106
RTAA-IOM-3
Chilled Water Reset (CWR)
As an option, the UCM will reset the
chilled water temperature setpoint,
based on either the return water
temperature, zone air temperature, or
outdoor air temperature. The following
are selectable:
1. One of four RESET TYPEs, from top
to bottom in order of reset:
no CWR
RETURN WATER
TEMPERATURE RESET ZONE
TEMPERATURE RESET
OUTDOOR AIR TEMPERATURE
RESET
The UCM will not permit more than
one type of reset to be selected in
Menu 3.
2. RESET RATIO Setpoints. For
OUTDOOR AIR TEMPERATURE
RESET, there are both positive and
negative reset ratios.
3. START RESET Setpoints.
4. MAXIMUM RESET Setpoints. The
maximum resets are with respect to
the chilled water setpoint.
No matter which type of reset is
selected, all parameters are factory set
to a predetermined set of values. Field
adjustment of 2, 3, or 4, above, is
usually not required.
The equations for each type of reset
are:
RETURN WATER TEMPERATURE
RESET
CWS’= CWS + RESET RATIO
[START RESET - (TWE - TWL)]
and CWS’> or = CWS
and CWS’- CWS < or = MAXIMUM
RESET
ZONE TEMPERATURE RESET
CWS’= CWS + RESET RATIO
[START RESET - TZONE]
and CWS’> or = CWS
and CWS’- CWS < or = MAXIMUM
RESET
OUTDOOR AIR TEMPERATURE RESET
CWS’= CWS + RESET RATIO
[START RESET - TOD]
and CWS’> or = CWS
and CWS’- CWS < or = MAXIMUM
RESET
CWS’ is the new chilled water setpoint.
CWS is the active chilled water setpoint
before any reset has occurred.
RESET RATIO is a user adjustable gain.
START RESET is a user adjustable
reference.
TZONE is the zone temperature.
TOD is the outdoor temperature.
TWE is the entering evaporator water
temperature.
TWL is the leaving evaporator water
temperature.
MAXIMUM RESET is a user adjustable
limit, providing the maximum amount
of reset.
Note: When any type of CWR is
enabled, the UCM will step the CWS
toward the desired CWS’ (based on the
above equations and setup
parameters) at a rate of 1 F every 5
minutes. This applies when the chiller
is both running and off. Normally the
chiller will start at the Differential-to-
Start value above a fully reset CWS or
CWS’.
The values for RESET RATIO for each type of reset are:
Reset Increment Factory
Reset Ratio English Increment Default
Type Range Units Sl Units Value
Return 10 to 120% 1% 1% 50%
Zone 50 to 300% 1 % 1 % 100%
Outdoor 80 to -80% 1 % 1 % 10%
The values for START RESET for each type of reset are:
Start Increment Factory
Reset Reset English Increment Default
Type Range Units Sl Units Value
Return 4 to 30 F 1 F 0.1 C 10 F (5.6 C)
(2.2 to 16.7 C)
Zone 55 to 85 F 1 F 0.1 C 78 F (25.6 C)
(12.8 to 29.4 C)
Outdoor 50 to 130 F 1 F 0.1 C 90 F (32.2 C)
(10 to 54.4 C)
The values for MAXIMUM RESET for each type of reset are:
Maximum Increment Factory
Reset Reset English Increment Default
Type Range Units Sl Units Value
Return 0 to 20 F 1 F 0.1 C 5 F (2.8 C)
(-17.8 to -6.7 C)
Zone 0 to 20 F 1 F 0.1 C 5 F (2.8 C)
(- 17.8 to -6.7 C)
Outdoor 0 to 20 F 1 F 0.1 C 5 F (2.8 C)
(- 17.8 to -6.7 C)
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