HVAC Measure Definitions

The heating and cooling business is filled with acronyms and fancy words. G & R Controls has compiled a glossary to help you understand the heating, cooling, and ventilation world.


Actuator – An electrically driven motor which moves valve stems and dampers to desired positions.

AHU – See Air Handling Unit

Air flow measuring station – Measures both supply and return air volumes and maintains a constant difference between them.

Air Handling Unit (AHU) – Regulates and directs air volume throughout the ductwork to spread heated or cooled air in a space. AHU’s are located outside the conditioned area.

Air leakage – see infiltration

American Society of Heating, Refrigerating and Air-Conditioning Engineers (ASHRAE) – Founded in 1894, it is a global society advancing human well-being through sustainable technology for the built environment.

Amp – See Ampere

Ampere (Amp) – A unit of electrical current.

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

BAS – See Building Automation System

Belts – A loop of flexible material used to link two or more rotating shafts mechanically, most often parallel. Belts may be used as a source of motion, to transmit power efficiently, or to track relative movement.

Blower/Motor circuit – The speed of the electrical current in a blower, fan or motor of a boiler.

Boiler – Pressure vessels designed to transfer heat, produced by combustion, to a fluid (water or steam). May burn coal, wood, fuel oil, waste oil or gas.

British Thermal Unit (BTU) – The amount of energy required to raise one pound of water one degree Fahrenheit. One BTU is the equivalent of the heat given off by a single wooden kitchen match.

BTU – See British Thermal Unit

Building Automation System (BAS) – Is the automatic centralized control system of a building’s heating, ventilation and air conditioning, lighting and other systems.

Building envelope – Refers to windows, doors, wall insulation, etc. which make up the exterior of the building.

Burner – Used to mix fuel gas and combustion air in the proper ratio before the flame in a boiler system

Cabinet unit heater – Free-standing appliances for heating large spaces without ductwork. They are often placed overhead, positioned to direct heat to specific areas. They incorporate fuel-fired heat exchangers and a fan or blower to move air through the exchangers and into the space.

Capacity control – On a refrigeration and air conditioning applications where the load may carry over a wide range, due to lighting, occupancy, product loading, ambient weather variations, or other factors, some means of compressor capacity control is desirable for optimum system performance.  The simplest form of capacity is on-off cycling of the compressor. 

CEM – See Certified Energy Manager

Certified Energy Manager (CEM) – The Association of Energy Engineers (AEE) Certified Energy Manager® (CEM®) certification program is accredited by the American National Standards Institute (ANSI) based on the International Standard ANSI/ISO/IEC 17024. ANSI Standard 17024 is well-recognized within the industry as the highest standard in personnel certification accreditation.

CFL – See Compact Fluorescent Lamp

CFM – See Cubic Feet per Minute

Chiller – Cools water, brine or other secondary coolant for air conditioning or refrigeration.

Closed loop – Hydronic fluids are self-contained and flow continuously within a piping system without coming into contact with air or any other liquid.  

Combustion – A chemical reaction in which an oxidant reacts rapidly with a fuel to liberate stored energy as thermal energy, generally in the form of high-temperature gases.

Compact Fluorescent Lamp (CFL) – A fluorescent light bulb that has been compressed into the size of a standard-issue incandescent light bulb. Modern CFLs typically last at least six times as long and use at most a quarter of the power of an equivalent incandescent bulb.

Compressor – A part of the outdoor unit that pumps refrigerant throughout a sealed system. The compressor maintains adequate pressure to cause refrigerant to flow in sufficient quantities in order to meet the cooling requirements of the system.

Condenser coil – Located in the outdoor unit, the coil dissipates heat from the refrigerant, changing the refrigerant from vapor to liquid.

Condensing unit – Heat exchange device that uses the refrigeration cycle to provide mechanical cooling through a pair of evaporator coils.

Control circuit – Measure of electrical current in controller of boiler.

Cooling Coil – Removes sensible and latent heat from the air using either chilled water or refrigerant.

CU – See condensing unit

Cubic Feet per Minute (CFM) – A standard of airflow measurement. A typical air conditioning system produces 400 CFM per ton of air conditioning.

CUH – See cabinet unit heater

Damper – Found in ductwork, this movable plate opens and closes to control airflow. Dampers are used effectively in zoning to regulate airflow to certain rooms.

DDC – See Direct Digital Control

Diffuser – Placed over ductwork, this part separates air with vanes going in differing directions.

Direct Digital Control (DDC) – A system that receives electronic signals from sensors, converts those signals to numbers, and performs mathematical operations on the numbers inside a computer. The computer output (in the form of a number) is converted to a voltage or pneumatic signal to operate an actuator.

Direct Expansion Cooling (DX Cooling) – Uses the vapor-compression refrigeration cycle to cool a building.

Ductwork – System of pipes transports heat from the central air conditioning or heating system to the rest of the building.

DX Cooling – See Direct Expansion Cooling

Economizer – Uses outdoor air to reduce refrigeration requirements. It takes advantage of cool outdoor air weather to assist mechanical cooling or, if outdoor air is cool enough, to provide total system cooling.

EER – See Energy Efficiency Ratio

EF – See exhaust fan

Energy Efficiency Ratio (EER) – Ratio of the cooling capacity (in BTU’s per hour) to the power input (in watts).  The higher the EER rating, the more efficient the air conditioner.

Energy management system – See Building Automation System

Energy saving opportunity – The equipment may have area(s) for improvement which could save you money or extend the life of the equipment.

ENERGY STAR Score – The ENERGY STAR Score is a measure of how well your property is performing relative to similar properties, when normalized for climate and operational characteristics.

The ENERGY STAR scores are based on data from national building energy consumption surveys, and this allows Portfolio Manager to control for key variables affecting a building’s energy performance, including climate, hours of operation, and building size. What this means is that buildings from around the country, with different operating parameters and subject to different weather patterns, can be compared side-by-side in order to see how they stack up in terms of energy performance. The specific factors that are included in this normalization (Hours, Workers, Climate, etc) will depend on the property type.

The 1-100 scale is set so that 1 represents the worst performing buildings and 100 represents the best performing buildings. A score of 50 indicates that a building is performing at the national median, taking into account its size, location, and operating parameters. A score of 75 indicates that at a property is performing in the 75th percentile and may be eligible to earn ENERGY STAR Certification.

Energy Use Index (EUI) – Expressed as energy per square foot per year. It’s calculated by dividing the total energy consumed by the building in one year (measured in kBtu or GJ) by the total gross floor area of the building.

EUI – See Energy Use Index

Evaporator coil – Located inside the building; removes heat and humidity from the air inside the building.

Exhaust fan – Fan that discharges air to the exhaust duct, or in the case of a rooftop unit, directly outdoors; used to balance the amount of air exhausted from the space with the amount of outside air introduced to the system.

Face & bypass dampers – Modulates a percentage of air the face of the heat coil and around the coil (bypass) to maintain the supply air temperature setpoint. The air passing through the hot water coil is mixed with the air bypassing the coil to produce a desired discharge air temperature.

Fan coil unit (FCU) – Introduce air into a conditioned space to obtain a desired indoor atmospheric environment.

Fan powered VAV – Terminal device that combines a true VAV box in parallel with a fan and optional heating coils.

FCU – See fan coil unit

Filter – A device that removes allergens, pollutants and other undesirable particles from air that is heated or cooled.

Fluorescent lamp – A light source consisting of a tube filled with argon, along with krypton or other inert gas.  When electrical current is applied, the resulting arc emits ultraviolet radiation that excites the phosphors inside the lamp wall, causing it to radiate visible light.

GPM – See Gallons Per Minute

Gallons Per Minute (GPM) – The flow rate needed to deliver the required amount of BTUs.

Heat/energy recovery – Process of recovering heat and/or moisture between two airstreams at different temperatures and humilities. The process is important in maintaining acceptable indoor air quality (IAQ) while maintaining low energy costs and reducing overall energy consumption. Can save energy and reduce the required capacity of primary cooling and heating plants by 20% or more.

Heat/energy recovery types – Heat pipes, runaround loops, fixed-late energy exchangers and rotary wheel energy exchangers.

Heat exchanger – Transfers heat from one fluid to another without the fluids coming in contact with each other. Heat transfer occurs in a heat exchanger when a fluid changes from liquid to vapor (evaporator), a vapor to a liquid (condenser) or when two fluids transfer heat without a phase change. The transfer of energy is caused by a temperature difference.

Hot water coil – One or two row coils with 60° F entering air and a hot water temperature of anywhere from 140° F to 200° F.

Horsepower (HP) – A unit of power equal to 550 foot-pounds per second (745.7 watts).

HP – See Horsepower

HVAC – The classic acronym for Heating, Ventilation and Air Conditioning. Today, an HVAC system also includes air cleaning and moisture control.

Hydronic systems – Water systems that convey heat to or from a conditioned space or process with hot or chilled water. Water flows through piping that connects a boiler, water heater or chiller to suitable terminal heat transfer units located at the space.

IAQ – See Indoor Air Quality

Indoor Air Quality (IAQ) – combination of thermal regulation, controls of internal and external factors of pollutants, supply of acceptable air, removal of unacceptable air, occupants’ activities and preferences and proper construction, operation and maintenance of building systems. Studies indicate an outdoor air supply of about 20cfm per person is very likely to provide acceptable perceived indoor air quality in office spaces, whereas lower rates may lead to increase sick building syndrome symptoms.

Infiltration (also known as air leakage) – The flow of outdoor air into a building through cracks and other unintentional openings and through the normal use of exterior doors for entrance and egress.

Kilowatt (kWh) – Unit of electrical power per 1,000 watts.

kWh – See Kilowatt

Lamp Lumen Depreciation Factor (LLD) – A factor the represents the reduction of lumen output over time.  The factor is commonly used as a multiplier to the initial lumen rating in illuminance calculations, which compensates for the lumen depreciation.  The LLD factor is a dimensionless value between 0 and 1.

Leadership in Energy and Environmental Design (LEED) – An ecology-oriented building certification program run under the auspices of the U.S. Green Building Council (USGBC). A LEED professional credential signifies that you’re a leader in the field and an active participant in the green building movement.

Lead/Lag – Using multiple pumps that run in sequence is a common way to meet varying pump system demand.  Cycling of the lead pump adds reliability in the form of redundancy and increases the lifespan of the system.  In a traditional lead-lag system, the lead pump runs until the demand on the system is too great for the pump to meet, at which point the lag pump(s) initiates until demand is met.

LED – See Light Emitting Diode

LEED – See Leadership in Energy and Environmental Design

Light Emitting Diode (LED) – An illumination technology used for exit signs.  Consumes low wattage and has rated life of greater than 80 years.

Lighting system – Consists of lighting sources (lamps), luminaries (fixtures) and ballasts. 

LLD – See Lamp Lumen Depreciation Factor

Lumen – A unit of light flow, or luminous flux. The lumen rating of lamp is a measure of the total light output of the lamp. 

MBH (Thousand BTU per Hour) – 1 MBH = 1000 BTU/hr.

  • Input – Amount of BTU’s of fuel in boiler system
  • Output – Actual amount of heat generated by input

Metal Halide – A type of high intensity discharge (HID) lamp in which most of the light is produced by radiation of metal halide and mercury vapors in the arc  tube.  Available in clear and phosphor-coated lamps.

Nominal full-load efficiency (Nom eff.) – What the average would be if a substantial number of identical fan were tested and the averages of the batch were determined. Some fans might have a higher value and others might be lower, but the average of all units tested is shown as the nominal nameplate value.

Operational risk – One or more components that need attention to keep the equipment running optimally.  A risk could also indicate the equipment may be past ASHRAE’s life expectancy recommendation.

Outside air fan – Fan which brings in 100% outside air

Pneumatic controls – Mechanical device designed to measure temperature or pressure and transmit a corrective air signal to the final control element.

Pounds per Square Inch (PSI) – Measure of pressure

Pounds per Square Inch Gauge Pressure (PSIG) – Measurement of pressure relative to ambient atmospheric pressure.

Preheat coil – Heating coils placed upstream of a cooling coil; they can use steam, hot water, or electric resistance as a medium. Some AHUs do not require a preheat coil at all, particularly if the percentage of outdoor air is low and if building heating is provided elsewhere.

Primary equipment – Includes refrigeration equipment; heating equipment; and air, water and steam delivery equipment.

Proper operation – Indicates the equipment turns on and runs.

PSI – See Pounds per Square Inch

Pumps – Primary force to distribute and recirculate hot and chilled water in a variety of space-conditioning and Hydronic plumbing systems.

Radiant heater – Heat energy is emitted from a warm element, such as a floor, wall or overhead panel, and warms people and other objects in rooms rather than directly heating the air.

Refrigerant – A substance that produces a cooling effect. It’s used in most air conditioning and cooling systems.

Reheat coil – Heating coiling placed downstream of a cooling coil. Reheat systems are discouraged unless recovered energy is used.

Relief valve – Opens or closes gradually in proportion to excessive pressure. Only used for liquids (unheated water).

Retro-commissioning – The application of the commissioning process to existing buildings. It is a process that seeks to improve how building equipment and systems function together.

Return fan – Provides a positive return and exhaust from conditioned area. Ensures proper volume of air returns from the conditioned space. It prevents excess building pressure when economizer cycles introduce more than the minimum quantity of outdoor air, and reduces the static pressure against which the supply fan must work.

Roof Top Unit (RTU) – Packaged unit that rests on a curb adapter and sits on the roof; supplies one or all the heating, ventilation and/or air-conditioning of a building.

Revolutions Per Minute (RPM) – Measurement of the rotational speed of a mechanical component.

RPM – See Revolutions Per Minute

RTU – See Roof Top Unit

Secondary equipment – Refers to the set or subset of components that perform a specific HVAC function including, but not limited to, exhaust fans, radiant heaters, unit ventilators, fan powered VAV boxes, shut-off VAV boxes, fan coil units and cabinet heaters.

Shut off VAV – Terminal device comprised of an air flow measuring station and a control damper to provide a variable air supply to a space. Box can include heating source.

Sick building syndrome – Sick building syndrome is used to describe a situation in which the occupants of a building experience acute health- or comfort-related effects that seem to be linked directly to the time spent in the building, according to the National Institutes of Health.

Site energy use – The annual amount of all the energy your property consumes on-site, as reported on your utility bills.

Source energy use – The total amount of all the raw fuel required to operate your property, including losses that take place during generation, transmission, and distribution of the energy.

Site EUI – The Site Energy Use divided by the property square foot.

Source EUI – The Source Energy Use divided by the property square foot.

 Supply fan – Straight-through flow fan used for drawing in air across coils.

  • Draw-through – supply fan placed downstream of cooling coil
  • Blow-through – supply fan placed upstream of cooling coil

Terminal equipment – See secondary equipment

Unit ventilator (UV) – Assembly whose principal functions are to heat, ventilate, and cool a space by introducing outdoor air in quantities up to 100% of its rated capacity.

UV – See Unit Ventilator

Valve – Manual or automatic fluid-controlling elements in a piping system constructed to withstand a specific range of temperature, pressure, corrosion and mechanical stress.

Variable Air Volume (VAV) System – Controls temperature in a space by varying the quantity of supply air rather than varying the supply air temperature.

Variable Frequency Drive (VFD) – Fast-switching power transistors generate a variable-voltage, variable-frequency waveform that changes the speed of the AC motor. As speed decreases, input power is reduced substantially because the power required varies as the cube of the speed.

VAV – See Variable Air Volume (VAV) System

VFD – See Variable Frequency Drive

V-PH-Hz (Volts-Phase-Hertz) – Electrical unit of measure

  • Volt – Measurement of electrical force given to the electrons in an electric circuit.
  • Phase – Distribution of alternating current electric power using a system in which all the voltages of the supply vary in unison
  • Hertz – Measurement of electrical energy per second. Standard frequency is 6- Hertz.

Ventilation system – System which intentionally introduces outside air into a building. Provides a comfortable and healthy indoor environment (indoor air quality - IAQ) for building occupants.

Watt (W) – Unit for measuring electrical power.  It defines the rate of energy consumption by an electrical device when it is in operation.  The energy cost of operating an electrical device is calculated as its wattage times the hours of use.  In single phase circuits, it is related to volts and amps by the formula: Volts x Amps PF = Watts. 

Weather normalized site energy – Energy use your property would have consumed during 30-year average weather conditions.

Weather normalized source energy – Source energy use your property would have consumed during 30-year average weather conditions.

Weather normalized source EUI – Weather Normalized Source Energy divided by property size or by flow through a water/wastewater treatment plant.