Application

Sequence of Operation
Alarms
33
Siemens Industry, Inc.
Application Note, App 6752
140-1322
2015-07-07
When the controller is operating in the constant volume mode, TEMP LOOPOUT is not
allowed to get numerically bigger than the value of START. This means that the
application will control the temperature only by modulating the reheat valve; it will make
no attempt to control temperature by varying the airflow in the space. During constant
volume control, TEMP CTL VOL will remain equal to zero.
Alarms
The controller is equipped with ventilation and pressurization alarms. It does not
contain temperature alarms. The controller’s alarms are designed to:
 Inform room occupants of hazards.
 Inform building operation personnel that the system is not functioning correctly.
 Supply data for documenting laboratory safety records through trending.
These alarms can be annunciated locally and/or broadcast across a network.
Ventilation Alarm
The alarm level depends on whether the room is occupied or vacant. When the
OCC.UNOCC point indicates occupancy, the OC V ALM LVL is used. When the
OCC.UNOCC point indicates vacancy, the UC V ALM LVL is used.
NOTE:
In the following discussion, the currently active supply flow minimum is OCC SUP
MIN during occupancy and UOC SUP MIN during the unoccupied period. Likewise,
the currently active general exhaust box minimum is OCC GEX MIN during
occupancy and UOC GEX MIN during the unoccupied period.
The ventilation alarm, VENT ALM, indicates that there is something wrong with the
ventilation to the room. VENT ALM has an adjustable alarm level that can vary with the
occupancy status of the room. An adjustable delay timer, VENT ALM DEL, prevents
nuisance alarms.
VENT ALM is turned on when at least one of the following conditions is true:
 The supply flow to the room, TOTL SUPPLY, stays below the alarm level, for a
time at least equal to VENT ALM DEL.
 SUP AIR VOL stays below the currently active supply minimum, for a time at least
equal to VENT ALM DEL.
 GEX AIR VOL stays below the currently active general exhaust box minimum, for a
time at least equal to VENT ALM DEL.
It is turned off only when all of the following conditions are true:
 The TOTL SUPPLY stays above the alarm level, for a time at least equal to the
alarm delay.
 SUP AIR VOL stays above the currently active supply minimum, for a time at least
equal to VENT ALM DEL.
 GEX AIR VOL stays above the currently active general exhaust box minimum, for a
time at least equal to VENT ALM DEL.
If the current conditions will neither turn on the ventilation alarm nor shut off the alarm,
then VENT ALM will keep its current value.
Setting the alarm level to zero means the ventilation alarm will not turn on just because
of a low value for TOTL SUPPLY.