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The Iso Valve Close Delay default value is 45 seconds and may be
adjusted between 0 and 180. NOTE: On a request for unit
shutdown (described previously), the valve will remain open until
all compression is disabled and then additionally until the
configured valve close delay expires.
COOLING (SINGLE MODULE)
When configured for "Single/Secondary Unit" operation, The
W2W control will operate up to four stages of compression to
maintain the desired chilled water setpoint. The control point can
be selected by setting the "Control Type" to either the leaving wa-
ter temperature sensor (default) or the entering water temperature
sensor. While in the cooling mode, the compressor outputs are
controlled by the PI (Proportional-integral) cooling loop and
cooling stages capacity algorithm. The algorithms calculate the
desired number of stages needed to satisfy the load water setpoint
by comparing the actual water temperature sensor value to the
chilled water setpoint.
Additionally, an independent leaving load water temperature
safety check is included to verify that sufficient load water flow is
provided. Regardless of the control type, as the leaving load water
temperature exceeds the configured "Chilled Water Supply
Setpoint", the compressor stages will start to be reduced as
required.
The following conditions must be true in order for the cooling
algorithm to run:
•
Unit is occupied (see Occupancy Section above).
•
The lockout temperature (OAT, SPT, or T1) is greater than
the Cooling lockout temperature setpoint.
•
Cooling mode is set to Enable.
•
Heat mode is not active, the compressor time guard has
expired.
•
The Changeover time guard has elapsed.
•
Load water temp is above the Chilled Water Supply
Setpoint.
If all the above conditions are met, the compressor(s) will be
energized as required to satisfy the cooling requirements. While
cooling, the reversing valve output will be held in the cooling
position (either B or O type as configured) even after compression
is stopped. The valve will not switch position until the heating
mode is required.
During cooling operation, the control employs a 5 minute time
delay before each subsequent stage of compression is energized to
allow the leaving water temperature (LWT) to achieve a stable
temperature and avoid overshoot. Once energized, a stage of
compression will have to run for at least 180 seconds before being
staged off to avoid short-cycling.
Once a compressor is staged off, it may not be restarted again until
a five minute time guard delay period expires.
The module provides three (binary) compressor status inputs to
monitor compressor operation. These inputs are monitored to
determine if the compressor status matches its commanded state.
These inputs are used to determine if a refrigerant safety switch or
other safety device has tripped and has caused the compressor to
stop operating normally. If this should occur, an alarm will be
generated to indicate the faulted compressor condition. The
Compressor Status Alarm includes a 6 minute delay to avoid
nuisance alarms.
ADDITIONAL COOLING FUNCTIONS
Cooling Setpoint OA Rest
The W2W control may be configured to reset the chilled water
setpoint proportionally based upon OAT. The amount of reset and
the reset range are both user configurable. This feature can provide
energy savings by operating the unit at a higher load water
temperature setpoint when the minimum chilled water
temperature is not required. It also can be used to better match the
chilled water temperature to the cooling load requirements.
The following conditions must be true in order for the Cooling
Setpoint OA Reset algorithm to run:
•
The preconditions for cooling operation (described
previously) have been met.
•
The module has a valid Outdoor Air Temperature.
•
Cooling Setpoint OA Reset is Enabled.
RH Cooling Setpoint Rest
When the OAT Cooling Setpoint Reset function (described
previously) is enabled, The W2W control has the capability to
limit the (OAT) chilled water setpoint reset based upon the differ-
ence between the High RH setpoint and the measured RH value
(Space RH or Outdoor Air RH).
The RH Cooling Setpoint reset override uses a PI function to
determine the limit applied to the Cooling Setpoint OA Reset
function.
The following conditions must be true in order for the RH Cooling
Setpoint Reset algorithm to run:
•
The preconditions for cooling operation (described
previously) have been met.
•
The preconditions for Cooling Setpoint OA Reset operation
(described previously) have been met.
•
The module has a valid Relative Humidity value.
•
RH Cooling Setpoint OA Reset is Enabled.
COOLING CONTROL MODES
The following control modes are available for cooling operation.
These modes may be selected by the installing technician, or by an
operator of sufficient access privileges using one of the available
man machine interfaces (BACview, Field Assistant, or i-Vu Open
Server). See the Controls Configuration sections for additional
instructions.
Manual Cool
Configuring the Control Mode Selection for Manual Cool places
the control in a cooling only configuration and the machine
operates as a chiller. The module will operate as described in the
Cooling section while configured for Manual Cool. The Outdoor
Air and Relative Humidity Cooling setpoint reset functions are
available provided their associated preconditions (described
previously) are met.
When the Control Mode Selection is set to Manual Cool, the lock-
out temperature (OAT, SPT, or T1) is overridden, even when the
input is available. The control will operate as required to maintain
the Chilled Water Supply Temperature setpoint, regardless of the
any previously configured lockout temperature.
The following conditions must be true in order for the cooling
algorithm to run in the Manual Cool control mode:
•
Control Mode Selection is set to Manual Cool.
•
Cool Enable is set to Enable.
•
Unit is occupied (see Occupancy Section above).
•
Load water temp is above the Chilled Water Supply
Setpoint.
OAT
Configuring the Control Mode Selection for OAT sets the control
to change modes between heating and cooling automatically
based on the Outdoor Air Temperature (OAT). The control will
compare the OAT to the configured lockout temperatures for
heating and cooling and select the appropriate operating mode. A
valid outdoor air temperature reading (hardware or network) is
required for this mode. Once enabled, equipment control is as
described for heating and cooling.
When conditions require cooling, the module will operate as
described in the Cooling section above. The Outdoor Air and
Relative Humidity Cooling setpoint reset functions are available
provided their associated preconditions (described previously) are
met.
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