Application

Room Segment Functions

Device Control Functions HVAC

Siemens Industry, Inc.

Application Note

A6V11692055

Smart Infrastructure – Building Products

2019-04-01

CAUTION

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.

Example: 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.)

Air Volume Flow Setpoint Calculation

In control mode the airflow setpoints (VavSuSpAirFl1 and VavSuSpAirFl2) are

configured to provide cooling or heating demand and the configured ventilation

demand.

Although request values are in percent, the comparison to select the max is made

in physical airflow units. This is because the airflow limits (minimum flow and

maximum flow) are different for each request type.

The mapping from percent to airflow units for cooling and ventilation includes a

step-up at the low end, with a switch on point of 4% and a hysteresis of 2%. Thus,

the request percent must rise above 4% before its corresponding airflow value rises

from 0 to the request’s minimum airflow.

Potential uses of the step-up include:

⚫ Keeping the flow setpoint high enough to support heat transfer from a coil

⚫ Keeping the flow setpoint high enough for effective sensing

Dual Duct Air Volume Flow Calculation

Air Volume Flow (VavSuAirFl1, VavSuAirFl2) and Relative Air Flow

(VavSuAirFlRel1, VavSuAirFlRel2) are calculated values computed from logic that

utilizes the following:

⚫ Separate Velocity pressure from the differential pressure sensor or other flow

sensing technology

⚫ Units conversion constant to convert to air velocity