AC (Alternating Current): Alternating Current is a type of electric current that constantly changes direction, causing the flow of electricity in a circuit to reverse at regular intervals.

ACCA: ACCA is a non-profit association with a membership of over 60,000 professionals and 4,000 businesses in the indoor environment and energy services community. Their website can be accessed at www.acca.org.

Acoustical: Relating to sound, the science of sound, or the sense of hearing.

AFUE: Annual Fuel Utilization Efficiency is a measurement used to assess furnace efficiency by calculating the ratio of heat output to heat input.

AGA: American Gas Association, Inc.

AHRI: The Air-Conditioning, Heating, and Refrigeration Institute (AHRI) is a trade association that represents manufacturers in the HVACR and water heating industry. Their website can be found at www.ahrinet.org.

Air Conditioner: An apparatus that alters humidity levels, temperature, or air quality.

Airflow Volume: Airflow volume is measured in cubic feet per minute (cfm) and indicates the amount of air circulated in a given space.

Air Handler: The indoor component of an air conditioning system, including the circulating fan and either the evaporator coil (in summer) or condenser coil (in winter).

ASHRAE: The American Society of Heating, Refrigerating and Air-Conditioning Engineers is a global society dedicated to advancing human well-being through sustainable technology for the built environment. The society and its members focus on building systems, energy efficiency, indoor air quality, refrigeration, and sustainability within the industry. Their website can be visited at www.ashrae.org.


BTU: British Thermal Unit is a unit of measurement used to quantify the amount of heat required to raise or lower the temperature of one pound of water by one degree Fahrenheit.

BTU/h: British Thermal Units per hour.

Burner: A device that facilitates the combustion of air and gas.

Burner Orifice: The opening in the burner through which the gas or fuel passes before combustion.


Capacity: HVAC capacity refers to the output produced by a heating or cooling unit and is measured in BTUs per hour.

Celsius: A temperature scale that sets the freezing point of water as 0° and the boiling point as 100° under normal atmospheric pressure.

CFM (Cubic Feet per Minute): A unit of measurement used to quantify airflow volume.

Charging a System: The process of adding coolant or refrigerant to an HVAC system.

Coil: The coil, also known as the evaporator coil, is connected to the airflow outlet of the furnace. It circulates conditioned refrigerant to cool the structure in summer and provide heat in winter. By passing warm indoor air through the coil, it removes temperature and humidity, resulting in cooler indoor air. Installing a properly sized and rated evaporator coil is crucial for achieving optimal performance and comfort from a central air conditioning or heat pump system.

Compressor: A pump that increases the pressure of refrigerant gas.

Condensate: As warm air passes over the cool evaporator coil, the coil becomes moist, generating liquid condensation, which is then mechanically drained away from the equipment.

Condenser Coil: Typically located outdoors, the condenser coil removes heat from the refrigerant during the summer, allowing the refrigerant to transition from vapor to liquid and complete the refrigeration cycle.

Condenser Fan: A fan that enhances the airflow over the condenser coil, facilitating the removal of heat from the refrigerant.

CSA: Canadian Standards Association


DC: Direct Current. It refers to an electrical current that flows in only one direction.

Damper: These are sheet metal plates located at the junction points of ductwork. They can be adjusted to open or close, controlling the flow of air into a specific zone.

Degree-Day: It is calculated by subtracting the average outdoor temperature for a particular area from 65º Fahrenheit. This measurement helps estimate the heating or cooling requirements of a home or building.

Dehumidifier: A device designed to remove moisture or humidity from the air.

Diffuser: A grille placed over an air supply duct, equipped with vanes that distribute the discharged air in a specific pattern or direction.

DOE: Department of Energy. Their website can be accessed at www.energy.gov.

Downflow Furnace: A furnace that intakes air from the top and discharges it from the bottom.

Drain Pan: Also known as a condensate pan, it collects the condensate as the refrigerant vapor is converted into liquid form and directs it to the drain line.

Dry Bulb Temperature: The temperature measured without taking humidity into consideration.

Ductwork: A network of metal, fiberboard, or flexible material that distributes air from an HVAC unit to different zones within a home or office.


EER: Energy Efficiency Ratio. It represents the ratio of output cooling energy (in BTU) to input electrical energy at a specific operating point for a cooling device.

Energy Star®: ENERGY STAR is a voluntary program by the U.S. Environmental Protection Agency (EPA) that promotes superior energy efficiency to save money and protect the climate. The Energy Star website is www.energystar.gov.

EPA: The United States Environmental Protection Agency is a federal government agency responsible for protecting human health and the environment through the enforcement of regulations based on laws passed by Congress. Their website can be found at www.epa.gov.

Evaporator Coil: Also known as an indoor coil, it absorbs heat from the air, causing the liquid refrigerant flowing through it to change into a vapor and initiate the cooling process.

Expansion Valve: A valve that controls the flow of refrigerant based on temperature or pressure settings.


Fahrenheit: A temperature scale where water freezes at 32 degrees and boils at 212 degrees under normal atmospheric pressure.

Fan: A device comprising a motor and a blower wheel that creates airflow.

Filter: A central heating and cooling system typically utilizes multiple filters. The air filter is an essential component located in the system’s intake ducting, preventing contaminants from entering the equipment. It requires regular maintenance or replacement. Additionally, there is a filter in the refrigeration system, also known as a drier, which acts as a strainer, removing dirt and unwanted particles from the system.

Flue: A vent responsible for removing combustion byproducts from a furnace.

Furnace: The primary gas-fired heating component in a home. It facilitates the combustion of fuel and air to generate heat, which is then circulated throughout the home using a fan.

Fuse: A delicate metal strip that connects two parts of an electrical circuit. It serves as a safety device or circuit protector and breaks or melts in the event of excess electrical charge, interrupting the electrical circuit.


GAMA: Gas Appliance Manufacturers Association


Heat Exchanger: A device that facilitates the transfer of heat to a colder or warmer area or surface.

Heat Gain: The quantity of heat that is added or generated within a specific area.

Heating Coil: A coil that serves as a heat source in a heating system.

Heat Loss: The amount of heat that is lost or removed from a designated area.

Heat Pump: A device used for heating or cooling a space by transferring heat between two reservoirs.

Heat Transfer: The movement of heat from one area to another through conduction, convection, and/or radiation. Heat naturally flows from a warmer region to a cooler one.

HSPF: Heating Seasonal Performance Factor (HSPF) is a term used to measure the efficiency of a residential heat pump system.

Humidifier: A device that adds moisture or humidity to the air.

Humidistat: A device that measures humidity levels and controls the operation of a humidifier.

Humidity: The presence of moisture or dampness in the air due to water vapor.


Ignition: The process of raising the temperature of a substance to a point where it undergoes a combustion reaction.

Interconnection Agreement: A connection or agreement between power systems that allows them to share reserve capacity during times of need.


Kilowatt (kW): A unit of power equal to 1,000 watts.


Latent Heat: The energy released or absorbed by a body or thermodynamic system during a constant-temperature process that leads to a change of state. An example is the latent heat of evaporation, which causes a phase transition from liquid to vapor at a specific temperature and pressure.


Media: The fine material used in a filter that captures dirt, dust, mildew, or bacteria.

Manufacturer Approved System: When replacing a condensing unit, furnace, or air handler, it is important to use a system that is approved by the manufacturer and matched by the Air Conditioning, Heating, and Refrigeration Institute (AHRI). Note: Installation of unmatched systems is strongly discouraged.


NATE: North American Technician Excellence is the largest non-profit certification organization for heating, ventilation, air conditioning, and refrigeration (HVACR) technicians in the nation. NATE is governed, owned, operated, developed, and supported by the HVACR industry. NATE’s website can be found at www.natex.org.

NEC: National Energy Council or National Electric Code.

NEMA: National Electrical Manufacturers Association.


Orifice: An opening or hole.


Package Unit: An integrated heating and cooling system contained within a single outdoor unit.

Particulates: Fine liquid or solid particles present in combustion gases. The quantity and size of particulates emitted by vehicles, power plants, industrial facilities, wood stoves, etc., are regulated by the U.S. Environmental Protection Agency.

Plenum: A pressurized enclosure that contains a gas, typically air, at a positive pressure higher than its surroundings. One of the functions of a plenum is to manage and equalize pressure for a more uniform distribution.

Programmable Thermostat: A thermostat type that allows users to program a pre-set schedule of times and temperatures, enabling or disabling HVAC equipment accordingly.

PSI: Pound per square inch, a unit of pressure resulting from the force of one pound-force applied to an area of one square inch.

PSIA: Pounds per square inch, absolute, used to clarify that the pressure is relative to a vacuum rather than ambient atmospheric pressure. Since atmospheric pressure at sea level is around 14.7 psi, this value is added to any pressure reading made in air at sea level.

PSIG: Pounds per square inch gauge, indicating that the pressure is relative to atmospheric pressure.

Psychrometric: The analysis of atmospheric conditions, particularly moisture content in the air.

PVC: Polyvinyl chloride, a type of plastic.


Radiant floor: A type of heating system where channels or tubes containing hot fluids, such as air or water, are embedded within the building floor for radiant heat transfer.

Radiation: The transfer of heat through matter or space via electromagnetic waves.

Reciprocating Compressor: A compressor type used in cooling systems that compresses refrigerant through a piston action.

Refrigerant: A compound or working fluid used in air conditioners, heat pumps, and refrigerators to transfer heat into or out of an interior space. This fluid has a low boiling temperature, enabling it to release and absorb heat.

Refrigerant Charge: The amount of refrigerant present in a system.


Scroll Compressor: A compressor type used in both low and high-efficiency air conditioners. Scroll compressors feature fewer moving parts compared to reciprocating compressors, resulting in more efficient operation, increased tolerance to liquid refrigerant, reduced mechanical failures, and smoother, quieter operation.

SEER: Seasonal Energy Efficiency Ratio, a rating that indicates the efficiency of air conditioners. It is defined by the Air Conditioning, Heating, and Refrigeration Institute as the cooling output during a typical cooling season divided by the total electric energy input during the same period.

Self-contained System: A package unit.

Sensible Heat: Heat that causes a change in temperature when added or removed.

Sensor: A device that detects and reacts to changes in conditions.

Single-Speed: A motor that operates at a fixed speed until it reaches the desired temperature, at which point it shuts off. Single-speed motors tend to have louder start-up, higher energy consumption, and can put more strain on mechanical components.

Split System: A configuration where an outdoor unit is paired with an indoor unit (in contrast to a package unit). Split systems generally offer increased efficiency and more installation options.


Thermostat: A wall-mounted device that monitors and controls the output of an HVAC system.

Thermostatic Expansion Valve: A device that regulates refrigerant flow through the system to maintain a constant evaporator temperature.

Ton: A unit of cooling capacity equal to 12,000 BTUs per hour.

Two-Speed: A motor configuration commonly used in high-efficiency air conditioners. Two-speed motors operate in low gear to meet the cooling load, switching to high gear if necessary. Once the desired temperature is reached, they cycle back to low speed before shutting off


Upflow Furnace: A furnace that draws in air from the bottom and releases it through the top.


Vacuum: An area where the pressure is significantly lower than standard atmospheric pressure.

Variable-Speed: Ideal for high-efficiency air conditioners, a variable-speed motor operates with multiple speeds, offering smoother cycling, precise performance control, quiet operation, high energy efficiency, and minimal stress on mechanical components compared to single- or two-speed motors.

Ventilation: The process of moving air into and out of an interior space, either through mechanical means or natural airflow.

Volt: The unit of measurement for electrical potential and electromotive force.

Voltage: The electrical force that drives the flow of current through wires and cables.


Watt: A unit of measurement for the rate of energy transformation per unit of time, equivalent to one joule per second.

Wet Bulb Thermometer: A thermometer used to measure the relative humidity in the air.


Zoning: A system that divides a home, office, or space into different zones to enable better control of temperature and the efficiency of the heating and cooling system.