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BAS-SVX084A-EN
Calibration, Operation Modes, and Control
Ventilation Flow Control (VFC)
Ventilation flow control is applied to a VAV terminal and used to temper cold outdoor air (OA) that
is brought into a building for ventilation purposes. The tempered air is intended to supply an air
handler unit (AHU), which provides comfort control to the zones it serves. The VAV terminal
supplies the correct amount of ventilation air and, when possible, tempers the ventilation air to
reduce the load on the air handler. Refer to the following table for ventilation flow control outputs.
The ventilation flow control process is a constant volume, variable temperature process. Single
duct VAV units with either electric or hot water reheat are used. Fan-powered units are not used
for ventilation flow control. Ventilation flow control must have a temperature sensor that is located
and setup as a discharge air temperature sensor. The required range of discharge air temperature
setpoints is from 19°F to 70°F (-7.22°C to 21.11°C).
Ventilation flow control staged reheat control (electric or hot water) achieves a 30-minute average
discharge air temperature to within ±5°F (±2.78°C) of the discharge air temperature setpoint when
the inlet temperature is within the control range. Ventilation flow control modulating reheat control
(hot water only) achieves a discharge air temperature to within ±5°F (±2.78°C) of the discharge air
temperature setpoint when the inlet temperature is within the control range.
Air Valve Control
Ventilation flow control uses the air valve as a constant volume device. The unit is given a constant
flow setpoint for air valve control (configured ventilation setpoint). The air valve only repositions
itself in response to changes in inlet static pressure. By using pressure-independent control for
ventilation purposes, a constant volume of fresh air can be maintained, regardless of small
fluctuations in inlet static pressure. Ventilation flow control unit can use a Ventilation Setpoint from
a BAS system, if it is valid. If the unit is standalone, the ventilation flow control uses one of the
following two (2) airflow setpoints:
•
If no reheat being used, it uses the configured Ventilation Setpoint Local.
•
If reheat being used, it uses the configured Airflow Setpoint Minimum Local Heat.
Staged Reheat Control (Electric and Hot Water)
The heat outputs of the controller are binary. Only discrete levels of discharge air temperature are
possible. Since discrete discharge air temperature levels do not always provide an instantaneous
temperature within the required band, staged reheat controls to a 30-minute average discharge air
temperature. The discharge air temperature setpoint is limited from 19°F to 70°F (-7.22°C to 21.11°C).
Table 13. Ventilation flow control outputs
Occupancy
Mode
Source Temperature
Air Valve Control
Reheat Control
Occupied, Occupied
Standby, or Occupied
Bypass
Any
Constant volume (if valid,
communicated
ventilation setpoint; if not
valid, configured
ventilation setpoint)
Electric
VFC staged reheat
control
Staged hot water
VFC staged reheat
control
Modulating hot water
VFC modulating reheat
control (same as STC
capacity control)
Unoccupied
Communicated source temperature (if valid; if
not valid, discharge air temperature) greater
than configured OA low limit.
Closed, 0%
Electric
Off
Staged hot water
Off
Modulating hot water Off
Communicated source temperature (if valid; if
not valid, discharge air temperature) less than
configured OA low limit.
Closed, 0%
Electric
Off
Staged hot water
On, 100% freeze
protection
Modulating hot water
On, 100% freeze
protection
