HVAC Glossary

Welcome to Smoak’s HVAC glossary page. This page will educate those unfamiliar with the heating, ventilation, and air conditioning industry on the terminology and jargon. Understanding the language is crucial for making informed decisions, maintaining your heating and cooling system, or effectively communicating with experts.

This glossary aims to provide clear and concise explanations of the key terms, concepts, and components related to heating and air conditioning helping you navigate the world of home comfort and maintenance with confidence. Whether you’re looking for explanations of some of the most common HVAC terms or diving into more technical aspects of these systems, you’ll find the information you need here.

Please email us if there are any additional terms that we may have missed.

HVAC Glossary

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Absolute Pressure

Absolute pressure is the total pressure of a fluid, including atmospheric pressure, and is used as a reference point for measuring pressures in a closed system, such as in refrigeration systems.

AFUE stands for Annual Fuel Utilization Efficiency. It’s a measure of the heating efficiency of a furnace or boiler. The higher the AFUE percentage, the higher its efficiency.

Air Conditioner
An air conditioner is a device that cools indoor air by removing heat and humidity. It works by circulating refrigerant through a cycle of compression and expansion.

Air Filter
An air filter is a component of HVAC systems that traps particles like dust, pollen, and pet dander, improving indoor air quality and protecting the equipment from debris.

Air Flow
Air flow refers to air movement through a heating, ventilation, and air conditioning system, crucial for distributing conditioned air throughout a building and maintaining indoor comfort and air quality.

Air Handler
An air handler is a crucial component of an HVAC system and is a device responsible for circulating conditioned air throughout a residential or commercial space. Air handlers are typically located indoors and are part of residential and commercial HVAC systems.

Ambient Temperature
Ambient temperature refers to the surrounding temperature and conditions of the environment in which the HVAC system is operating.

ASHRAE is an acronym for the American Society of Heating, Refrigerating, and Air-Conditioning Engineers. This prominent professional organization sets industry standards and guidelines for heating, ventilation, air conditioning, and refrigeration systems to ensure efficiency and sustainability.


A capacitator is an electrical component that stores and releases electrical energy, providing the necessary boost to start the motors of various HVAC equipment like air conditioners and furnaces.

Carbon Dioxide (CO2)
A gaseous, nontoxic combination of carbon and oxygen that results from combustion and respiration.

Carbon Monoxide (CO)
Carbon monoxide is a colorless and odorless gas that’s a byproduct of incomplete combustion, posing a potentially life-threatening safety hazard when present in indoor environments, and is often monitored and controlled through ventilation and detectors.

Central Air Conditioning
Central air conditioning is a system that cools an entire building by distributing conditioned air from a central unit to various rooms or spaces through ductwork and vents.

Combustion Blower
A motor-driven fan that brings fresh air into a heating system’s combustion chamber and expels combustion gases out through the heat exchanger and flue.

Cubic Feet Per Minute (CFM)
Cubic Feet Per Minute, or CFM, measures the air volume an HVAC system can move or circulate in one minute, indicating its capacity for air delivery or extraction.

A coil is a heat exchange component typically made of copper or aluminum that either absorbs or releases heat, depending on whether it is an evaporator coil (absorbing heat) or a condenser coil (releasing heat).

Commissioning ensures that an HVAC system is installed, configured, and tested to operate efficiently and meet design specifications before it becomes operational.

A compressor is a critical component that pressurizes refrigerant, enabling it to circulate through the air conditioning or refrigeration system, facilitating heat transfer.

Condensate is the liquid that forms when water vapor in the air condenses into a liquid state, often occurring as a byproduct of the cooling process in air conditioning systems.

Condensate Pan
A condensate or drain pan is a shallow, typically metal or plastic tray located beneath the evaporator coil of an air conditioning system, designed to collect and drain the condensate (water) that forms during the cooling process.

A heavy-duty type of electromagnetic relay that can handle high-current loads, such as motors. Contactors can have a variety of contact configurations, from a single set of contracts to five or six sets of contacts.

Crawl Space
Type of building foundation that leaves just enough space between the floor and the ground to allow access.


Efficiency refers to a heating, ventilation, or air conditioning system’s ability to produce the desired heating or cooling level while minimizing energy consumption and waste effectively and economically.

Energy Star
ENERGY STAR is a program initiated by the U.S. Environmental Protection Agency (EPA) that identifies and promotes energy-efficient heating, ventilation, and air conditioning products and systems to help consumers reduce energy consumption and lower utility costs.

Environmental Protection Agency (EPA)
The EPA is the U.S. Environmental Protection Agency, which plays a significant role in regulating and establishing guidelines for refrigerants and environmental standards in heating, ventilation, and air conditioning systems.

Evaporator Coil
An evaporator coil absorbs heat from the indoor air during the cooling process located inside the indoor unit of an HVAC system.

Expansion Valve
An expansion valve is a device that regulates the refrigerant flow into an evaporator coil, controlling the cooling process and reducing the refrigerant’s pressure and temperature, thus enabling efficient heat exchange.


Fahrenheit is a temperature scale commonly used in the United States to measure temperatures, including those relevant to heating and cooling systems, with freezing at 32° and boiling at 212° under standard atmospheric pressure. 

A filter-drier is a component within a refrigeration system that filters and dries the refrigerant, removing contaminants and moisture to maintain system efficiency and prevent damage.

Freon is a common term for refrigerants like chlorofluorocarbons (CFCs) and hydrochlorofluorocarbons (HCFCs) that were historically used in air conditioning systems but are being phased out due to environmental concerns.


Heat Exchanger
A heat exchanger is a device or component that transfers heat from one fluid or substance to another, typically between combustion gases and the air in a furnace or between the refrigerant and the surrounding air in an air conditioning system. 

Heat Gain
Heat gain refers to increased heat within a space due to factors like solar radiation, occupants, equipment, or external conditions, which cooling systems must counter to maintain a comfortable indoor environment.

Heat Loss
Heat loss is the amount of thermal energy that escapes a building or system, typically through factors like poor insulation, air leaks, or inadequate heating, requiring additional power to maintain desired indoor temperatures.

Heat Pump
A heat pump is an HVAC system that can heat and cool an indoor space by transferring heat from one location to another. It’s energy-efficient and often used in residential and light commercial properties in Charleston, SC. 

Heat Transfer
Heat transfer is moving thermal energy from one area to another, from a heat source to a living space, often involving conduction, convection, and radiation. 

Horizontal Flow
Horizontal flow refers to the orientation of an air handler or furnace. Air is drawn in from one side, conditioned, and then discharged horizontally from the other, often used in attic or crawl space installations.

A humidifier is a device that adds moisture to the air to maintain optimal indoor humidity levels. It can improve comfort and health during dry winter months.

Humidity is the amount of moisture or water vapor present in the air, impacting indoor comfort, air quality, and the efficiency of heating and cooling systems.

HVAC is an acronym for Heating, Ventilation, and Air Conditioning. It encompasses the systems and technology used to control indoor temperature by heating or cooling, humidity, and air quality.


Indoor Air Quality (IAQ)
The quality of the air within buildings and structures, especially as it relates to the health and comfort of building occupants.


Kilowattt (kW)

A kilowatt (kW) is a unit of measurement for electrical power that quantifies the rate at which energy is consumed or produced by equipment, such as heating or cooling systems, to indicate their energy usage or capacity to provide heating or cooling.


Latent Heat
Latent heat is the energy absorbed or released during a phase change of a substance, such as the condensation or evaporation of water in an air conditioning system, playing a crucial role in dehumidification and cooling processes.

Load Calculation
A load calculation determines the heating and cooling needs of a building. It considers factors like insulation, windows, climate, and occupancy to size the HVAC system correctly.


MERV Rating
MERV (Minimum Efficiency Reporting Value) rating is a standardized scale that measures the efficiency of an air filter in capturing and removing particles from the air that helps to improve indoor air quality and protect HVAC systems.

A micron is a unit of measurement used to quantify tiny particles or contaminants in the air, with one micron equal to one-thousandth of a millimeter or 0.00003937 inches. 

Mini-splits or ductless mini-splits are ductless heating and cooling systems comprising a compact outdoor unit connected to one or more indoor air handlers, providing zoned heating and cooling without traditional ductwork.


Packaged Unit
A packaged unit is a self-contained heating and cooling system that combines multiple components, including the furnace, air conditioner, or heat pump, into a single, compact unit typically installed outside or on rooftops.

A box or separate space that connects to the HVAC system for air circulation and has a critical function and that’s to bring it, distribute, and remove air. Each HVAC system typically consists of two plenum boxes: a supply and a return. Supply plenums work by taking in the new cool or warm air from the heater or AC system. Then distribute it throughout the home or building using the HVAC’s ductwork. A return plenum takes used air, using the ductwork, and sends it back to the core HVAC system. Which is recycled as a fresh batch later.

Programmable Thermostat
A programmable thermostat is a device that allows users to preset temperature settings for different times of the day, providing energy savings by automatically adjusting heating and cooling as needed. 

PSI is an acronym for pounds per square inch, a unit of measurement used to quantify pressure, often relevant when assessing refrigerant pressure levels in air conditioning and refrigeration systems.

PVC stands for polyvinyl chloride, a plastic pipe used for venting high-efficiency condensing furnaces and water heaters, as it is resistant to corrosion and heat.


R-22 (Refrigerant)
R22 refrigerant is a hydrochlorofluorocarbon (HCFC) refrigerant typically used in air conditioning and refrigeration systems. Still, it is being phased out due to its detrimental environmental impact, as it depletes the ozone layer. 

R-410A (Refrigerant)
R-410A is a hydrofluorocarbon (HFC) refrigerant commonly used as a more environmentally friendly alternative to R-22, offering better cooling efficiency and less impact on the ozone layer.

Reclaim refers to recovering, purifying, and reusing refrigerants, such as R22, rather than releasing them into the atmosphere, contributing to environmental conservation and compliance with regulations.

Recovery refers to capturing and removing refrigerants from a system for recycling or disposal to prevent their release into the environment, ensuring compliance with environmental regulations. 

Refrigerant is a chemical fluid used in air conditioners and heat pumps to transfer heat. It goes through a cycle of compression and expansion to cool or heat the air.

Refrigerant Charge
A refrigerant charge refers to the quantity of refrigerant in an air conditioning or refrigeration system, which must be at the correct level to ensure optimal cooling or heating performance.

Refrigerant Lines
Refrigerant lines are the copper or aluminum pipes that carry refrigerant between the indoor and outdoor units of an air conditioning or refrigeration system, facilitating heat exchange.

A register is a louvered or slatted cover, often with an adjustable damper, installed in the wall, ceiling, or floor to control and direct the flow of conditioned air from the ducts into a room. 

Relative Humidity
Relative humidity is a measurement of the moisture content in the air expressed as a percentage, indicating how close the air is to being fully saturated with water vapor at a specific temperature, affecting indoor comfort and air quality.

Retrofit is upgrading or modifying an existing HVAC system to improve energy efficiency, performance, or compliance with newer regulations or standards.

Return Air
The air that is returned to the evaporator coil from inside the building.

Return Duct
The duct through which warm air is drawn into the air conditioning system to be cooled and blown through the ductwork.

Return registers are the openings or grilles through which air from the conditioned space is drawn into the heating or cooling system for recirculation and conditioning.


SEER stands for Seasonal Energy Efficiency Ratio. It measures the cooling efficiency of an air conditioner or heat pump. Higher SEER ratings indicate higher energy efficiency. SEER2 is the current standard.

SEER2 is the updated version of SEER and was enacted on January 1, 2023.

Short Cycling
Short Cycling refers to a malfunction in which an air conditioning or heating system turns on and off more frequently than necessary, leading to reduced energy efficiency and potential wear and tear on the equipment.

Single-Stage Operation
A single-stage operation is a system, such as a furnace or air conditioner, that operates at only one fixed level of heating or cooling capacity, often resulting in less precise temperature control and potentially higher energy consumption. 

Split System
A split system consists of two separate units, one installed indoors (the air handler or furnace) and the other installed outdoors (the condenser unit or heat pump) to provide heating and cooling for a building.

Sub Cooling
Sub cooling refers to cooling a liquid refrigerant below its saturation temperature to ensure it is in a liquid state before entering the evaporator coil, essential for efficient heat transfer in air conditioning systems. 

Super Heat
Super heat refers to the temperature increase of a vapor refrigerant above its saturation temperature as it absorbs additional heat, typically occurring in the evaporator coil of an air conditioning system. 

Supply Registers
Supply registers are openings or grilles through which conditioned air is released into the living space from the heating or cooling system.


Thermostatic Expansion Valve (TXV)
A thermostatic expansion valve or TXV regulates refrigerant flow into the evaporator coil of an air conditioning system based on temperature and pressure, allowing for precise control of cooling capacity and efficiency.

A thermostat is a device that regulates the temperature in a particular space by controlling the heating and cooling systems. Modern thermostats often come with programmable features for energy efficiency. 

Ton (or Tonnage)
A ton is a unit of measurement for cooling capacity and is equivalent to the amount of heat required to melt one ton (2000 pounds) of ice in 24 hours, often used to rate the cooling output of air conditioners. 

Two-Stage Operation
A two-stage operation refers to a system, such as a furnace or air conditioner, that can operate at two different levels of heating or cooling capacity, providing better temperature control, improved efficiency, and quieter operation.


Upflow Unit
An upflow unit is a heating and cooling system where air is drawn in from the bottom and discharged through the top, commonly used in basement or first-floor installations.


Variable Speed Motor
An electric motor that can adjust its speed and airflow to maintain consistent and energy-efficient temperature control, resulting in improved comfort and reduced energy consumption.


Zoned System
A zoned system divides a building into separate heating and cooling zones, allowing for individual temperature control in different areas. This can increase comfort and energy efficiency.