Honeywell ST74A1053 User Manual

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

X-XX UL

69-0990-1

ST74A1053 DEFROST CONTROLLER

TEST

T3 T1

RST

HLD 24V OUT

ST74A

DEFROST RELAY CONTACT

OUTDOOR COIL THERMOSTAT

INDOOR WALL THERMOSTAT

COM

DEFROST RELAY

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

+ 2 hidden pages