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Fig. 28 — Defrost Control Board (DFB) Arrangement
REVERSING VALVE CONTROL
The DFB has an output for unit reversing valve control. Opera-
tion of the reversing valve is based on internal logic; this appli-
cation does not use an "O" or "B" signal to determine reversing
valve position. Reversing valve is energized during the Cool-
ing stage and de-energized during Heating cycles. Once ener-
gized at the start of a Cooling stage, the reversing valve will re-
main energized until the next Heating cycle demand is re-
ceived. Once de-energized at the start of a Heating cycle, the
reversing valves will remain de-energized until the next Cool-
ing stage is initiated.
COMPRESSOR CONTROL
The DFB receives inputs indicating Stage 1 or 2 Cooling and
Stage 1 Heating from the space thermostat or third-party Build-
ing Management System; it generates commands to start com-
pressor with or without reversing valve operation to produce
Stage 1 or 2 Cooling, or Stage 1 Heating.
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 the defrost thermostat (DFT1) 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. 29 and Table 16). The
default run period is 60 minutes.
Fig. 29 — DIP Switch Settings — Defrost Board
At the end of the continuous run period, the defrost control will
test for a need to defrost. DFT1 controls the start and termina-
tion of the defrost cycle. If DFT1 is still open, the defrost
test/run window is closed and the control repeats the continu-
ous run period. If DFT1 is closed, the defrost cycle is initiated.
The defrost period will end when DFT1 opens (indicating the
outdoor coil has been cleared of frost and ice) or a 10 minute
elapsed period expires, whichever comes first. At the end of
the unit defrost cycle, the unit will be returned to Heating cycle
for a full continuous run period. If the space heating load is sat-
isfied and compressor operation is terminated, the defrost con-
trol will remember where the run period was interrupted. On
restart in Heating, the defrost control will resume unit opera-
tion at the point in the run period where it was last operating.
DEFROST THERMOSTAT
The DFT is a temperature switch that monitors the surface tem-
perature of the outdoor coil circuits. The switch is mounted on
DIP
Switches
a vapor header tube above the vapor header's center check
valve. See Fig. 30 for specific unit size locations.
During the Heating mode, this tube location contains cold
evaporating fluid entering the outdoor coil's evaporator circuit;
Speed-Up
during Defrost/Cooling mode, this tube location contains con-
Jumpers
densed liquid or mixed-phase condensing refrigerant entering
the subcooler circuit. A surface temperature below 30°F during
Heating mode indicates the potential to form frost or ice on the
outdoor coil; this
condition will initiate a Defrost cycle at the next DFB test peri-
od. During the Defrost mode, this tube location will warm as
the frost and ice on the outdoor coil are removed, until the
switch opens and terminates the Defrost cycle.
The DFT switch closes on temperature drop at 30°F (–1°C) and
resets open on temperature rise at 80°F (27°C).
DEFROST SPEEDUP FUNCTIONS
The DFB permits the servicer to speed-up the defrost cycle.
There are two speed-up sequences: relative speed-up and an
immediate forced defrost. Speed-up sequences are initiated by
shorting jumper wires JMP17 and JMP18 together (see
Fig. 28); use a straight-edge screwdriver.
Shorting the jumpers for a period of 1 to 3 secs reduces the de-
frost timer periods by a factor of 0.1 sec/minute. (For example,
the 90 min run period is reduced to 9 secs.) The DFB will step
the unit through a Heating cycle and a Defrost cycle using
these reduced time periods. This mode ends after the Defrost
cycle.
Shorting the jumpers for a period of 5 to 20 secs bypasses the
remaining continuous run period and places the unit in a
Forced Defrost mode. If the DFT is closed when this mode is
initiated, the unit will complete a normal defrost period that
will terminate when the DFT opens or the 10 minute defrost
cycle limit is reached. If the DFT is open when this mode is
initiated, the Defrost cycle will run for 30 secs. Both modes
end at the end of the Defrost cycle.
29
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