Application

Room Segment Functions
Device Control Functions HVAC
60
Siemens Industry, Inc.
Application Note
A6V11692053
Smart Infrastructure – Building Products
2019-04-01
Fan coasting feature
Note
The fan coasting feature applies to duct 1 and duct 2 separately: there is a fan
coasting object for each duct (FanSuCstVal1, FanSuCstVal2).
The fan coasting feature (FanSuCstVal) prevents premature closing of the room
supply damper while the AHU fan is coasting down. This might occur during a
switch between operating modes (for example, switching from Comfort to
Economy).
The fan coasting feature is active when:
⚫ The room supply air damper is the only damper that is open – all other VAV
supply air dampers are closed.
⚫ The AHU supply fan has been turned off and is coasting down.
⚫ 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.