iWorx® MPU2
28
505-009, Effective: April 28, 2014
© 2014 Taco Electronic Solutions, Inc.
Dry Bulb Comparisons
Free cooling can be determined based on a comparison of outdoor air temperature and return air temperature. When
the outdoor air temperature is a programmable amount below the return air temperature, free cooling is enabled. When
the outdoor air temperature rises above the return temperature free cooling is disabled.
Enthalpy Calculation
An enthalpy calculation is performed periodically to determine if “free cooling” is available from the outside air. The out-
side enthalpy is calculated based on the outside air temperature and humidity. The outside temperature and humidity
are measured by an external device (such as an ASM2 [Auxiliary Sensor Module]) on the network and sent to the con-
troller. The same calculation is performed on the inside air based on the space temperature and the return air humidity.
The inside enthalpy minus the outside enthalpy must be greater than the
Free Cooling Setpoint
in order for the econo-
mizer to be used for free cooling.
Optionally, an ASM2 can measure the indoor air humidity globally. In this case, the return air humidity sensor would not
be required at each MPU2.
Two-position Economizer
If configured, the two-position economizer is enabled when there is “free cooling” available as determined by the
enthalpy or dry bulb calculations. When the economizer is enabled, the economizer triac output is energized. When the
economizer is disabled, the economizer output is de-energized. A configuration parameter is available to optionally dis-
able the economizer during unoccupied periods.
Modulated Economizer
If configured, the modulated economizer is enabled when there is “free cooling” available as determined by the
enthalpy or dry bulb calculations. When the economizer is enabled, the economizer triac output is energized. When the
economizer is disabled, the economizer output is de-energized. A configuration parameter is available to optionally dis-
able the economizer during unoccupied periods.
When the economizer is enabled, a Propor Integral (P+I) control loop modulates the economizer output position
to maintain a constant mixed air temperature.
The P+I control loop is based on the mixed air temperature setpoint and economizer setpoint. As the temperature
increases above the economizer setpoint, the economizer is modulated open. The economizer is modulated closed as
the temperature decreases below the economizer setpoint.
To prevent the integral component from becoming too large, there is anti-wind up reset protection. This protection
clamps the integral value when all of the components add up to more than 100% or less than 0%. The following equa-
tions are used for P+I control:
Kp = Proportional Gain
Ki = Integral Gain
Error = EconSP - MixedAirTemp
When the economizer is disabled, it modulates to the minimum position. A configuration parameter is available to
optionally disable the economizer during unoccupied periods.
Cutoff Temperatures
The cutoff temperatures can be set for heating, cooling and for the economizer to suppress mechanical equipment
from activating if the outside air temperature is above or below a cutoff setpoint.
Heating will be suspended when the outside air temperature rises above
Max OAT Heat
. It resumes when the OAT falls
5
°F below the setting. Cooling is suspended when the OAT fails below
Min OAT Cool
. It resumes when OAT rises 5
°F above the setting. The economizer will close the dampers to the Minimum fresh air position when the OAT is out-
side the
Max OAT
and
Min OAT Economizer
temperature limits.
