Operating Instructions

Laboratory Room Controller Owner's Manual
3-8 Siemens Building Technologies, Inc.
Setting the Unoccupied Flow Rates
The LRC supports ventilation setback by providing two sets of airflow minimums and
maximums, one for occupied mode and another for unoccupied mode. Each mode contains
the following:
• Supply Minimum
• General Exhaust Minimum
• Supply Maximum
• General Exhaust Maximum
For the occupied mode, the HVAC engineer usually selects the supply minimum to maintain
the minimum ventilation rate, and sets the other three values according to equipment sizing.
For example, the general exhaust maximum is usually determined by the maximum required
cooling flow, less the minimum flows through the other exhaust devices. The supply
maximum is usually set to balance the exhaust flows when they go to design flow.
When selecting flow limits for the unoccupied mode, it is important to know how those values
were determined for the occupied mode. Assuming the supply minimum was selected to
maintain the required air change rate, lowering the unoccupied value is the first step toward
implementing a ventilation setback scheme. However, that may not actually lower the flows. It
is possible that the general exhaust minimum must also be lowered. Again, it is important to
know how the occupied value was selected.
Lowering supply and general exhaust minimums will usually result in lower airflows, but it is
possible that the supply flow may still be high because fume hoods are being left open. The
facility monitoring capabilities of the APOGEE can help pinpoint that problem (which hoods,
which days, etc.) so corrective steps can be taken. Refer to
Chapter 5, Monitoring Laboratory
Operation
.
It is also possible that airflows will stay high because the air is needed to cool the room.
Some building operators choose to limit this cooling capability by lowering the general
exhaust maximum. Some have even chosen to completely eliminate general exhaust during
unoccupied periods by setting the general exhaust maximum to zero. These approaches are
not usually recommended. In most cases, it is better to lower the minimums and let the
controller reduce the airflow if that is consistent with the cooling requirements.
Remember that it is recommended that the APOGEE be set up to alarm when the ventilation
is inadequate. Refer to
Using Airflow
for more information. It is possible that the ventilation
rate selected for the unoccupied periods is lower than the alarm level chosen for the occupied
mode. To prevent meaningless alarms, you should select a separate alarm level appropriate
for the unoccupied mode.
Table 3-3 shows the points in the LRC that define a ventilation setback strategy. Select and
set values for these points according to safety requirements and HVAC design criteria. The
APOGEE provides access to these points through the Start-up report. This report is available
through CIS, the field panel and Insight.