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iWorx® VPU2
26
505-010-2, Effective: April 29, 2014
© 2014 Taco Electronic Solutions, Inc.
After the supply temperature has dropped below the cooling setpoint, the first available stage is turned off. If the supply
air temperature remains below the cooling setpoint for an additional time-period, the next available stage is turned off.
This cycle continues until all available stages have been de-energized. If the supply air temperature drops below the
cooling setpoint plus control band all of the stages immediately cycle off.
Figure 11: Staged Cooling Sequence (Occupied)
Calculated
Cooling
Setpoint
Cooling
Control
Band
Stage 3 Off
Stage 1 Off
Stage 2 On
Stage 1 On
T1
T2
T1
T2
T3
T4
Stage 3 On
Stage 4 On
T1 = Stage Cycle Time
T2 = Stage Cycle Time x 2
T3 = Stage Cycle Time x 3
T4 = Stage Cycle Time x 4
T3
T4
Stage 2 Off
Stage 4 Off
Temperature
Space
Setpoint
Cooling with Floating Point Control
The cooling outputs can be configured for floating point control of a cooling valve. Floating point control is enabled
when
Cooling Stages
is set to zero and the
Cooling Valve Travel Time
is non-zero. The C1 output is the valve open sig-
nal and the C2 output is the valve close signal.
After a reset, the floating point valve is calibrated by closing the valve for a period of the travel time. This ensures that
the valve is fully closed. When the valve is at its calculated 0% or 100% position, the valve is overdriven for 30 seconds
to ensure that the valve is fully closed or open.
The floating point control is similar to the cooling modulated algorithm. If the space temperature is above the cooling
setpoint, the valve is driven open. When the space temperature is below the cooling setpoint, the valve is driven
closed. There is a +/- 1 °F
(0.55
°C) deadband around the setpoint to prevent the valve from dithering. During mixed air
low limit alarms, the cooling valve is driven to 100%.
Cooling with Modulated Output (Valve or Variable Speed Circulator)
The calculated cooling loop setpoint is derived from the cooling setpoint and the loop proportional gain.
CalcCoolingLoopSp
=
CalcCoolingSp
- ½(Kp)
The cooling output is modulated by a P+I control loop based on the cooling loop setpoint and the space temperature.
The P+I control loop will modulate the output to maintain a constant space temperature. As the temperature increases
above the cooling loop setpoint, the cooling valve will be modulated open. The cooling output will be modulated closed
as the temperature decreases below the cooling loop setpoint. When unoccupied mode is entered, the cooling loop
setpoint is set up through a separate unoccupied cooling setpoint.