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Fig. 38 — Defrost Control Board Arrangement
The DFB is located in the 50TCQ unit's main control box (see
Fig. 40). All connections are factory-made through harnesses
to the unit's CTB, to IFC (belt-drive motor) or to ECM (direct-
drive motor), reversing valve solenoids and to defrost thermo-
stats. Refer to Table 6 for details of DFB Inputs and Outputs.
REVERSING VALVE CONTROL
The DFB has two outputs for unit reversing valve control. Op-
eration of the reversing valves is based on internal logic; this
application does not use an "O" or "B" signal to determine re-
versing valve position. Reversing valves are energized during
the cooling stages and the defrost cycle and de-energized
during heating cycles. Once energized at the start of a cooling
stage, the reversing valve will remain energized until the next
heating cycle demand is received. Once de-energized at the
start of a Heating cycle, the reversing valves will remain de-en-
ergized until the next cooling stage is initiated.
COMPRESSOR CONTROL
The DFB receives inputs indicating Stage 1 Cooling, Stage 2
Cooling and Stage 1 Heating from the space thermostat or unit
control system (PremierLink™ controller or RTU Open con-
troller); it generates commands to start compressors with or
DIP
Switches
Speed-Up
Jumpers
Fig. 40 — Defrost Control Board Location
27
without reversing valve operation to produce Stage 1 Cooling
(one compressor runs), Stage 2 Cooling (both compressors run)
or Stage 1 Heating (both compressors run).
AUXILIARY (ELECTRIC) HEAT CONTROL
The 50TCQ unit can be equipped with one or two auxiliary
electric heaters, to provide a second stage of heating. The DFB
will energize this Heating System for a Stage 2 Heating Com-
mand (heaters operate concurrently with compressor(s) in the
Stage 1 Heating cycle), for an Emergency Heating sequence
(compressors are off and only the electric heaters are ener-
gized) and also during the Defrost cycle (to eliminate a "cold
blow" condition in the space).
DEFROST
The defrost control mode is a time/temperature sequence.
There are two time components: The continuous run period
and the test/defrost cycle period. The temperature component
is provided by Defrost Thermostat 1 and 2 (DFT1 and DFT2)
mounted on the outdoor coil.
The continuous run period is a fixed time period between the
end of the last defrost cycle (or start of the current Heating cy-
cle) during which no defrost will be permitted. This period can
be set at 30, 60, 90 or 120 minutes by changing the positions of
DIP switches SW1 and SW2 (see Fig. 39 and Table 7). The de-
fault run period is 60 minutes for unit sizes 08, 09, and 12.
Fig. 39 — DIP Switch Settings — Defrost Board
Shorting the jumpers for a period of 5 to 20 seconds bypasses
the remaining continuous run period and places the unit in a
Forced Defrost mode. If the controlling DFT is closed when
this mode is initiated, the unit will complete a normal defrost
period that will terminate when the controlling DFT opens or
the 10 minute defrost cycle limit is reached. If the controlling
DFT is open when this mode is initiated, the Defrost cycle will
run for 30 seconds. Both modes end at the end of the Defrost
cycle.
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