HVAC Definitions

Many HVACR terms and acronyms defined by ANSI/ACCA 5 QI – 2010

AABC: Associated Air Balance Council

ACCA: Air Conditioning Contractors of America

airflow:  duct airflow balance: a condition that exists when the duct system has been properly designed and assembled (i.e., sizing, friction loss, balance dampers, etc.) to ensure that the correct volume of air (in CFMs) is delivered to each room or space. This term also is used to describe work associated with the measurement and adjusting of the airflow rates at various points in an air distribution system to provide correct airflow delivery to the rooms or spaces as proscribed during the design process.

fan airflow: the total volume of air (in CFM) that exits the fan assembly or blower unit during
operation at design conditions. [Fan airflow is a function of static pressure resistance presented by
the duct system and any and all appliances connected within the subject duct system. A fan motor is designed to provide optimal airflow within a specified range of acceptable total static pressures. If a fan is installed in a duct system with appliances that exceed this total static pressure threshold, the fan cannot deliver proper airflow, and the systems capacity will be reduced. Variable speed fans do not save energy when installed in duct systems that exceed total static pressure limits – they only provide more options for multi-stage equipment.]

room airflow balance: a condition that exists when the airflow rate (CFM) entering a room or other enclosed space equals the airflow rate leaving the room, space or equipment

AHRI: Air-Conditioning Heating, and Refrigeration Institute

amps (ampere; A): A unit of electric current.

ASHRAE: American Society of Heating, Refrigerating, and Air-Conditioning Engineers

blower: see fan

boiler: vessel in which a liquid is heated with or without vaporization; boiling need not occur
bonding: (electrical ground) connection to ground potential of a metal part on an appliance or component which may become energized by an electric fault, or develop a static charge

Btu: British thermal unit, the amount of heat that must be added or removed to/from one pound of water to raise or lower its temperature one degree Fahrenheit

Btuh or Btu/h: British thermal units added or removed per hour

built-up system: see system

CEE: Consortium for Energy Efficiency

CFM: cubic feet per minute (ft 3)

clearance*: clearance for maintenance or repair: the distance between the item requiring maintenance and the closest interfering surface

combustion*: chemical process of oxidation that occurs at a rate fast enough to produce heat and usually light either as a glow or flame

combustion analysis: analysis of combustion as defined above

contractor*: the person or entity responsible for performing the work and identified as such in an ownercontractor agreement

control*: device for regulation of a system or component in a normal and safe operation, manual or automatic; if automatic, the implication is that it is responsive to changes of pressure, temperature, or other variable whose magnitude is to be regulated

diffuser: an outlet designed to discharge air in a spreading pattern

DOE: United States Department of Energy

duct modification: A change in the air distribution network that includes additions or deletions of duct runs or changes register/grille location(s). This does not include transitions at the air handler supply and return. Additionally, simple repairing or replacing damaged duct runs with like-size ducts are excluded from this definition.

EPA: United States Environmental Protection Agency

ESP: external static pressure APPENDIX B | DEFINITIONS Page 23 ANSI / ACCA 5 QI – 2010 (HVAC QI Specification)

expansion coil: an evaporator (heat exchanger) constructed of bare or finned pipe or tubing in which direct expansion of liquid refrigerant occurs

fan*: device for moving air by two or more blades or vanes attached to a rotating shaft

fan airflow: see airflow / fan airflow


  1. part of a boiler or warm air heating system in which energy is converted to heat;
  2. enclosed chamber or structure in which heat is produced, as by burning fuel, or by converting electrical energy

GAMA: Gas Appliance Manufacturers Association

Geothermal Heat Pump System: A geothermal heat pump system rejects heat to (in cooling mode) or extracts heat from (in heating mode) various ground resources, including the shallow surface of the Earth, ground water, surface water, etc. A geothermal heat pump system consists of the following three major components, a water source heat pump unit operable over an extended range of entering fluid temperatures, a ground heat exchanger, and a circulation system. Additionally, for ground water heat pump systems that do not use the direct expansion type of ground heat exchanger, a pump or pumps are usually needed to circulate the heat transfer medium (water or aqueous antifreeze solution) through the geothermal heat pump and the ground heat exchanger.

Geothermal Heat Pump**: A geothermal heat pump uses the thermal energy of the ground or
groundwater (or otherwise wasted resources) to provide residential or commercial space conditioning  and/or domestic water heating. A geothermal heat pump normally consists of one or more factory made assemblies that include indoor conditioning and/or domestic water heat exchanger(s), compressors, and a ground-side heat exchanger. A geothermal heat pump may provide space heating, space cooling, domestic water heating, or a combination of these functions and may also include the functions of liquid circulation, thermal storage, air circulation, air cleaning, dehumidifying or humidifying.

Ground Heat Exchanger: The method by which heat is exchanged with the ground, groundwater, or surface water. Geothermal heat pumps may use any form of ground heat exchange, which includes horizontal, vertical, or submerged surface water closed loops; open loops using ground water, reclaimed water, or surface water; or direct refrigerant-to-ground or refrigerant-to-water heat exchange.

Closed Loop: A ground heat exchange method in which the heat transfer fluid is permanently
contained in a closed piping system.

Open Loop: A ground heat exchange method in which the heat transfer fluid is part of a larger
environment. The most common open loop systems use ground water, reclaimed water, or surface
water as the heat transfer medium.

GPM: Gallons per minute

grille: a covering for an opening through which air passes

heat gain: The instantaneous flow (BTU/H) of sensible or latent heat entering the conditioned space or passing through a structural component. (A gain may or may not be equivalent to a space load, see Load Calculation)

heat loss: The instantaneous flow (BTU/H) of sensible or latent heat leaving the conditioned space or passing through a structural component. (Losses are equivalent to space loads because thermal mass effects are ignored for winter heat loss calculations, see Load Calculation).

heat pump*: thermodynamic heating/refrigerating system to transfer heat in either direction. By
receiving the flow of air or other fluid, a heat pump is used to cool or heat.

cooling and heating heat pump*: system designed to utilize alternately or simultaneously the heat
extracted at a low temperature and the heat rejected at a higher temperature for cooling and heating functions, respectively.

heating heat pump*: refrigerating system designed primarily to utilize the heat rejection from the
system for a desired heating function.

HIA: Hydronics Industry Alliance

HVAC: heating, ventilating and air conditioning

HVAC system*: a system that provides either collectively or individually the processes of comfort
heating, ventilating, and/or air conditioning within, or associated with, a building

HVACR: heating, ventilating, air conditioning, and refrigeration

IAQ: indoor air quality

IBR or I=B=R: AHRI training program for fossil fuel and hydronic appliances. Page 24 APPENDIX B | DEFINITIONSANSI / ACCA 5 QI – 2010 (HVAC QI Specification)

IFGC: International Fuel Gas Code

IGSHPA: The International Ground Source Heat Pump Association installation file: The information left with or attached to the installed equipment. Owner’s information.

kilowatt-hour: Energy used in the marketing of electrical power. Units are Kilowatt (i.e., 1000 watts) per hour of usage.


air leakage: the uncontrolled exchange of air between conditioned and unconditioned building spaces (or the interior and the outdoors) through unintended openings in the building envelope and/or unintended openings in duct runs through unconditioned spaces

distribution leakage: leakage of the ambient air through the cracks and openings in supply and/or return ducts or the supply and/or return-side of HVAC equipment cabinetry

load calculation: A systematic method of evaluation that uses mathematical models (equations,
databases, defaults and protocols) to estimate heat loss, sensible and latent heat gain, heating load, humidification load, sensible and latent cooling load, and related issues like infiltration, CFM minimum ventilation rate, month-hour temperature adjustments, building construction materials, building solar orientation, etc.

block analysis: a load calculation approach where the total space heat loss/heat gain load imposed on equipment is determined on a space that may include more than one room or more than one zone room-by-room analysis: a load calculation approach where the combined space heat loss/heat gain load imposed on equipment is determined on a room-by-room basis

system load: Heat loss (sensible BTU/H) or heat gain (sensible and latent Btu/H) required for
engineered ventilation, air or water distribution, relevant ancillary devices (e.g., blowers, motors,
pumps), reheat and humidification.

magnehelic: a diaphragm-type pressure differential sensor with a direct reading gauge

manometer*: instrument for measuring head or pressure; traditionally, a U-tube partially filled with a liquid, usually water, mercury, or manometer gauge oil, so constructed that the difference in level of the  liquid legs indicates the pressure exerted on the instrument


  1. act or result of determining the characteristics of some thing;
  2. extent, capacity, or amount ascertained by measuring;
  3. system of measures

nameplate rating: full-load continuous rating of a compressor, motor, or other equipment under
specified conditions, as designated by the manufacturer, and usually indicated on an attached plate

NATE: North American Technician Excellence

NEBB: The National Environmental Balancing Bureau

NEC: National Electrical Code

NFGC: National Fuel Gas Code

OEM: original equipment manufacturer

on-rate (also known as fuel flow rate): refers to the volume of fuel flowing into the combustion process at steady-state operation. Once the measured flow is corrected for temperature and altitude, the on-rate (for gas, rated in Btu/ft3; for oil, rated in Btu/gal) can be established utilizing a fuel’s heat content in Btu.


  1. system of pipes for carrying fluids;
  2. pipe or tube mains for interconnecting the various parts of a refrigerating system

pitot tube*: small bore tube inserted perpendicular to a flowing stream with its orifice facing the stream to measure total pressure


  1. in a refrigerating system, the medium of heat transfer which picks up heat by evaporating at a low temperature and pressure, and gives up heat on condensing at a higher temperature and pressure;
  2. (refrigerating fluid) fluid used for heat transfer in a refrigerating system that absorbs heat at a low temperature and low pressure of the fluid and transfers heat at a higher temperature and a higher pressure of the fluid, usually involving changes of state of the fluid


  1. actual amount of refrigerant in a closed system.
  2. weight of refrigerant required for proper functioning of a closed system

reclaim: (as in “reclaim refrigerant”) to reprocess refrigerant to new conditions, by means which may include distillation; require chemical analysis of the contaminated refrigerant to determine that appropriate process specifications are met (This term usually implies the use of processes or procedures available only at a reprocessing or manufacturing facility)APPENDIX B | DEFINITIONS Page 25 ANSI / ACCA 5 QI – 2010 (HVAC QI Specification)

recover: (as in “recover refrigerant”) to remove refrigerant in any condition from a system and to store it in an external container without necessarily testing or processing it in any way
recycle: (as in “recycle refrigerant”) to clean refrigerant for reuse by oil separation and single or multiple passes through moisture absorption devices, such as filter driers with replaceable cores. This procedure is usually implemented at the field site or at a local service shop
safety/safeties: see control / safety control

SMACNA: Sheet Metal and Air Conditioning Contractors National Association

steady state: HVAC system operating in equilibrium (generally operating constantly for over 10 minutes) A system operating in a stable condition over time; where the change in one direction is balanced by change in another

subcool(ing): removal of heat from a liquid when at a temperature lower than the saturation temperature corresponding to its pressure.

superheat(ing)*: extra heat in a vapor when at a temperature higher than the saturation temperature corresponding to its pressure


  1. organized collection of parts united by regular interaction;
  2. a heating or refrigerating scheme or machine, usually confined to those parts in contact with a heating or refrigerating medium

control system: see control

cooling system*: apparatus for lowering the temperature of a space or product to a specified
temperature level

duct system: A network of tubular or rectangular pipes and connectors(elbow, tees, branch fitting, and boot fitting) used to more air from one point to another
existing system: one that has existed previously geothermal

heat pump system: see geothermal heat pump

heating system*: one in which heat is transferred from a source of energy through a distribution network to spaces to be heated

matched system: The components of a split system are matched, rated, and have certified
performance through the AHRI and/or CEE databases

multi-zone: HVAC system capable of handling variable loads from different sections of a building simultaneously or independently

new system: one that has not previously been in existence

split system: (as in split system air conditioner) a two component system with the condensing unit installed outside, remote from the evaporator section, which is installed in a conditioned space, and uses interconnecting refrigerant lines to connect the condensing unit to the evaporator

venting system: A venting system is designed in accordance with OEM and code requirements to direct flue or combustion gases from a fossil fuel burning appliance to the outside atmosphere

TABB: Testing, Adjusting and Balancing Bureau

total load: Sensible plus latent requirements in BTU/hr.

thermal envelope*: elements of a structure that enclose conditioned spaces and control transmission of heat, air, and water vapor between the conditioned spaces and the exterior

unitary air conditioner*: one or more factory-made assemblies which normally may include an evaporator or cooling coil, a compressor and condenser combination, and may include a heating function.

voltage: electric potential or potential difference expressed in volts

watts (W)*: A power term that reflects the work done or energy generated by one ampere induced by an emf of one volt (P = EI = I 2 R)

zoning*: 1. division of a building or group of buildings into separately controlled spaces (zones), where different environmental conditions can be maintained simultaneously; 2. practice of dividing a building into smaller sections for control of heating and cooling (each section is selected so that one thermostat can be used to determine its requirements)

* Definition adapted from ASHRAE Terminology of Heating, Ventilation Air Conditioning & Refrigeration
Second Edition 1991.
** Definition adapted from Energy Star Program Requirements for Geothermal Heat Pumps Partner Commitments Version 3 Definitions section.

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About The Author

Ron Walker

After retiring from the U.S. Marines and achieving his B.S. degree, Ron Walker entered the HVAC field. He has been an HVAC technician, service manager, and business owner. Working as a service manager, he spent many years training HVAC technicians to be more technically competent and really understand their trade. His passion for teaching and helping others resulted in the creation of HVAC Training Solutions, LLC.