69-0990 - ST74A1053 |Defrost Controller
Contents
1. the four mounting eyelets Use No 6 or No 8 sheetmetal screws in the four mounting eyelets The control can be mounted in any position 45 degrees to vertical Wiring Disconnect power supply to prevent electrical shock or equipment damage All wiring must comply with local codes and ordinances Refer to Fig 1 for wiring hookup The Off cycle time is determined by a selectable jumper that allows a choice of 50 70 or 90 minutes when connected to T1 T2 or T3 respectively Select the timing that is closest to the timing of your original control If the timing does not exactly match choose a longer timing rather than a shorter one See Table 3 for a description of the terminal functions 69 0990 1 ST74A1053 DEFROST CONTROLLER DEFROST RELAY CONTACT INDOOR WALL THERMOSTAT OUTDOOR COIL THERMOSTAT DEFROST RELAY CONNECT JUMPER LEAD FOR DESIRED OFF TIME AS FOLLOWS MINUTES ON STANDARD MODEL T1 50 T2 70 T3 90 24 VAC Fig 1 ST74A wiring hookup Table 3 Terminal Functions COM 24V Provides power to the ST74A defrost system Provides power to the defrost relay when the control relay contacts are closed and the heat pump outdoor thermostat contacts are closed 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 th2. Honeywell TRADELINE APPLICATION The ST74A is a low voltage time temperature electronic defrost controller for heat pumps This device provides timed low voltage relay switching to control an external heat pump defrost relay A complete list of the specific Honeywell and other controllers that the ST74A Defrost Controller is designed to replace is provided in Tables 1 and 2 Table 1 OEM Replacement Part Numbers Manufacturer Service Part Number C6431001 Table 2 Controller Manufacturer Replacement Part Numbers Honeywell ST74A 1004 ST74A 1020 ST74A 1038 U S Registered Trademark Copyright 1996 Honeywell Inc All Rights Reserved ST74A1053 Defrost Controller INSTALLATION INSTRUCTIONS INSTALLATION When Installing this Product 1 Read these instructions carefully Failure to follow them could damage the product or cause a hazard ous condition 2 Check the ratings given in the instructions and on the product to make sure the product is suitable for your application 3 Installer must be a trained experienced service technician 4 After installation is complete check out product operation as provided in these instructions CAUTION Disconnect power supply before beginning installation to prevent electrical shock or equipment damage Mountin Mount the ST74A in the wiring compartment of the outside section of the heat pump Use the ST74A as a template to mark the position of
3. e duty cycle until the compressor comes on again O H 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 See Fig 2 for the typical operation sequence of the ST74A time temperature defrost control 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 69 0990 1 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 ST74A1053 DEFROST CONTROLLER Table 4 ST74A Operation Process Smolin SSSS S 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 resum
4. ed to complete the total Off cycle time setting On ae Coi thermostat contacts closed Upon completion of the Off cycle time the defrost relay is pulled in for ten il thermostat contacts closed minutes unless terminated by the opening of the coil thermostat contacts thermostat contacts open If the thermostat contacts open during the defrost cycle the defrost relay il thermostat contacts closed 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 thermostat contacts closed If the coil thermostat contacts open during the defrost cycle defrost is thermostat contacts open 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 Shorting the TEST pin terminals speeds up the control by a factor of 256 Test pins shorted for example an Off time setting of 90 minu
5. tes is reduced to a time of 21 seconds making it easy to check out device operation 3 69 0990 1 T74A1053 DEFROST CONTROLLER CONTACTS CLOSED STARTS OFF CYCLE TIMER OFF CYCLE TIMING MODE SELECTABLE TIME WALL OFF CYCLE THERMOSTAT HOLDS OFF CYCLE COMPLETE CONTACTS OFF CYCLE TIMER HOLD MODE p COIL THERMOSTAT CONTACTS CLOSED WALL THERMOSTAT CONTACTS CLOSED ACCUMULATES ON CYCLE TIME ON CYCLE HOLD MODE HOLDS ON CYCLE TIMER ON CYCLE DEFROST TIMING MODE INITIATED 10 MIN PULLS IN DEFROST RELAY COIL THERMOSTAT CONTACTS OPEN ON CYCLE COMPLETE DROPS OUT DEFROST RELAY M7827 DEFROST TERMINATED Fig 2 Typical operation sequence of ST74A time temperature defrost control Honeywell Home and Building Control Home and Building Control Honeywell Inc Honeywell Limited 1985 Douglas Drive North 155 Gordon Baker Road Golden Valley MN 55422 North York Ontario M2H 2C9 69 0990 1 J S Rev 5 96 Printed in Mexico
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