Install Instructions

69-0990—13
ST74A1053 DEFROST CONTROLLER
Table 4. ST74A Operation.
Process Function
Power Up
• Wall thermostat contacts closed With 24 Vac applied, the Off cycle timing starts.
Time Cycle
Off Cycle
• Wall thermostat contacts closed The defrost control time-cycles when the wall thermostat contacts are
closed. The Off cycle time is selected by moving the jumper to the desired
Off cycle time.
• Wall thermostat contacts open When the wall thermostat is satisfied, the compressor stops running and
the Off cycle timing is held at this point in the duty cycle until the
compressor comes on again. Timing is then resumed to complete the total
Off cycle time setting.
On Cycle
• Wall thermostat contacts closed
• Coil thermostat contacts closed
Upon completion of the Off cycle time, the defrost relay is pulled-in for ten
minutes unless terminated by the opening of the coil thermostat contacts.
• Wall thermostat contacts open
• Coil thermostat contacts closed
lf the thermostat contacts open during the defrost cycle, the defrost relay
remains in.
On time is held at this point in the duty cycle until the wall thermostat
contacts close again to resume the defrost cycle or the coil thermostat
contacts open and terminate the defrost cycle.
• Wall thermostat contacts closed
• Coil thermostat contacts open
If the coil thermostat contacts open during the defrost cycle, defrost is
terminated and the timer is immediately reset to zero. If the RST (reset)
terminal is not used, the On cycle timer times out. The defrost relay
remains de-energized unless the coil thermostat remakes during the
remainder of the On cycle.
Coil Thermostat
• Contacts open The system does not go into defrost when the coil thermostat contacts are
open.
• Contacts closed The coil thermostat contacts close when the heat pump outdoor coil
temperature falls below the Original Equipment Manufacturer (OEM)
selected temperature.
Defrost Test
• Coil thermostat contacts closed
• Test pins shorted
Shorting the TEST pin terminals speeds up the control by a factor of 256;
for example, an Off time setting of 90 minutes is reduced to a time of
21 seconds, making it easy to check out device operation.