iWorx® MPU2
505-009, Effective: June 30, 2015
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© 2015 Taco Electronic Solutions, Inc.
Figure 13: Modulated Staging with 2 Stages; 1st Stage Modulating
Economizer Operation
The controller may be configured for either a two-position or a modulated economizer. The economizer is enabled
based on the availability of “free cooling” from the outdoor air. Free cooling is determined by dry bulb or enthalpy com-
parisons. To provide maximum energy savings, the cooling stages are delayed for three minutes to allow free cooling to
cool the space.
Dry Bulb Comparisons
Free cooling is determined based on a comparison of outdoor air temperature and indoor air temperature. When the
outdoor air temperature is a programmable amount below the indoor air temperature, free cooling is enabled. When the
outdoor air temperature rises above the indoor temperature, free cooling is disabled.
Enthalpy Comparison
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 ASM) on the network and sent to the controller. The same calculation
is performed on the inside air based on the space temperature and return air humidity. The inside enthalpy minus the
outside enthalpy must be greater than the Free Cooling Setpoint in order for the economizer to be used for free cooling.
Optionally, an ASM can measure the indoor air humidity globally. In this case, a return air humidity sensor would not be
required at each MPU2.
Two-position Economizer Control
When configured, the two-position economizer is enabled when free cooling is available as determined by the enthalpy
or dry bulb calculations. When the economizer is enabled, the economizer triac output is energized. When the econo-
mizer is disabled, the economizer output is de-energized. A configuration parameter is available to optionally disable
the economizer during unoccupied periods.
