UC800 resets the chilled water temperature setpoint
based on return water temperature. Return Reset is stan-
dard.
The following shall be selectable:
One of three Reset Types: None, Return Water Tem-
perature Reset, or Constant Return Water Tempera-
ture Reset.
Reset Ratio Set Points.
Start Reset Set Points.
Maximum Reset Set Points.
The equations for each type of reset are as follows:
CWS' = CWS + RATIO (START RESET - (TWE - TWL))
and CWS' > or = CWS
and CWS' - CWS < or = Maximum Reset
CWS' is the new chilled water set point or the "reset
CWS"
CWS is the active chilled water set point before any reset
has occurred, e.g. normally Front Panel, Tracer, or ECWS
RESET RATIO is a user adjustable gain
START RESET is a user adjustable reference
TWE is evaporator entering water temperature
TWL is evaporator leaving water temperature
MAXIMUM RESET is a user adjustable limit providing
the maximum amount of reset.
For all types of reset, CWS' - CWS < or = Maximum Re-
set.
The values for "RESET RATIO" for each of the reset types are:
Reset Type
Reset Ratio Range
Increment English Units
Increment SI Units
Factory Default Value
Return
10 to 120%
1%
1%
50%
The values for "START RESET" for each of the reset types are:
Reset Type
Reset Ratio Range
Increment English Units
Increment SI Units
Factory Default Value
Return
2.2 to 16.67 C (4 to 30F)
0.1 F
0.1 C
5.56 C (10 F)
The values for “MAXIMUM RESET” for each of the reset types are:
Reset Type
Reset Ratio Range
Increment English Units
Increment SI Units
Factory Default Value
Return
0.0 to 11.11 C (0 to 20 F)
0.1 F
0.1 C
2.78 C (5 F)
In addition to Return, the MP provides a menu item for the operator to select a Constant Return Reset. Constant Re-
turn Reset will reset the leaving water temperature set point so as to provide a constant entering water temperature.
The Constant Return Reset equation is the same as the Return Reset equation except on selection of Constant Return
Installation - Electrical
Reset, the MP will automatically set Ratio, Start Reset, and Maximum Reset to the following.
RATIO = 100%
START RESET = Design Delta Temp.
MAXIMUM RESET = Design Delta Temp.
The equation for Constant Return is then as follows:
CWS' = CWS + 100% (Design Delta Temp. - (TWE - TWL))
and CWS' > or = CWS
and CWS' - CWS < or = Maximum Reset
When any type of CWR is enabled, the MP will step the Active CWS toward the desired CWS' (based on the above
equations and setup parameters) at a rate of 1 degree F every 5 minutes until the Active CWS equals the desired CWS'.
This applies when the chiller is running.
When the chiller is not running the CWS is reset immediately (within one minute) for Return Reset and at a rate of 1
degree F every 5 minutes for Outdoor Reset. The chiller will start at the Differential to Start value above a fully reset
CWS or CWS' for both Return and Outdoor Reset.
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RTAG-SVX001H-EN