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ST74A1053
Defrost Controller
INSTALLATION INSTRUCTIONS
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
Amana C64301-1
Artesian 10321-00
Arcoaire 32312-00
Carrier 621
Coleman 3030A374
Goodman B12260-06
Heil Quaker HQ1052757
ICP 1052757
Intertherm 6208800
Lennox 33G9501
Rheem 47-21776-01
SnyderGeneral 1395-329
Table 2. Controller Manufacturer
Replacement Part Numbers.
Manufacturer Part Number
Honeywell ST74A 1004
White-Rodgers/Steveco 90-621
ICM 300C
Therm-o-Disc 26E-10
Robertshaw TD-10
C6431001
ST74A 1020
ST74A 1038
INST ALLA TION
When Installing this Product…
1. Read these instructions carefully. Failure to follow
them could damage the product or cause a hazardous 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.
CA UTION
Disconnect power supply before beginning
installation to prevent electrical shock or equipment
damage.
Mounting
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 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,
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.
+ 45 degrees
®U.S. Registered Trademark
Copyright © 1996 Honeywell Inc. • All Rights Reserved
X-XX UL
69-0990-1
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ST74A1053 DEFROST CONTROLLER
TEST
T3 T1
RST
HLD 24V OUT
T2
ST74A
DEFROST RELAY
CONTACT
1
OUTDOOR COIL
THERMOSTAT
INDOOR WALL
THERMOSTAT
COM
DEFROST
RELAY
1
CONNECT JUMPER LEAD FOR DESIRED OFF TIME AS FOLLOWS
(MINUTES ON STANDARD MODEL): T1 = 50, T2 = 70, T3 = 90.
24 VAC
M7826
Fig. 1. ST74A wiring hookup.
Table 3. Terminal Functions.
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
HLD Operates the timer. When energized by the indoor thermostat, the timer accumulates defrost Off
RST Returns the timing circuit to the start of the Off cycle when the defrost cycle is interrupted by the
OPERA TION
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.
outdoor thermostat contacts are closed.
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.
outdoor thermostat or by the On cycle timer.
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.
69-0990—12
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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
• Wall thermostat contacts open When the wall thermostat is satisfied, the compressor stops running and
On Cycle
• Wall thermostat contacts closed
• Coil thermostat contacts closed
• Wall thermostat contacts open
• Coil thermostat contacts closed
• Wall thermostat contacts closed
• Coil thermostat contacts open
Coil Thermostat
• Contacts open The system does not go into defrost when the coil thermostat contacts are
• Contacts closed The coil thermostat contacts close when the heat pump outdoor coil
Defrost Test
• Coil thermostat contacts closed
• Test pins shorted
closed. The Off cycle time is selected by moving the jumper to the desired
Off cycle time.
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.
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.
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.
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.
open.
temperature falls below the Original Equipment Manufacturer (OEM)
selected temperature.
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.
69-0990—13
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ST74A1053 DEFROST CONTROLLER
POWER
UP
SETS ALL TIMERS TO ZERO
OFF-CYCLE
TIMING MODE
(SELECTABLE
TIME)
ON-CYCLE
TIMING MODE
(10 MIN)
STARTS OFF-CYCLE TIMER
ACCUMULATES OFF-CYCLE TIME
STARTS ON-CYCLE TIMER
DEFROST
INITIATED
DEFROST
TERMINATED
WALL
THERMOSTAT
CONTACTS
CLOSED
YES
OFF-CYCLE
COMPLETE
YES
COIL
THERMOSTAT
CONTACTS
CLOSED
YES
PULLS IN
DEFROST
RELAY
COIL
THERMOSTAT
CONTACTS
OPEN
YES
DROPS OUT
DEFROST
RELAY
NO
NO
THERMOSTAT
ACCUMULATES
ON-CYCLE TIME
NO
WALL
CONTACTS
OPEN
THERMOSTAT
CONTACTS
CLOSED
ON-CYCLE
COMPLETE
NO
NO
NO
WALL
YES
YES
YES
NO
HOLDS
OFF-CYCLE TIMER
HOLDS
ON-CYCLE TIMER
OFF-CYCLE
HOLD MODE
ON-CYCLE
HOLD MODE
M7827
Fig. 2. Typical operation sequence of ST74A time/temperature defrost control.
Home and Building Control
Honeywell Inc.
1985 Douglas Drive North
Golden Valley, MN 55422
Home and Building Control
Honeywell Limited
155 Gordon Baker Road
North York, Ontario
M2H 2C9
69-0990—14
69-0990—1 J.S. Rev. 5-96 Printed in Mexico
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