Viconics VBZS Application Guide - Manual (Page 23)

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Also, the recorded RTU supply delta temperature and demand variances will always be higher when using
a highest demand control type operation versus an average demand method. Energy consumption is also
expected to be higher with a highest demand control type operation versus an average demand method of
calculating the system requirements.
3G) By-Pass Damper / VFD Control and Operation
The RTU controller has a built in static pressure control loop with an analog 0 to 10 VDC output that can be
configured to control a by-pass damper or a variable frequency drive (VFD). In order to operate, the static
pressure control loop needs to have a static pressure sensor connected to the static pressure input on the
RTU controller (terminal SP).
The type of pressure transducer used needs to be of voltage type (0 to 5 VDC) and have a 24 VAC half-
bridge power supply.
The range of the pressure transducer needs to be one of the following and needs to be properly configured
using the static pressure configuration parameter (SP range).
Static pressure transducer range.
Voltage input range is 0 to 5 VDC.
- 0 = 0 to 1.5 in WC
- 1 = 0 to 2 in WC
- 2 = 0 to 3 in WC
- 3 = 0 to 4 in WC
- 4 = 0 to 5 in WC
Typically, the static pressure sensor probe is installed 2/3 of the way down the main ventilation duct.
The static pressure set point is set by the configuration parameter (Pressure). The default value is 0.8” WC.
The range and adjustability of the set point is: 0 to 2 in WC (0 Pa to 500 Pa) (increments: 0.1” WC or 25
Pa).
Please note that the static pressure scale will automatically change from inches of WC to PA (Pascals)
when the local units configuration parameter is changed.
- 0 = SI for Celsius / Pa pressure scale
- 1 = Imp for Fahrenheit / in. WC pressure scale
Operation of the static pressure control loop is dependent on the fan running or not, and the setting of the
pressure control type parameter SP Cntrl. For proper operation of the control loop, the static pressure
control actuator or VFD need to be properly installed.
By-Pass Damper Operation:
- Control signal = 0 VDC = Static pressure by-pass damper fully closed = No air recirculation from
supply to return
- Control signal = 10 VDC = Static pressure by-pass damper fully opened = Maximum air
recirculation from supply to return
When the fan output is off (Terminal G), the static pressure control loop is off and the by-pass damper is
fully opened to 10 VDC output. This will minimize the air pressure related noise during initial fan start-up.
Please note that the fan is ALWAYS on during occupied periods and that it will cycle on demand with the
heating and cooling stages only during unoccupied periods.
When the fan output is on (Terminal G), the static pressure control loop is enabled and the by-pass damper
will modulate to maintain the desired static pressure set point according to the static pressure input reading
at the RTU controller. The current static pressure value can be read at the RTU controller at any time by
using the manual scroll function and displaying the pressure prompt.