A list of industry terms and definitions to help you brush up on your HVAC lingo!



Agency Compliance
3rd party certifications that the product meets. Agency compliance refers to third-party certification from organizations that verify a product meets industry standards for performance in specific areas such as safety. These organizations include the Air Movement and Control Association International (AMCA), Underwriters Laboratories (UL) and ASTM International.
Specific usage for a particular fan or ventilator, regardless of building type. Examples include pressurization, car park ventilation, kitchen ventilation, fire and smoke ventilation, etc.
Describes whether the drive package or accessories are shipped assembled to the fan, or if they are shipped separately/independently from the fan.


Base Height (power roof ventilators)
Height of the curb cap on power roof ventilators (vertical mount units).
Base Length x Base Width
Length and width of a ventilator curb cap.
Base Top to Shroud
Distance between the top of the curb cap to the point where the shroud begins on downblast power roof ventilators.
Base Top to Shroud Top
The dimension on a downblast exhaust ventilator measuring from the top of the base to the top of the shroud.
Bearing Size
Bore size of the bearings utilized on a ventilator.
Bearing Type
Style of bearings utilized on a ventilator or a motor.
Blade Gauge
Blade material thickness on a particular damper. Blade gauge is the thickness of the blade material used on a damper
Blade Material
Construction material of the blades on a product.
Building Type
Common building style on which ventilator and other products are installed. Examples include hospital, restaurant, shopping malls, offices, warehouses, etc.


CFM (Flow Rate)
CFM (Cubic Feet per Minute) - a measure of volume air flow rate, or air moving capability, of an air moving device. Volume of air moved per minute. Other common units of measure are CMH (cubic meters per hour), as well as CMS and m3/s (cubic meters per second).
Type of material used to manufacture a product. For example, several products are manufactured using stainless steel material.
Curb Cap Size (Purling Box Size)
Square base dimension on power roof ventilators (see Base Length x Base Width).Indicates what size/area roof curb/upstand is needed.
Curb Type
Type of curb or upstand (i.e. - fixed, adjustable, ventilated, non-ventilated).


Damper Actuator
Small power supply used to operate damper opening and closure. Types include electric and manual.
Damper Depth
Maximum depth of a damper product.
Drive Type
Transmission type of a ventilator (belt drive or direct drive).


Effective Damper Width x Damper Height
The "ACTUAL" damper width and height. Dampers are undersized to allow mounting into the duct work/wall opening.


Fits Fan Base Size (sq)
Describes the square base dimension a roof curb fits.
Flange Depth
The depth of the mounting flange on a wall propeller fan panel.
Flange Width
The width of the flange on the wall propeller fan panel, fan inlet or outlet flange, or dampers flange.
Flashing Flange
Portion of the roof curb/upstand that mounts to the roof deck.
Flow Rate
See CFM.
Fan Diameter
The diameter of the propeller compatible with an accessory.
For Use With
Describes the fans or ventilators a particular accessory can be used with. Indicates which fan or ventilator compatible with an accessory.
Frame Gauge
Material thickness of the frame on a particular damper.
Frame Material
Construction material of the frame of a given product.


The height of a given product.
Hertz (Cycles)
Measure of the frequency or cycles per second for motor or actuator power supply.
Unit of measure for power required to drive an air moving device. Horsepower required varies with system conditions. Kilowatt is another unit of measure for power. (1 Kilowatt = 1.34 horsepower)
Housing Material
Type of material used in the construction of a ventilator housing.


Impeller Diameter
Dimension of the impeller (propeller or wheel). Common units of measure are inches, centimeters, and millimeters.
Impeller Material
Type of material used in the construction of an impeller for a fan or ventilator.
Impeller Type
Style of impeller used on a product (i.e. centrifugal or axial).
Describes items that a particular product has in addition to or part of, such as mounting hardware, etc.
Inlet Diameter
The diameter of the fan opening where air is drawn in.
Inlet Width x Inlet Height
Describes the size (width x height) of the opening where air is drawn into a ventilator.


The end to end measurement of a given product.
The connecting point location of a mechanical device and damper blades.


Max Inlet Temp
The maximum entering air temperature a fan or ventilator can withstand while in constant operation, without compromising the life of the unit.
Max Operating Temp
The maximum air temperature a fan or ventilator can withstand while in constant operation, without compromising the life of the unit.
Max Air Velocity
The recommended maximum speed at which air should pass through an HVAC opening.
Max. Operating Amps
The maximum amperage a motor on a given fan or ventilator will draw while in operation.
Max. Operating Watts, Max Operating Power or Absorbed Power
A fan requires mechanical power to provide a certain flow rate at a certain pressure. This is the maximum wattage or power that a given fan or ventilator requires while in operation.
Motor Enclosure
The design of the motor casing. Common types are totally-enclosed fan cooled (TEFC) and totally-enclosed air over (TEAO).
Mounting Location
Refers to the location for the mount of a fan or ventilator.
Mounting Position
Refers to the mounting orientation for a product.


Number of Blades
The number of blades an axial or centrifugal fan product or damper product has.


OCI (also see RRL)
Open/Closed Indicator, which contains a single pole, double throw switch used to indicate damper blade position and provides a positive open or closed signal which can be used in conjunction with remote indicator lights (furnished by others). Can be combined with RRL in an RRL/OCI device.
Outlet Width x Outlet Height
This dimension refers to the size of the discharge (where air leaves the ventilator) on a particular unit.
Overall Diameter
Describes the diameter at the widest point for a given product.
Overall Height
Describes the maximum height of a product.


Power Supply Phase
Indicates the space relationships of windings and changing values of the recurring cycles of AC voltages and currents. The most common power supplies are either single- or three-phase.
Pressure Differential Max
Describes the maximum static pressure that a given damper can withstand and still be fully operational in an HVAC system.
Purling Box Size
See Curb Cap Size.


Required Accessory
Describes what accessory(s) must be purchased in addition to make the product fully functional.
Roof Curb
The square mounting stand holding the fan above the roof deck on roof-mounted fans. Also called “upstand” in some markets.
Roof Opening Width x Roof Opening Length
Describes the size (width x length) of the roof opening required to install a given fan or ventilator.
Revolutions Per Minute - The number of times per minute the shaft of the motor (machine) rotates. This is a function of design and power supply, and pulley/drives ratio on belt-driven fans.
RRL (also see OCI)
Resettable link for a combination fire smoke damper. It replaces the fusible link with an electric sensor (thermostat) which interrupts power to the actuator, causing the actuator’s spring return mechanism to close the damper. The sensor can reset after the temperature cools below the sensor set point. Can be combined with OCI in an RRL/OCI device.


The type of seals used on a damper blade to minimize leakage.
Shaft Diameter
Describes the diameter of the shaft on a given damper, fan, ventilator or motor.
Accessory used to reduce sound power (dB).
Sound Attenuator
See Silencer.
Sound Power
The acoustic power radiating from a sound source (expressed in watts or decibels. Sound power cannot be measured directly, heard by the human ear or measured with a microphone. It is a logarithmic ratio of the sound power emitted to a reference sound power. Sound Power is preferred by architects and acoustical engineers to be submitted in eight (8) octave bands, as it is independent of application and can be used for direct comparison between alternate fan/product selections.
Sound Pressure
The acoustic pressure at a point in space where a microphone or listener’s ear is situated (expressed in microbars or decibels) dependent upon acoustic environment and distance from the source, and is the value that may be requested as overall sound level at a point in a room.
Speed Controller
Describes whether a fan or ventilator can be used with a solid state speed control. (NOTE: A speed control cannot be used with any belt-drive fan or ventilator.)
Supply Frequency (Hz)
Frequency is the number of cycles per second in an alternating current (AC), supplied to the motor or actuator. See Hertz.
Surface Finish
Type of coating or material appearance used on a product.


Tip speed (TS)
Also called peripheral velocity, equals the circumference of the fan wheel time the RPM of the fan, and is expressed in m/s (ft/min)
Two Speed Motor (Double Polarity Motor)
Typically 2/4, 4/6, 4/8 and 6/8 poles motors are used. 4/6, 6/8 poles motors have two separate winding while 2/4, 4/8 poles motors can have a single winding (Dahlander) or two windings.


Universal Mounting
Brackets capable of mounting fans in multiple configurations.
See Roof Curb.


Warranty Period
The length of time a product can be returned/repaired as covered under the agreement.
Wide Open Volume (% WOV)
The ratio percent of the operating point compared to the airflow when the static pressure across the fan is zero. The point with static pressure of zero is 100% WOV.
Describes the side-to-side measurement of a given product.