Carrier 38AUQ - Manual (Page 24)

24
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. 22); 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.