iWorx® VPU2
505-010-2, Effective: April 29, 2014
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© 2014 Taco Electronic Solutions, Inc.
If configured for a floating point valve control the heating valve is calculated by a P + I control loop based on the supply
temperature and cooling setpoint. As the temperature decreases below the heating setpoint, the valve will be modu-
lated open. The valve will be modulated closed as the temperature increases above the heating setpoint.
In both the heating and cooling modes the supply air temperature setpoint may be reset by the zone demand.
The VPU2 can also monitor the supply air temperature to determine if the heating and cooling are operating properly.
During the cooling mode, if the supply air temperature fails to drop below the cooling operational limit after a pre-deter-
mined time period, a cooling failed alarm is reported to the LCI.
During the heating mode, if the supply air temperature fails to rise above the heating operational limit after a pre-deter-
mined time period, a heating failed alarm is reported to the LCI.
As a safety device, the controller can optionally monitor the supply air temperature to determine if the heating stages
have failed on. If the supply air temperature rises above the heating high limit setpoint, the fan energizes. If the supply
air temperature does not drop below the setpoint after a pre-programmed time delay, the fan speed is overridden off. A
heating high limit exceeded alarm is reported to the LCI and all of the zone controllers.
An indoor air quality input is provided. If an indoor air quality alarm is detected, the supply air fan is energized and the
economizer is overridden to supply fresh air to the zones. In addition, an alarm condition can be signaled by one of the
zone controllers. When an alarm condition exists, the controller energizes the supply air fan and overrides the static
pressure setpoint to the IAQ alarm setpoint. The controller attempts to clear the IAQ condition by allowing the econo-
mizer to open more than usual. If the condition has not been cleared after a programmable delay, an alarm is sent to
the LCI.
The VPU2 monitors a digital input to determine the presence of smoke. When the input indicates smoke, the controller
immediately turns off the fan and all stages of heating and cooling. An alarm is reported to the LCI when this condition
exists. The VPU2 remains in the shutdown alarm state until it is reset.
A digital input is provided on the VPU2 to monitor the status of the air filter. An external pressure switch is wired to the
input to determine when the filter becomes dirty. An alarm is reported to the LCI when this condition exists.
Mixed air low limit protection is provided through a digital input. If a low limit condition exists, the VPU2 turns off all
stages of heating and cooling along with the supply air fan. If heating is controlled by a floating-point valve, the valve is
opened fully to prevent equipment damage. An alarm is reported to the LCI when this condition exists. The VPU2
remains in the shutdown alarm state until it is reset.
The VPU2 monitors an input to determine if the fan is operating properly. When the input indicates a fan failure, the
controller immediately turns off the fan and all stages of heating and cooling. An alarm is reported to the LCI when this
condition exists.
The VPU2 monitors the runtime of the cooling stages, heating stages and fan. When any one of the runtimes exceeds
a programmable limit, a maintenance alarm is reported to the LCI.
When the Return Air humidity rises above the humidity setpoint, dehumidification is enabled by enabling the cooling
stages, if modulated cooling is enabled; the cooling output goes to 100%. Dehumidification is disabled, when return air
humidity drops below the setpoint by 3%.
SEQUENCE OF OPERATION
This section describes the detailed sequence of operation for the VPU2 control algorithms.
Occupancy State
The VPU2 operates in one of three occupancy states: extended occupancy, occupied, or unoccupied. 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.
