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Operation Manual
D
–EOMWC00A07-16_02EN
Reset varies from 0 to Max Reset set point as the Evaporator EWT
– LWT (Evap delta t) varies from the Start Reset Delta
T set-point to 0.
7.11.3
4-20 mA External Signal Reset
The Active Leaving Water variable is adjusted by the 4 to 20 mA reset analog input.
Parameters used:
1. Cool LWT set point
2. Max Reset set point
3. LWT Reset signal
Reset is 0 if the reset signal is less than or equal to 4 mA. Reset is equal to the Max Reset Delta T set point if the reset
signal equals or exceeds 20 mA. The amount of reset will vary linearly between these extremes if the reset signal is
between 4 mA and 20 mA. An example of the operation of 4-20 reset in Cool mode follows.
20
(54)
Cool LWT Set
Point (44)
4
4-20 mA Reset - Cool Mode
0
Max Reset
(10)
Active
LWT
(
o
F)
Reset Signal (mA)
7.12 Unit Capacity Control
Unit capacity control is performed as described in this section.
7.12.1
Compressor Staging in Cool Mode
The first compressor on the unit is started when evaporator LWT is higher than the target plus the Startup Delta T set point.
An additional compressor is started when Evaporator LWT is higher than the target plus the Stage Up Delta T set point.
When multiple compressors are running, one will shut down if evaporator LWT is lower than the target minus the Stage
Down Delta T set point.
The last compressor running will shut down when the evaporator LWT is lower than the target minus the Shut Down Delt a
T set point.
7.12.2
Compressor Staging in Heat Mode
The first compressor on the unit is started when condenser LWT is lower than the target minus the Startup Delta T set
point.
An additional compressor is started when condenser LWT is lower than the target minus the Stage Up Delta T set point.
When multiple compressors are running, one will shut down if condenser LWT is higher than the target plus the Stage
Down Delta T set point.
The last compressor running will shut down when the condenser LWT is higher than the target plus the Shut Down Delta
T set point.
Stage Up Delay
A minimum amount of time will pass between compressors starting, which is defined by the Stage Up Delay set point. This
delay will only apply when at least one compressor is running. If the first compressor starts and quickly fails on an alarm,
another compressor will start without this minimum time passing.
Required Load For Stage Up
An additional compressor will not be started until all running compressors are at a capacity higher than the Loa d Stage Up
set point, or running in a limited state.
