Install Instructions

69-0990—12
ST74A1053 DEFROST CONTROLLER
T2
T3 T1
TEST
RST
HLD 24V OUT
COM
DEFROST RELAY
CONTACT
INDOOR WALL
THERMOSTAT
OUTDOOR COIL
THERMOSTAT
DEFROST
RELAY
24 VAC
1
1
CONNECT JUMPER LEAD FOR DESIRED OFF TIME AS FOLLOWS
(MINUTES ON STANDARD MODEL): T1 = 50, T2 = 70, T3 = 90.
ST74A
M7826
Terminals Function
COM, 24V Provides power to the ST74A defrost system.
OUT Provides power to the defrost relay when the control relay contacts are closed and the heat pump
outdoor thermostat contacts are closed.
HLD Operates the timer. When energized by the indoor thermostat, the timer accumulates defrost Off
cycle time when the compressor is running. When the compressor is off, the timing is held at a
point in the duty cycle until the compressor comes on again.
RST Returns the timing circuit to the start of the Off cycle when the defrost cycle is interrupted by the
outdoor thermostat or by the On cycle timer.
Fig. 1. ST74A wiring hookup.
Table 3. Terminal Functions.
OPERATION
The ST74A energizes the defrost relay in a heat pump
system by relating time and temperature. Defrost takes
place when the compressor reaches a predetermined run
time and the heat pump outdoor coil temperature is below
the predetermined setpoint. The control time-cycles the
heat pump defrost relay On and Off starting with an Off
cycle on power-up. See Table 4 for a description of the
time cycles, coil thermostat and defrost functions.
See Fig. 2 for the typical operation sequence of the ST74A
time/temperature defrost control.
CHECKOUT
Power up the ST74A Defrost Controller and set the wall
thermostat to call for heat. Observe the controller through
one complete cycle to make sure it operates as intended.