from a full system replacement. For contractors, retrofits present opportunities to expand project scope through efficiency upgrades, IAQ improvements, or smart controls, all of which can significantly increase the average order value of a job.
A packaged HVAC system installed on a building roof, often serving multiple zones with heating, cooling, and ventilation. Common in U.S. commercial buildings.
A positive-displacement compressor using two meshing helical rotors to compress refrigerant. Typical capacities 50–1,000 kW, with a modulating range of 10–100%. Particularly suited to large chillers due to efficient part-load performance.
A compressor using two intermeshing spiral scrolls — one fixed, one orbiting — to compress refrigerant progressively. Quiet, efficient, and mechanically simple. Widely used in split systems and precision air conditioning. Available in fixed or variable (inverter) capacity versions.
An air conditioning system with two main components: an outdoor condensing unit and an indoor evaporator/air handler, connected by refrigerant pipes. Typically used for residential and smaller commercial applications.
Factory-packaged split system with separate indoor evaporator and outdoor condenser sections. Often easier to transport and install than full central systems.
Thermostatic Expansion Valve. A self-regulating metering device that controls refrigerant flow into the evaporator by sensing suction superheat via a remote sensing bulb. Maintains optimal evaporator performance over varying load conditions.
A device for controlling HVAC operation based on temperature setpoints. It may be programmable or smart-enabled for energy savings.
A semiconductor switching device used to control electrical current. In HVAC, thyristors are used to control the output of electric heaters by regulating the portion of each AC cycle passed to the heater elements.
Aerodynamic guide vanes installed inside ductwork at direction changes to smoothly redirect airflow, reducing pressure drop and turbulence. Particularly important in high-velocity systems, where sharp bends would otherwise cause significant energy losses.
An air conditioning unit that discharges supply air upward through the top of the unit into a ceiling void or via overhead ductwork. Common in comfort cooling, where a raised floor is not available.
A test performed on a refrigeration circuit before charging by drawing a deep vacuum (typically below 500 microns). Verifies that the system is leak-free and that moisture has been removed, both of which are critical to long-term circuit reliability.
A physical barrier applied to walls, floors, ceilings, and pipe insulation that prevents moisture and air infiltration into a critical space such as a data center. Prevents condensation damage to sensitive electronic equipment.
An HVAC system that varies the volume of supply air to individual zones to maintain setpoint temperatures, rather than varying temperature at a fixed flow rate. VAV systems save fan energy at part load and enable zone-by-zone control.
A multi-split system using variable-speed compressors and electronic expansion valves to control refrigerant flow to multiple indoor units from a single outdoor unit. Highly efficient at part load, heat-recovery VRF systems can simultaneously heat and cool different zones.
Pumps or fans controlled by Variable Frequency Drives to modulate output for energy savings.
A refrigeration or chiller system that rejects heat to water (from a cooling tower, river, or other source) rather than to air. Water-cooled condensers allow lower condensing temperatures and therefore better efficiency than air-cooled systems in most climates.
A building HVAC strategy where different areas (zones) are independently controlled for temperature, humidity, or air quality using VAV boxes, zone dampers, fan coil units, or VRF indoor units managed by a central BMS or individual thermostats.
The division of a building into independently controlled temperature areas (zones) using motorized dampers, multiple thermostats, and a zone controller. Allows different areas (bedrooms, offices, or sun-facing rooms) to maintain different setpoints simultaneously, improving comfort and reducing energy waste. Common implementations include bypass damper zoning (single system), multi-zone AHUs, VRF systems, and multiple independent split systems.
A mixture of two or more refrigerant compounds that behaves as a single substance, having identical vapor and liquid compositions at a given temperature. Azeotropes cannot be separated by distillation and show minimal temperature glide.
The specific temperature at which a liquid mixture boils and produces vapor with the same composition as the liquid phase. At this point, the blend behaves as a single-component refrigerant.
A mixture of two or more refrigerants in a specific ratio that, unlike azeotropes, can be separated by distillation. Regular blends may have a temperature glide of up to 10°C or more during phase change.
The temperature and pressure at which a refrigerant liquid begins to vaporize. For blended refrigerants, this differs from the dew point, creating a temperature glide range during phase change.
Chlorofluorocarbon refers to a class of refrigerants (e.g., R12) with high ozone-depleting potential (ODP) and high global warming potential (GWP). Phased out globally under the Montreal Protocol by the early 2000s, with limited essential-use exemptions.
Clean, empty cylinders used for temporary refrigerant storage during maintenance operations. Dedicated decant cylinders prevent cross-contamination and enable safe reuse or recycling of recovered refrigerant.
A refrigeration system in which the refrigerant itself is the primary cooling medium, evaporating directly in the coil that cools the air or process load. Contrasts with chilled water systems, which use a secondary fluid.
A replacement refrigerant that can be retrofitted into an existing system designed for a different refrigerant, with minimal system modifications.Note: Hydrocarbon refrigerants are not drop-ins for most fluorocarbon systems due to flammability differences.
The temperature at which a given refrigerant vaporizes within the evaporator at a specific operating pressure. Directly related to suction pressure and is a key diagnostic parameter in system commissioning and fault finding.
Fluorinated greenhouse gases — a family of synthetic gases including HFCs, PFCs, and SF₆ — are used widely in refrigeration and air conditioning. Subject to increasingly strict regulation due to their high global warming potential (GWP). These regulatory shifts are part of broader HVAC industry trends that will shape equipment selection and technician skills over the next decade.
ASHRAE system for classifying refrigerants based on flammability and toxicity.
A chemical compound containing fluorine and carbon. Most synthetic refrigerants are fluorocarbons. CFCs, HCFCs, HFCs, and HFOs all belong to this family. Their properties make them effective refrigerants, but many carry significant environmental concerns.
The difference in boiling temperatures between the bubble point and dew point of a refrigerant blend during phase change. Observed as a continuous change in temperature as the blend evaporates or condenses. This is important for system design and analysis.
An antifreeze additive (typically ethylene or propylene glycol) mixed with water in chilled water systems to lower the freezing point and allow sub-zero operation (e.g., in free cooling systems or cold-climate installations).
Global Warming Potential. A relative measure of the heat a greenhouse gas traps in the atmosphere over 100 years, compared to CO₂ (GWP = 1). CFCs: 5,000–10,000. HCFCs and HFCs: typically 1,000–4,000. Natural refrigerants and HFOs: typically <20.
Hydrochlorofluorocarbon (e.g., R22). It has a lower ozone-depleting potential than CFCs but is still regulated under the Montreal Protocol.
Hydrofluorocarbon (e.g., R134a, R410A). Zero ozone-depleting potential but high global warming potential. HFCs replaced CFCs and HCFCs as the dominant refrigerant family, but are now subject to phase-down under the Kigali Amendment and F-Gas regulations.
Hydrofluoroolefin (e.g., R1234ze, R1234yf). Fourth-generation refrigerants with zero ODP and very low GWP (typically <10). Increasingly used as replacements for high-GWP HFCs. Most are mildly flammable (A2L safety classification).
A capacity control method where hot discharge gas is re-routed back into the suction side of the circuit. Allows a compressor to run at reduced effective capacity during low-load conditions without cycling on/off.
Refrigerants containing only carbon and hydrogen, including propane (R290), isobutane (R600a), and propylene (R1270). Zero ODP and very low GWP, but require careful system design due to flammability characteristics.
A petroleum-derived lubricant used in older refrigeration compressors designed for CFC and HCFC refrigerants. Has low affinity for moisture but limited solubility with HFC refrigerants, which require synthetic POE (polyolester) oils.
Refrigerants that occur naturally in the environment, including ammonia (R717), carbon dioxide (R744), hydrocarbons (R290, R600a), and water. Typically have very low or zero GWP and ODP, but may require specialist system design due to pressure, toxicity, or flammability.
A refrigerant blend whose vapor and liquid compositions are nearly identical. Per ASHRAE 34, near azeotropes have a temperature glide of less than 2°C and behave almost like single-component refrigerants in normal operation.
A gas in a refrigeration system that does not condense at the operating condenser temperature and pressure, typically air that has leaked into the circuit. Non-condensables raise head pressure, reduce efficiency, and cause long-term reliability issues.
Ozone Depletion Potential. A relative measure of a substance's ability to deplete stratospheric ozone, compared to R11 (ODP = 1.0). CFCs: 0.1–1.0; HCFCs: 0.01–0.1; HFCs, HFOs, and natural refrigerants = 0.
An HCFC refrigerant widely used in residential and commercial air conditioning systems installed before 2010. R22 has ozone-depleting potential and is regulated under the Clean Air Act.In the United States, production and import of virgin R22 were fully phased out as of January 1, 2020, under the EPA’s implementation of the Montreal Protocol. Since that date, only reclaimed or recycled R22 may be used for servicing existing systems. While operating existing R22 equipment remains legal, no new R22 can be manufactured or imported, and the limited supply has made the refrigerant increasingly expensive for maintenance and repair.
An HFC refrigerant (Tetrafluoroethane) with zero ODP, developed to replace R12. Widely used in automotive air conditioning, refrigerators, and centrifugal chillers. Now subject to phase-down due to its GWP of 1,430.
A zeotropic HFC blend of R32, R125, and R134a. Developed as a replacement for R22. Has a significant temperature glide (~7°C), which must be accounted for in system design and charging procedures.
A near-azeotropic HFC blend of R32 and R125. The dominant refrigerant in split systems and precision air conditioning has been R-410A since the late 1990s. High GWP (~2,088) means it is now subject to phase-down under F-Gas and Kigali Amendment regulations.
An HFO fourth-generation refrigerant with a GWP below 1, lower than CO₂. Used as a replacement for R134a in large centrifugal chillers and some DX applications. Mildly flammable (A2L safety classification).
The reprocessing of recovered refrigerant to a virgin quality standard via filtration, distillation, drying, and chemical treatment. Reclaimed refrigerants must pass laboratory analysis before resale. Normally processed off-site at a specialist facility.
The removal of refrigerant from a system and transfer into recovery cylinders — preventing release to the atmosphere, which is environmentally damaging and illegal. Required during maintenance, decommissioning, or refrigerant conversion.
The cleaning of recovered refrigerant via oil separation, filtration, and drying to improve its quality for reuse. Unlike reclaim, recycling may be done on-site and does not need to meet virgin refrigerant quality standards.
The working fluid in a refrigeration cycle that absorbs heat at a low temperature (evaporating) and rejects it at a high temperature (condensing). Designated by ASHRAE R-numbers (e.g., R410A, R32, R744). Selection balances efficiency, safety, GWP, and ODP.
The precise quantity of refrigerant (in kg) contained within a closed refrigeration system. The correct charge is critical. Undercharge causes poor performance and compressor damage; overcharge raises head pressure and reduces efficiency.
The vapor compression cycle — comprising compression, condensation, expansion, and evaporation — that continuously transfers heat from a low-temperature source to a high-temperature sink. The foundation of all mechanical refrigeration and air conditioning.
Standard procedures for safely recovering, storing, transporting, and charging refrigerants to prevent accidents and environmental release.
The official designation assigned to a refrigerant upon ASHRAE accreditation. R-numbers provide a standardized, internationally recognized way to identify refrigerants unambiguously.
The cooling of liquid refrigerant below its saturation (bubble point) temperature. Ensures only pure liquid — no vapor — enters the expansion device, improving system efficiency and capacity. Typically 3–8°C of subcooling is desirable.
The heating of refrigerant vapor above its saturation (dew point) temperature. Suction superheat ensures dry vapor enters the compressor, protecting it from liquid slugging. Discharge superheat is a key diagnostic indicator of system performance.
The temperature difference between the bubble point and dew point of a zeotropic refrigerant blend during phase change. Blends with significant glide require careful system design, particularly in flooded evaporators and heat recovery applications.
The gaseous phase of a substance. In refrigeration, vapor refers to refrigerant gas near its saturation point. At sub-critical temperatures, it does not strictly follow ideal gas laws. Managing vapor quality at the compressor inlet is critical for system reliability.
A refrigerant mixture of two or more compounds that separates into its constituent parts during phase change, causing a composition shift and temperature glide. Examples: R407C, R404A. Fractionation must be considered when topping up or recovering these blends.
A process of reducing heat through a change in air pressure caused by volume expansion, without any heat exchange with the surrounding environment. Used in evaporative cooling strategies and some free-cooling pre-cooler applications.
The normal surrounding atmospheric conditions of temperature and pressure at a given location. Equipment performance ratings are stated relative to defined ambient conditions.
A physical transition between phases of matter — solid, liquid, and gas. The refrigeration cycle relies on the refrigerant changing state (evaporating and condensing) to absorb and reject heat.
Moisture removed from air when it is cooled below its dew point during the dehumidification process. Condensate must be collected and safely drained away from air handling and cooling equipment.Condensate discharge and roof‑mounted equipment can create conditions where HVAC problems intersect with roofing needs. Coordinating responses to HVAC and roofing emergency calls ensures both systems are protected and repairs are handled efficiently.
The transfer of heat energy through direct contact between substances via the movement of electrons or the vibration of molecules. The rate of conduction depends on the material's thermal conductivity and the temperature gradient.
The transfer of heat through the bulk movement of a fluid (liquid or gas). Natural convection is driven by density differences due to temperature; forced convection uses fans or pumps to enhance the heat transfer effect.
The mass per unit volume of a substance. In HVAC, air density varies with temperature, pressure, and humidity, affecting fan performance, duct sizing, and heat transfer calculations.
The temperature at which air becomes fully saturated with water vapor and condensation begins to form. Also used in refrigeration to describe the temperature at which refrigerant vapor starts condensing to liquid.
The total heat content of a substance — its internal energy plus the product of pressure and volume. In HVAC, enthalpy is used to calculate total cooling or heating loads in psychrometric calculations (sensible + latent heat).
A thermodynamic property representing the degree of disorder or randomness in a system. In practical HVAC terms, entropy changes describe the direction and efficiency of heat transfer processes within the refrigeration cycle.
Pressure measured relative to atmospheric pressure (approximately 1 bar absolute / 14.7 psi), commonly zeroed so that readings below atmospheric pressure appear negative. Denoted with "g" suffix, e.g., barg, psig.
The amount of heat energy required to raise the temperature of a substance by one degree Kelvin. Specific heat capacity expresses this per unit mass (kJ/kg·K). A key property in calculating heating and cooling loads for all HVAC applications.
The engineering process of determining the total amount of heating or cooling energy a building requires under worst-case design conditions. Takes into account factors such as building envelope (insulation, windows, walls), occupancy, internal heat gains (lighting, equipment), ventilation requirements, and local climate data. Residential calculations typically follow ACCA Manual J. Results directly determine appropriate equipment sizing.
The exchange of thermal energy between two interacting media — such as refrigerants, air, and water — in a heat exchanger. Occurs via conduction, convection, radiation, or a combination.
The broader process of determining system sizing requirements based on a building's heating and cooling loads. Encompasses residential (Manual J), commercial, and industrial methodologies. An accurate load calculation prevents the common problems of oversized systems (short-cycling, humidity issues) and undersized systems (inability to maintain setpoint). Distinct from a heat load calculation in that it typically covers both heating and cooling across all building types and system configurations.
The heat energy absorbed or released during a change of state at constant temperature. In air conditioning, the latent heat component relates to the addition or removal of moisture (dehumidification), distinct from sensible heat (temperature change).
The section of a refrigeration system from the outlet of the expansion device to the compressor suction inlet, where refrigerant exists at low pressure and temperature. In multi-stage systems, this is where the lowest temperature is achieved.
A graphical representation of the thermodynamic properties of moist air, including dry-bulb temperature, wet-bulb temperature, dew point, relative humidity, humidity ratio, and enthalpy. Based on 1 kg of dry air, used to analyse air treatment processes.
The resistance to airflow within a duct system, measured in inches of water column (in. w.c.) or Pascals. Every component in an air distribution system — ducts, fittings, coils, filters, registers, grilles — contributes to total external static pressure. Excessive static pressure forces the blower to work harder, reduces airflow, increases energy consumption, causes noise, and accelerates equipment wear. Accurate static pressure measurement is a foundational diagnostic skill in HVAC service and commissioning.
Thermal transmittance is the rate of heat transfer through a building element (wall, roof, glazing), expressed in W/m²K. Lower U-values indicate better insulation and reduced fabric heat gains/losses, directly reducing HVAC heating and cooling loads.
The temperature a parcel of air would reach if cooled to saturation by evaporating water into it, with latent heat supplied by the air parcel. Always equal to or lower than dry-bulb temperature. Critical for evaluating cooling tower and adiabatic cooler performance.
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