iWorx® MPU2
18
505-009, Effective: June 30, 2015
© 2015 Taco Electronic Solutions, Inc.
SEQUENCE OF OPERATION
This section describes the detailed sequence of operation for the MPU2 control algorithms.
Operational Mode
The MPU2 operates in one of four operating modes: primary heating, primary cooling, primary fan only, and primary off.
The operating mode determines whether warm or cool air is supplied to the zone controllers. The MPU2 determines
the operational mode based on the zone demand information supplied by each of the associated zone controllers. At
least once every 5 seconds, a different zone controller is polled. Each zone controller transfers its zone demand infor-
mation to the MPU2 over the communications network. The following information is transferred to the MPU2 controller:
• Zone temperature
• Calculated Heating Setpoint
• Calculated Cooling Setpoint
• IAQ Sensor Status (safe, alarm)
• Local Alarm (VAVD shutdown)
• Occupancy Mode (occupied, unoccupied, occupied extension)
• Supplemental heat status (on, off)
A zone heating demand is recognized when the zone temperature of a zone is at least 1.0
°
F
below
the
calculated
heating setpoint. The total heating demand is the sum of all zones requiring heating.
A zone cooling demand is recognized when the zone temperature of a zone is at least 1.0
°
F
above
the
calculated
cooling setpoint. The total cooling demand is the sum of all zones requiring cooling.
The system operational mode is determined by the greatest total demand value. When there is demand for both heat-
ing and cooling, the system switches between heating and cooling based on a configurable changeover time. Note that
minimum cycle times for the heating and cooling stages are enforced before an operational mode change can take
place. Also, a minimum 5-minute off-cycle is enforced before switching modes and following controller startup or reset.
When all zone demands have been satisfied (zone demand = 0), the operational mode is set to primary off indicating
no heating or cooling is being provided. When there is neither heating nor cooling, but the supply fan is on, the opera-
tional mode is set to primary fan only.
The heating mode can be disabled during warm weather by setting the outdoor air temperature heating lockout
Max
OAT Heat
. If the OAT is above the heating lockout temperature, the primary heating mode is disabled and the MPU2
can only be in the primary off or primary cooling modes. This feature requires that a controller that broadcasts the OAT,
such as an iWorx® ASM2, be installed on the network.
The cooling mode can be disabled during cold weather by setting the outdoor air temperature cooling lockout
Min OAT
Cool
. If the OAT is below the cooling lockout temperature, the primary cooling mode is disabled and the controller can
only be in the primary off or primary heating modes. This feature requires that a controller that broadcasts the OAT,
such as an ASM2 be installed on the network.
The economizer can be disabled during warm or cold weather by setting the outdoor air temperature economizer
Max
and
Min OAT Econ
settings. If the OAT is above the Max lockout temperature or below the Min OAT temperature, the
economizer is disabled and forced to its minimum fresh air position. The controller can only be in the primary off, pri-
mary heating or primary cooling modes. This feature requires that a controller that broadcasts the OAT, such as an
ASM2 be installed on the network.
The MPU2 can also be configured to only enter primary cooling or primary heating mode when a minimum number of
zones require cooling or heating. Setting the
Zone Limit
to a higher number prevents a small number of zones from
affecting the desired operating mode of the whole space by instructing the MPU2 to not change operating modes until
at least that number of zones require heating or cooling.
The current operational mode information is periodically transferred to the VAVD over the communications network.
The following information is transferred to the VAVD from the primary air source controller:
• Operational Mode (primary cool, primary heat, primary fan only, primary off)
