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M−Defrost System HP27−4 and −6 ONLY
HP27−4 and −6 units are equipped with a demand defrost
system. The self−calibrating defrost control board includes
defrost relays, sensors (two) which monitor coil and out-
door ambient temperatures, a timed−off control, protection
circuits (two), a 3−strike lockout feature, a test mode jump-
er and a terminal strip. The demand defrost control board
initiates a defrost cycle based on temperature differential
and compressor run time. This type of system allows
greater frost accumulation on the outdoor coil and initiates
fewer defrost cycles than the time/temperature defrost
system. The defrost board is shown in figure 10.
Temperature Sensors
The demand defrost control board includes two perma-
nently attached sensors which monitor coil and outdoor
ambient temperatures. The coil temperature sensor is
equipped with a spring clip to allow proper positioning on
the outdoor coil. These sensors must not be detached
from the control board and must be replaced as part of the
control board. Do not attempt to cut or splice the tempera-
ture sensor wires. See figure 11 for ambient and coil tem-
perature sensor location.
Timed−Off Control
The control board includes a 5−minute timed−off delay
which protects the compressor from short cycling. The
5−minute delay is initiated at the end of a compressor
cycle, any time a system protection switch is reset, or if the
Y1 circuit is interrupted for more than two continuous line
cycles.
If a system protection switch opens while the Y1 OUT" cir-
cuit is energized, the timed−off control will initiate a 5−min-
ute delay when the pressure switch closes.
The timed−off control run times can be bypassed by
shorting the TEST" pins.
Protection Circuits
The control board includes two protection circuits.
The unit high pressure switch is factory−wired to the HI −
PS terminals. The circuit through Y1" (input and Y1
OUT") is completed through the high pressure switch.
When the high pressure switch opens, the control board
de−energizes the compressor and the 3−strike lockout
counter registers one strike.
If, for any reason, the high
pressure switch is removed, a jumper must be applied
across the HI − PS terminals to complete the circuit.
The second protection circuit is not used in this application
A jumper must be applied across the LO − PS terminals
to ensure proper control board operation.
Inputs (24V Terminal Strip Connections)
Y1
Thermostat input, controls unit operation
O
Thermostat input, reversing valve
R
24VAC power
C
24VAC common
W1
24VAC input/output from indoor thermostat to indoor unit
L
Service light thermostat
T
Ambient sensor (connection for Room Thermostat with
ambient compensation)
(1/4" Quick Connect Terminal Factory Connections)
HI−PS
Pressure switch / system fault sensor
LO−PS
Pressure switch / system fault sensor
Y1 Out
Compressor contactor
O Out
Reversing valve
FAN
Condenser fan
W/L/C
Service light thermostat
T/T
Ambient sensor
Defrost Control Board
FIGURE 10
Y1
O
W1
R
CL
T
W1
C
L
T
FAN
FAN
CC
Y
HI−PS
LO−PS
RV
C
O−OUT
Y1−OUT
LED1
LED2
AMB
COIL
PS2
AUX
PS1
DIS
IND
CUR
70
80
90
100
TEST
PRESS
