AC-SVX002C-EN
43
maximum rate of 1°F every 5 minutes until the actual
CWR equals the desired CWR. When the chiller is not
running, actual CWR shall be set equal to the desired
CWR within one minute (no maximum rate is in effect).
If Chilled Water Reset is disabled, desired CWR is 0.
A
Ad
dd
diittiio
on
na
all V
Va
arriia
ab
blle
e D
De
effiin
niittiio
on
nss
•
RESET RATIO is a user adjustable gain
•
START RESET is a user adjustable reference
•
TOD is the outdoor air temperature
•
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 Reset.
The equations for each type of reset are as follows:
R
Re
ettu
urrn
n
CWR = RESET RATIO * + [START RESET - (TWE - TWL)]
with limits:
•
CWR > or = 0
•
CWR < or = Maximum Reset
O
Ou
uttd
do
oo
orr
CWR = RESET RATIO * + (START RESET - TOD)
with limits:
•
CWR > or = 0
•
CWR < or = Maximum Reset
C
Co
on
nsstta
an
ntt R
Re
ettu
urrn
n W
Wa
atte
err T
Te
em
mp
pe
erra
attu
urre
e R
Re
esse
ett
CWR = 100% * [Design Delta Temperature — (TWE-
TWL)]
with limits:
•
CWR > or = 0
•
CWR < or = Maximum Reset
Using Equations for Calculating CWR
Degrees of Reset
•
OUTSIDE AIR: Degrees of Reset = Reset Ratio *
(Start Reset — TOD)
•
RETURN RESET: Degrees of Reset = Reset Ratio *
[Start Reset — (TWE-TWL)]
•
DEGREES OF RESET = 100% * [Design Delta Temp
— (TWE-TWL)]
Active CWS from Degrees of Reset
Active CWS = Degrees of Reset + Arbitrated CWS
N
No
otte
e:: Arbitrated CWS can either be Front Panel, BAS,
or External
Reset Ratio
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.
Diagnostic
If any sensor measurement needed to perform the
currently selected chilled water reset type is invalid due
to loss of communication or sensor failure, the desired
CWR will be set to 0. The actual CWR is subject to
maximum rate limits described earlier.
Communications Interface
LonTalk Interface (LCI-C)
UC800 provides an optional LonTalk
®
Communication
Interface (LCI-C) between the chiller and a building
automation system (BAS). An LCI-C LLID shall be used
to provide "gateway" functionality between a LonTalk
®
compatible device and the chiller. The inputs/outputs
include both mandatory and optional network variables
as established by the L
ON
M
ARK
®
Functional Chiller
Profile 8040.
N
No
otte
e:: For more information see ACC-SVN100*-EN.
BACnet Interface (BCI-C)
Optional BACnet
®
Communication Interface for
Chillers (BCI-C) is comprised of a Tracer
®
UC800
controller with interface software. It is a non-
programmable communications module that allows
units to communicate on a BACnet
®
communications
network.
N
No
otte
e:: For more information, see BAS-SVP01*-EN.
Modbus Remote Terminal Unit Interface
Modicon Communication Bus (Modbus
™
) enables the
chiller controller to communicate as a slave device on a
Modbus
™
network. Chiller setpoints, operating modes,
alarms and status can be monitored and controlled by
a Modbus
™
master device.
N
No
otte
e:: For more information, see BAS-SVP01*-EN.
