iWorx® MPU2
16
505-009, Effective: June 30, 2015
© 2015 Taco Electronic Solutions, Inc.
When “free cooling” is available, the modulated economizer position is calculated by a Propor Integral (P+I)
control loop based on the mixed air temperature and setpoint. As the temperature increases above the mixed air set-
point, the economizer damper is modulated open. The economizer is modulated closed as the temperature decreases
below the mixed air setpoint. The economizer is modulated to its minimum position when the economizer is disabled.
The economizer can optionally be disabled during unoccupied periods.
The bypass damper operates to maintain a configurable system static pressure setpoint. The bypass damper position
is calculated by a Propor Integral (P+I) control loop based on the measured static pressure and setpoint. As the
pressure increases above the pressure setpoint, the bypass damper is modulated open. The bypass damper is modu-
lated closed as the pressure decreases below the pressure setpoint.
Heating and cooling changeover setpoints are provided to prevent zone thermal shock during mode changes.
Figure 6: Multiplexed Zone Control System
An indoor air quality input is provided to monitor the Indoor Air Quality (IAQ). It can accept a digital CO2 sensor provid-
ing a contact closure, or an analog CO2 sensor. In addition, an alarm condition can be signaled by one of the Zone
Controllers. When an alarm condition exists, the MPU2 energizes the supply air fan and override the static pressure
setpoint to the IAQ alarm setpoint. The controller attempts to clear the IAQ condition by allowing the economizer to
open more than usual. If the condition has not been cleared after a programmable delay, an alarm is sent to the LCI.
The MPU2 scans all associated Zone Controllers to collect system demand data. The total heating and cooling
demands are accumulated. The greatest demand determines the control mode.
When the system cooling demand is greater than the system heating demand, the system enters the cooling mode.
When the system heating demand is greater than the system cooling demand, the system enters the heating mode.
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 (valve or variable speed circulator). 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).
The cooling stages are sequenced with a timed-proportioned control algorithm to minimize excessive cycling. The
sequencing is based on the measured supply air temperature, and the cooling setpoint. The cooling and heating
demands are continually re-evaluated during the cooling mode of operation. The controller is capable of switching to
the heating mode when the temperature demand is greater for heating. The cooling stages are interlocked with the
economizer control. If the two-position economizer is employed, the stages sequence on after the economizer.
The heating stages are sequenced with a timed-proportioned control algorithm to minimize excessive cycling. The
sequencing is based on the measured supply air temperature, and the heating setpoint. The cooling and heating
demands are continually re-evaluated during the heating mode of operation. The controller is capable of switching to
the cooling mode when the temperature demand is greater for cooling.
