68 RT-SVX34C-EN
Startup
Coil Frost Protection
The Frostat™ control monitors the suction line temperature to prevent the evaporator from
freezing due to low operating temperatures whenever there is a demand for cooling. When a closed
circuit has occurred for 5 seconds minimum, the RTRM turns off all of the cooling outputs providing
the 3 minute minimum "On" time for the compressor(s) has elapsed. The Supply Fan will be held
"On" until the Frostat has been open for 5 continuous seconds or for 60 seconds after the last
compressor was shut "Off", whichever is the longest. The compressor shutdown is communicated
to Tracer, if applicable. There is no local diagnostic for this condition.
Dehumidification Frost Protection
Two control schemes will be active on units configured for Dehumidification. The first employs the
use of the Frostat function. The second scheme takes precedence over Frostat. Operation will be
as described below.
The second scheme is only in control during active dehumidification and includes the use of an
Entering Evaporator Temperature sensor (EET). If the EET drops below 35°F for 10 continuous
minutes compressors will stage off. For dual circuit units one circuit will be staged off. For single
circuit units one compressor will be staged off. The compressors which have been staged off will
not be energized until the unit leaves the current dehumidification cycle or a dehumidification
purge is initiated.
VFD Programming Parameters
See System Troubleshooting section.
Condenser Fan Sequencing Control
The condenser fans are cycled according to the outdoor air temperature and the number of cooling
steps that are operating. Table 25, p. 68 lists the temperatures at which the A and B Condenser Fan
Outputs on the RTRM switches the fans "Off". The fans are switched back "ON" when the outdoor
temperature rises approximately 5° F above the "Off" temperature.
Figure 42, p. 69 shows the condenser fans as viewed from the top of the unit facing the control
panel. Whenever a condenser fan is cycled back "On", the condenser fan Outputs A and B and the
compressor steps are de-energized for approximately seven seconds to prevent problems with fan
windmill.
Table 25. Condenser Fan/Compressor Sequence
Unit Size Compressor Staging Sequence Condenser Fan Output
O/A Temp.
(°F)
(Tons) Step 1 Step2 Step 3 Output A Output B Fans "Off"
27.5 - 30
CPR 1*
CPR 1, 2
N/A
Fan #2
Fan #3
70
90
Fan #2
Fan #3
-10
60
35
CPR 1*
CPR 1, 2
Fan #2
Fan #3
65
85
N/A Fan #2
Fan #3
-20
55
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