Application

Room Segment Functions
Device Control Functions HVAC
60
Siemens Industry, Inc.
Application Note
A6V11692055
Smart Infrastructure – Building Products
2019-04-01
⚫ The airflow setpoint signal (VavSuSpAirFl), while on its way to zero, falls below
the airflow demand hysteresis setting (HysAirFlDmd).
When these conditions are true, the boolean supply fan coasting signal
(FanSuCstVal) will equal 1 (True) and the damper is reset to its last (previous)
operating position / value. This keeps the damper open while the fan coasts down
to prevent supply duct over-pressure.
In addition to the fan coasting signal freezing the damper in place, an internal timer
function was activated when VavSuSpAirFl initially dropped below HysAFlSta. The
fan coasting signal and the timer (60 seconds) must both finish (return to False)
before the room supply damper can close. See the figure below.
NOTE:
The internal timer operates independently and regardless of FanSuCstVal. Once
the timer is activated, the room supply air damper is frozen in place until the
counter reaches zero, after which the damper may close.
Fan supply (FanSuCstVal) coast value comes from the primary AHU.
Configuration
Parameters
Description
Parameter
Preloaded value
Switch-on point for air flow demand
SwiOnAirFlDmd
4%
Hysteresis for air flow demand
HysAirFlDmd
2%