Application

Sequence of Operation

Ventilation – VAV Mode

Siemens Industry, Inc.

Application Note, App 6752

140-1322

2015-07-07

Ventilation – VAV Mode

During VAV operation, the ventilation works as follows:

OCC SUP MIN, the occupied supply minimum, is used to ensure that the room

receives enough supply air for proper ventilation during the occupied mode. UOC SUP

MIN is used to ensure that the room receives enough supply air for proper ventilation

during the unoccupied mode. If necessary, the application raises the general exhaust

flow to keep the supply flow from dropping below the minimum. See

Calculating

Exhaust Flow Setpoint

for more information.

NOTE:

Since Application 6752 places a higher priority on pressurization than it does on

ventilation, the currently active supply minimum (OCC SUP MIN or UNOCC SUP

MIN) may be overridden to maintain negative pressurization.

Ventilation Setback in VAV Mode

Ventilation setback allows the minimum and maximum flows (air change rate) for each

VAV terminal to vary based on occupancy mode (OCC.UNOCC). This allows several

options for reducing the ventilation for unoccupied periods, including:

 Lowering the minimum supply flow, which allows a lower air change rate, but

maintains cooling control.

 Lowering the maximum flow, which limits the air change rate and reduces cooling

capacity.

 Closing the general exhaust, which lowers airflow and disables cooling completely.

Ventilation – Constant Volume Mode

During Constant Volume (CV) operation, the active supply airflow minimum is used to

ensure that the room always receives enough supply air for proper ventilation. If

necessary, the application raises the general exhaust flow to keep the supply flow from

dropping below the minimum. See

Calculating Exhaust Flow Setpoint

for more

information.

In CV mode the active supply minimum is the setpoint. The active supply airflow

minimum is used to ensure that the room always receives enough supply air for proper

ventilation.

During CV operation, the air volume out of the supply box will equal the active supply

airflow minimum as long as this is sufficient to maintain proper room pressurization.

NOTE:

Regardless of the operation—VAV or CV temperature control—airflow out of both the

supply and general exhaust box will vary as needed in order to maintain proper room

pressurization whenever HOOD VOL changes.