Installation Electrical
RTHD-SVX02H-EN 71
Flow Overdue” shall be generated which terminates the
prestart mode and de-energizes the condenser water
pump relay This diagnostic is automatically reset if flow
is established at any later time.
Note: This diagnostic would never automatically reset
if UC800 was in control of the condenser pump
through its condenser pump relay since it is com-
manded off at the time of the diagnostic. It could
however reset and allow normal chiller operation
if the pump was controlled from some external
source.
Heat Recovery Pump Control
UC800 provides a closed contact output for starting and
closing the heat recovery pump.
Heat Recovery Water Flow Interlock
UC800 on one input port, receives the user installation
flow test device, such as flow switch contact signal and
provide pump starter auxiliary contact signal, to interlock
cooling water flow. This input must be filtered to prevent
the instantaneous disconnection and closure caused by
eddy current. The filtering time constant is 6 seconds. The
heat recovery flow switch sensor voltage is 115/240vac.
When heat recovery is required, the UC800 unit will
energize the heat recovery pump relay, and then check
the heat recovery flow switch and pump starter input to
confirm the flow. The compressor must not be started
before confirming the flow.
If heat recovery water pump relay live within 1200 sec-
onds (20 minutes) after flow can’t established, will ap-
pear automatically restore the diagnosis of fault - “delay”
heat recovery water information, thus to suspend the pre
startup mode and disconnect heat recovery pump relay.
The fault will be automatically reset after the flow is es-
tablished in the following time.
Note: if UC800 is used to control the heat recovery
pump through a heat recovery pump relay, the
diagnostic failure will not be automatically reset
because it is set to be closed in case of a diagno-
sis. If the pump is controlled by external source,
the fault can be automatically reset and the chiller
can be started normally.
Chilled Water Reset (CWR)
The MP will reset the chilled water temperature setpoint
based on either return water temperature, or outdoor air
temperature. The Return Reset option is standard, Out-
door Reset is optional.
The following is selectable:
• RESET TYPE Setpoint.
This can be set to: NO CWR, OUTDOOR AIR TEM-
PERATURE RESET, RETURN WATER TEMPERATURE
RESET, or CONSTANT RETURN WATER TEMPERA-
TURE RESET. The MP shall not permit more than one
type of reset to be selected.
• RESET RATIO Setpoints.
For outdoor air temp. reset, both positive and nega-
tive reset ratios will be allowed.
• START RESET Setpoints.
• MAXIMUM RESET Setpoints.
The maximum resets shall be with respect to the
chilled water setpoint.
When the chiller is running, if any type of CWR is en-
abled, the MP will step the CWS toward the desired
CWS’ (based on the below equations and setup param-
eters) at a rate of 1 degree F every 5 minutes until the
Active CWS equals the desired CWS’. When the chiller
is not running the CWS will be fully reset immediately
(within one minute). The chiller will then start at the Dif-
ferential to Start value above a fully reset CWS or CWS’
for Outdoor, Return, and Constant Return Reset.
Equations for calculating CWR
Equation used to get Degrees of Reset:
Outdoor Air:
Degrees of Reset = Reset Ratio * (Start Reset - TOD)
Return Reset:
Degrees of Reset = Reset Ratio * (Start Reset - (TWE -
TWL))
Constant Return:
Degrees of Reset = 100% * (Design Delta Temp - (TWE -
TWL))
To obtain Active CWS from Degrees of Reset:
Active CWS = Degrees of Reset + Previous CWS
Note: Previous CWS can either be Front Panel, BAS, or
External
Reset Ratio calculation:
The Reset Ratio on the User Interface is displayed as a
percentage. To use it in the above equation it must be
converted to its decimal form.
Reset Ratio percent / 100 = Reset Ratio decimal
Example of converting Reset Ratio:
If the Reset Ratio displayed on the User Interface is 50%
then use (50/100) = .5 in the equation
TOD = Outdoor Air Temp
TWE = Evap Entering Water Temp
TWL = Evap Leaving Water Temp
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