iWorx® VPU2
16
505-010-2, Effective: April 29, 2014
© 2014 Taco Electronic Solutions, Inc.
If a modulated economizer is employed and “free cooling” is available the modulated economizer position is calculated
by a Propor Integral (P+I) control loop. The control is based on the mixed air temperature and setpoint. As the
temperature increases above the mixed air setpoint the economizer is modulated open. The economizer is modulated
closed as the temperature decreases below the mixed air setpoint. The economizer is modulated to its minimum posi-
tion when the economizer is disabled. The economizer can optionally be disabled during the unoccupied periods.
When free cooling is available, mechanical cooling will not be enabled until the economizer is fully open (100%) for
three minutes at which time mechanical cooling will be enabled. This will help in preventing low suction and icing of the
cooling coils.
Either type of economizer can be disabled during unoccupied periods.
The VPU2 operates in one of four states: morning warm-up/cool-down, occupied, unoccupied, or shutdown. A host
device on the network determines the active operating mode. An optional backup schedule is provided for cases when
the host device is not available. The operating state determines if the VPU2 is in heating mode or cooling mode.
Figure 6: VAV Zone Control System
Cooling is accomplished through control of up to four stages of cooling, or one floating point cooling valve or control of
one analog cooling output (valve or variable speed circulator).
When the operating mode is cooling (occupied and extended occupancy), the cooling stages are sequenced on and off
with a time-proportioned control algorithm to minimize excessive cycling. The sequencing is based on the supply air
temperature and cooling setpoint. The cooling stages are interlocked with the economizer control. If the economizer is
enabled, the cooling stages do not sequence on until the economizer has reached its open position.
If configured for modulated analog output the cooling output position is calculated by a P + I control loop based on the
supply temperature and the cooling setpoint. As the temperature increases above the cooling setpoint, the cooling out-
put will be modulated open. The cooling output will be modulated closed as the temperature decreases below the cool-
ing setpoint.
If configured for a floating point valve control the cooling valve is calculated by a P + I control loop based on the supply
temperature and cooling setpoint. As the temperature increases above the cooling setpoint, the valve will be modulated
open. The valve will be modulated closed as the temperature decreases below the cooling setpoint.
Heating is accomplished through control of up to two stages of electric heating, or control of one floating point heating
valve or control of one analog output.
When the operating mode is heating (morning warm-up), the heating stages are sequenced on and off with a time-pro-
portioned control algorithm to minimize excessive cycling. The sequencing is based on the supply air temperature and
heating setpoint. Heating can also be controlled with a floating-point control of a valve.
The heating output position is calculated by a P +I control loop based on the supply temperature and the heating set-
point. As the temperature decreases below the heating setpoint, the heating output will be modulated open. The heat-
ing output will be modulated closed as the temperature increases above the heating setpoint.
