Daikin WGZ030DW Operating And Maintenance Manual page 14

Start Up Temp is the LWT at which the first compressor starts. The start up temperature equals the stage
up temperature plus the Start Delta temperature. A high Start Delta will keep the unit off longer and
reduce unit cycling at low loads. However, this high Start Delta will cause a larger excursion from the
LWT setpoint before the unit starts.
Stated another way, the Start Delta is the number of degrees above the Evap LWT setpoint, plus ½ the
Dead Band, that determines when the first compressor starts. The Start Delta is in effect for only the first
start after all compressors have been off. Additional compressor starts and stops are determined by the
LWT in respect to the dead band only. The dead band is automatically set at 30% of the EvapDeltaT
selected in menu 3. The following sequence would occur for the settings shown below:
EvapDelta T=10.0°F
Figure 7, Staging/Starting Temperatures
EWT
50.0°F
Evap Delta-T Set 10.0°F
LWT Set 40.0°F
At some point, the chilled water temperature will be dropping and begin to approach the point when
compressors should begin staging off, which is the LWT setpoint minus ½ of the Dead Band, 38.5°F in
this case. If the LWT remains below LWT setpoint minus ½ Dead Band and the Stage Down Timer times
out, additional compressors will stage off. The last compressor will stage off when the LWT falls below
the LWT Setpoint minus ½ the Dead Band minus the Stop Delta T. The stop Delta T is in effect for only
the last compressor running.
If the temperature climbs above 38.5°F all running compressors will remain on. No compressor staging
occurs within the Dead Band. The next-on compressor will start when the chilled water temperature
reaches 41.5°F and the Stage Up Timer times out.
However, in some circumstances this methodology can cause the LWT to drop to dangerously low levels,
with the evaporating temperature below the freeze point, before stopping. In the example shown in
Figure 7, the Shutdown Temp (last compressor off) would be 36°F.
This would result in a refrigerant evaporating temperature approaching freezing, so the rule is amended to
read:
If the Cool Leaving Water Temperature (LWT) set point is less than half the Control Band
above 39.0° F the Stage Down temperature is calculated as:
Stage Down Temperature = Cool LWT – (Cool LWT - 39.0
Shutdown Temperature = Cool LWT – (Cool LWT - 39.0
This keeps the Stage Down Temp above 39°F and the Shutdown Temp above 36°F, as the maximum Stop
Delta T allowed is 3-degrees.
Which compressor starts and stops. One compressor per circuit will start before starting the second
compressor (or third) on any circuit. In other words, the compressor with the lowest number of starts will
start first. The compressor with the lowest number of starts on the other circuit will start next, so that one
compressor on each circuit will be running. The third compressor on will be the compressor on either
circuit with the fewest starts. The remaining compressor will be the last on. If a circuit is unavailable for
any reason, the second compressor on the operating circuit will stage on.
compressors (on the one circuit) will be operating.
There is a 150 second delay after power-up before any compressor is allowed to start.
14
Dead Band=3.0°F StartDelta=5.0°F
Start Delta T
½ DB
½ DB
Stop Delta T
WGZ030D through WGZ200D
StopDelta=2.0°F
For a warm start-up (no compressors
running), the first compressor will start at any
46.5°F
temperature above 46.5°F. Each subsequent
compressor will start after the Stage Up
Timer has timed out and if the temperature is
41.5°F
above the dead band, 41.5°F in this case.
40.0°F
If the LWT stays above 41.5°F, all of three (or
38.5°F
5) remaining compressors will eventually
36.5°F
stage on after the Stage Up Timer times out
between each stage.
°
F), and the
°
F) – Stop Delta T
LWT=40.0°F
Only two (or three)
OMM 1130-2