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 60 second 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 de-
lay feature. Setting 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 jumper wires JMP17 and JMP18 together (see
Fig. 27); use a flat-blade screwdriver.
Shorting the jumpers for a period of 1 to 3 seconds reduces the
defrost timer periods by a factor of 0.1 sec/minute. (For exam-
ple, the 90 minute run period is reduced to 9 seconds) The DFB
will step the unit through a Heating cycle and a Defrost cycle
using these reduced time periods. This mode ends after the De-
frost 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.
ELECTRIC HEATERS
50KCQ units can be equipped with field-installed accessory
electric heaters. The heaters are modular in design, with heater
frames holding open coil resistance wires strung through ce-
ramic insulators, line-break limit switches and a control con-
tactor. One or two heater modules can be used in a unit.
Heater modules are installed in the compartment below the in-
door (supply) fan outlet. Access is through the indoor access
panel. Heater modules slide into the compartment on tracks
along the bottom of the heater opening. See Fig. 30-32.
Not all available heater modules can be used in every unit. Use
only those heater modules that are UL listed for use in a specif-
ic size unit. Refer to the label on the unit cabinet regarding ap-
proved heaters.
Unit heaters are marked with Heater Model Numbers. Howev-
er, heaters are ordered by and shipped in cartons marked with a
corresponding heater Sales Package part number. See Table 10
for correlation between heater Model Number and Sales Pack-
age part number.
NOTE: The value in position 9 of the part number differs between
the sales package part number (value is 1) and a bare heater model
number (value is 0).
Fig. 30 — Typical Access Panel Location
Fig. 31 — Typical Component Location
Fig. 32 — Typical Module Installation
Table 9 — Dip Switch Position
SWITCH NO.
1 2 12 12 1 2 3
11·1·1··1On
0· ·0· 0 ·0 0 Off
90 minutes 60 minutes 30 minutes 120 minutes Fan Delay
DISCONNECT MOUNTING
LOCATION
UNIT BLOCK-OFF
PANEL
OUTDOOR
ACCESS PANEL
INDOOR
ACCESS
PANEL
DISCONNECT
MOUNTING
LOCATION
EMT OR RIGID CONDUIT
(FIELD-SUPPLIED)
SINGLE
POINT BOX
CENTER
POST
HEATER
COVERS
HEATER
MOUNTING
BRACKET
HEATER
MODULE
(LOCATION 2)
HEATER
MODULE
(LOCATION 1)
SINGLE POINT
BOX
MOUNTING
SCREW
BRACKET AND
CONDUIT
DRIP BOOT
MAIN
CONTROL
BOX
CONTROL WIRE TERMINAL BLOCK
MANUAL RESET
LIMIT SWITCH
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