Defrost; Defrost Thermostats - Carrier 38AUQ 25 Series Installation, Start-Up And Service Instructions Manual

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 cycle)
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 defrost
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 outdoor 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 circuit
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 cy-
cle, Circuit 1 will also enter a defrost cycle. Circuit 1's defrost cy-
cle will end when DFT1 opens (indicating the upper portion of the
outdoor coil is cleared of frost and ice) or the Circuit 2 defrost cy-
cle 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 terminated, the de-
frost control 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 THERMOSTATS

These are temperature switches that monitor the surface tempera-
ture of the outdoor coil circuits. These switches are mounted 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 ther-
mostat's fan selector switch is set on AUTO control. DIP Switch
SW3 on the DFB selects use of the fan off time delay feature. Set-
ting SW3 in the OPEN position turns the Fan Off Delay feature
on; setting SW3 in the CLOSED position disables 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 jump-
er 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 defrost
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 re-
maining continuous run period and places the unit in a Forced De-
frost mode. If the controlling DFT is closed when this mode is ini-
tiated, the unit will complete a normal defrost period that will ter-
minate 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