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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 thermostats (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. 26 and Table 13). The
default run period is 60 minutes.
Fig. 26 — DIP Switch Settings — Defrost Board
At the end of the continuous run period, the defrost control will
test for a need to defrost. DFT2 (located on the bottom circuit
of the outdoor coil) controls the start and termination of the de-
frost cycle. If DFT2 is still open, the defrost test/run window is
closed and the control repeats the continuous run period. If
DFT2 is closed, the defrost cycle is initiated in Circuit 2. The
defrost period will end when DFT2 opens (indicating the out-
door coil has been cleared of frost and ice) or a 10 minute
elapsed period expires, whichever comes first.
Circuit 1's defrost thermostat DFT1 (located on the upper cir-
cuit of the outdoor coil) cannot initiate a unit defrost cycle;
only DFT2 may do this. But once Circuit 2 is in defrost, the
DFB will monitor the status of DFT1. If DFT1 closes during a
Circuit 2 defrost cycle, Circuit 1 will also enter a defrost cycle.
Circuit 1's defrost cycle will end when DFT1 opens (indicating
the upper portion of the outdoor coil is cleared of frost and ice)
or the Circuit 2 defrost cycle is terminated.
1
2
1
•
0
•
30 minutes
Table 13 — Dip Switch Position
1
2
1
•
1
0
•
0
60 minutes
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 satisfied and compressor operation is terminat-
ed, the defrost control will remember where the run period was
interrupted. On restart in Heating, the defrost control will re-
sume unit operation at the point in the run period where it was
last operating.
DEFROST THERMOSTATS
These are temperature switches that monitor the surface tem-
perature of the outdoor coil circuits. These switches are mount-
ed on the liquid tube exiting the outdoor coil heating circuits.
These switches close on temperature drop at 30°F (–1°C) and
reset open on temperature rise at 80°F (27°C).
INDOOR FAN OFF DELAY
The DFB can provide a 30 sec delay on Indoor Fan Off if the
thermostat's fan selector switch is set on AUTO control. DIP
Switch SW3 on the DFB selects use of the fan off time delay
feature. Setting SW3 in the OPEN position turns the Fan Off
Delay feature on; setting SW3 in the CLOSED position dis-
ables this feature. The delay period begins when Y1 demand or
W1 demand by the space thermostat is removed.
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. 24); 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 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 secs. Both modes end at the end of the Defrost cycle.
SWITCH NO.
1
2
1
1
•
•
0
90 minutes
27
1
2
3
•
•
1
0
•
Fan
120 minutes
Delay
On
Off
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