Nominal air volume flow refers to the "nominal" flow rating of the terminal box, the maximum flow for which it is rated.

Nominal air volume flow (AirFlNom) is a parameter located in the VAV supply or extract damper AF ("VavSu.xx"). (Exceptions: not in VavSu11 or 15; not in VavEx11 or 15)

Default value of AirFlNom

  • Standard preloaded VAV / FPB templates: AirFlNom default = 1200 ft3/min
  • Raw factory value in unconfigured VAV or FPB type: AirFlNom default = 100 m3/h (59 ft3/min)
  • Lab (CET) type: see Lab documentation



The guidelines for configuring AirFlNom in HVAC Types 12 and 13 (VAV and FPB respectively) differ depending on the version number of the application. The version number can be read by going online with the DXR using ABT Site. Access path: Device > List view > Infrastructure > your application .[properties] > "Present application version"

AirFlNom is used to define the scaling value that converts physical flow values to their relative (percent) flow values. When AirFlNom is used, all of the application’s air volume flow percent calculations are based on the value of AirFlNom. This means that the value of AirFlNom - when used - is crucial for the proper functioning of the automation station’s air volume flow control algorithms. Application version numbers and lower must use AirFlNom. Application version numbers and higher have the option of setting AirFlNom to zero, in which case it is not used. See the following guidelines for specific information.

Guidelines for application versions and lower

For application versions and lower, AirFlNom must be set at least equal to the largest max flow setpoint. It can be set equal to or slightly larger than the largest max flow setpoint. The best setup for application versions and lower is usually about 1.1 times the flow value of the largest max flow setpoint. For example if max cooling flow setpoint is 1000 ft3/min, AirFlNom could be set to 1100. This allows some flexibility for the balancer who may need to increase the flow slightly above the max setpoint during balancing procedures.

In versions and lower, do not set AirFlNom larger than 1.2x the value of the largest max flow setpoint. (see also the Caution note below)

Guidelines for application versions or higher

For application versions or higher, AirFlNom can be set to 0. If AirFlNom is set to 0 in an application version or higher, the application will not rely on AirFlNom as the basis for its air volume flow percent calculations. Instead, the application will use the largest max flow setpoint for this purpose (typically either max cooling or max ventilation flow setpoint).

If AirFlNom is to be used in an application version or higher (i.e., set to a non zero value), then set it equal to the largest max flow setpoint. In versions or higher, AirFlNom should not be set larger than the largest max flow setpoint (setting it higher may adversely affect control functions). (see also the following Caution note)

Duct size must be considered when setting AirFlNom

For all application version numbers, when AirFlNom is used (i.e., set to a non zero value), duct size must be considered.
a 6-inch round duct (approx 0.2 sq ft) with a typical maximum velocity of 3000 ft/min suggests a nominal air volume flow of 600 cfm (3000 ft/min * 0.2 sq ft = 600 cfm (approximately 300 lps)).

In this example, AirFlNom must not be left at the preloaded template default value of 1200 cfm! It requires a different value following the guidelines above.

(AirFlNom is not a BACnet object. It is a configuration parameter that can only be set using ABT Site or ABT-SSA.)