Viconics VWZS Application Guide - Manual (Page 25)

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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.
Variable Frequency Drive Operation:
In this case the G fan output is used as an enable / disable output for the VFD.
The VFD should be configured to be reverse acting:
- Control signal = 0 VDC = VFD at high flow = Higher pressure.
- Control signal = 10 VDC = VFD at low flow = Lower pressure.
When the VFD is disabled (Terminal G is off), the static pressure control loop is off and the VFD output is at
0VDC. Please note that the fan is ALWAYS on during occupied periods and that it will cycle on demand
with the heating and cooling staged only during unoccupied periods.
When the VFD is enabled (Terminal G is on), the static pressure control loop is enabled and the VFD 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.
3H) Minimum Supply Heat Temperature Control and Lockout
The VZ7656F1000W master controller features a minimum supply air temperature PI control loop that is
used to prevent the supply air temperature from dropping below the Min SH (Minimum Supply Heat
Temperature) parameter value.
This control sequence is only available in the VZ7656F1000W modulating heat rooftop controller.
An outdoor temperature supply air lockout is also one of the VZ7656F1000W features. This lockout only
affects the minimum supply heat control sequence. Room temperature control is not affected.
The SH Lock is the parameter used to set the outdoor air temperature above which the minimum supply
heat temperature control sequence is disabled.
Example 1:
Outdoor Temperature:
60°F
SH Lock: 55°F
Minimum Supply Heat Temperature Control is disabled
Example 2:
Outdoor Temperature:
50°F
SH Lock: 55°F
Minimum Supply Heat Temperature Control is enabled