iWorx® MPU2
24
505-009, Effective: June 30, 2015
© 2015 Taco Electronic Solutions, Inc.
Cooling Sequence
The controller provides support for either four compressor stages of cooling or one floating point or 0-10V modulating
cooling valve. You can specify which type of cooling you are using through configuration parameters.
Cooling Stages
The cooling sequence is initiated when the current operating mode calls for cooling. The cooling compressor stages
are sequenced based on the supply air temperature, cooling setpoint and the control band. When the supply air tem-
perature rises above the cooling setpoint plus the control band, a stage is turned on. If the supply air temperature
remains above the cooling control band for an additional time-period, the next available stage is turned on. If all zone
temperature readings are within 0.5
°F of their setpoints, the next stage does not cycle on. This cycle continues until all
available stages have been energized.
As the supply temperature drops below the cooling setpoint, the first available stage is turned off. If the supply air tem-
perature 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 minus control band all of the stages immediately cycle off.
Figure 11: Staged Heating and Cooling
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
are set to zero and the
Cooling Valve Travel Time
is non-zero. The
Cooling Stage 1
output is the
valve open signal and the
Cooling Stage 2
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%.
