Written on: April 7, 2025 by Timmie McElwain
This is the second part of a two-part article; Part 1 appeared in the January/February 2025 issue of Indoor Comfort Marketing.
The ST9120U-1011 will replace all the other Electronic Fan Timers produced by Honeywell (including the ST9120, ST9141, ST9101 and ST9160 covered in previous issues). This is very helpful in reducing the amount of necessary truck stock.
The ST9120U Universal Electronic Fan Timers integrate control of all combustion blower and circulating fan operations in a gas warm air appliance. This control is the central wiring point for most of the electrical components in the furnace. Its basic purposes are to monitor the thermostat for heat, cool and fan demands, run the induced draft blower motor and run a circulating fan (up to two speeds) as required. The ST9120U also monitors limit switch strings and energizes separate ignition control systems through pressure switches. It features a field-adjustable heat “Fan-On” delay, a field adjustable heat “Fan-Off” delay, a field adjustable cool “Fan-On” delay and a field-adjustable cool “Fan-Off” delay. This universal control can replace any Honeywell ST9101, ST9120, ST9141 or ST9160.
It now has a replaceable fuse and DIP switch with selectable heat and cool delay for added flexibility. The post-purge timing is five seconds. Electronic air cleaner (EAC) and humidifier (HUM) convenience terminal connections and continuous low speed indoor air circulation are provided.
Adjustable DIP Switch
The ST9120U control provides a wide variety of configurable Heat-On, Heat-Off, Cool-On and Cool-Off delays. Set the delay DIP switches as shown in Figure 1. The Heat-On delay time starts when the main gas valve is energized at the start of a thermostat call for heat; the Heat-Off delay time starts when the main gas valve is deenergized at the end of a thermostat call for heat. The fan Cool-On delay time starts when the compressor is energized at the start of a thermostat call for cool. The fan Cool-Off delay time starts when the compressor is deenergized at the end of thermostat call for cool.
Twinning
Twinning mode is used when two ST9120U controls operate in parallel. In this mode, blower outputs (heat, cool and continuous speed) are activated or deactivated at both controls at the same time. Both ST9120U are synchronized through a communication bridge utilizing TWIN quick connect terminals, communication hardware and special software package.
To get into twinning mode, both ST9120U must share common 24 VAC ground, both must be supplied off the same 24 VAC transformer or two identically-phased transformers, and the TWIN terminals must be interconnected together. One or both ST9120U must be connected to the thermostat.
If two 24 VAC transformers are used (one for each ST9120U), then the transformers must be phased in (24 VAC HOT legs connected to X terminals at each ST9120U unit must be in the same phase relative to the 24 VAC common ground). If the transformers are phased out, the twinning mode will not be achieved.
It is permissible to interconnect just two ST9120U to operate in the twinning mode. The ST9120U Twinning mode is not backwards compatible with legacy Electronic Fan Timers.
1. Take the two remaining white wires on the ST9101 wiring adapter and connect to “Neutral” quick connect terminals on the ST9120U.
2. Take the black wire on the ST9101 wiring adapter and connect to the L1 quick connect terminal on the ST9120U.
3. Take the blue wire on the ST9101 wiring adapter and connect to the IND quick connect terminal on the ST9120U.
4. Identify the model number of the board being replaced and set the dip switches.
5. Check proper connection.
ST9141 to ST9120U Wiring Conversion Instructions
1. Turn off power to appliance and carefully remove each wire and connect directly to the new ST9120U control board before removing the existing control. Determine the proper ST9120U terminal for each corresponding ST9141 connection. Be careful to directly connect the new terminal or label each wire prior to removal from original board.
2. Unplug the 9-pin connector and plug existing wiring harness directly into the 9-pin connector found on the ST9141 wiring adapter 50034962. See Figure 2 and Table 5 for further wire harness adapter description.
3. Plug the 6-pin connector found on the ST9141 wiring adapter directly to the open 6-pin connector on the new ST9120U control board.
4. Take the gray wire on the ST9141 wiring adapter and connect to the LIM quick connect terminal on the ST9120U.
5. Take the brown wire on the ST9141 wiring adapter and connect to the PRSW quick connect terminal on the ST9120U.
6. Identify the model number of the board being replaced and set the dip switches. See Table 3.
7. Check proper connection.
ST9160 to ST9120U Wiring Conversion Instructions
1. Turn off power to appliance and carefully remove each wire and connect directly to the new ST9120U control board before removing the existing control. Determine the proper ST9120U terminal for each corresponding ST9160 connection. Use Table 1 to determine the proper ST9120U terminal for each corresponding ST9141 connection. Be careful to directly connect the new terminal or label each wire prior to removal from original board.
2. Unplug the 6-pin connector and plug it into the J3 connector found on ST9120U.
3. Plug the jumper plug into the J1 connector at ST9120U.
4. Identify the model number of the board being replaced and set the dip switches.
5. Check proper connection.
A comparison is made to each board the ST9120U replaces in Table 1 as to wiring connections. Most of the terminals are the same. Refer to the section above for twinning connections. Table 2 provides the Led diagnostic troubleshooting codes. It is important that if twinning is not correct you will get a 6 flash code. Table 3 lists all the boards the ST9120U replaces along with the OEM, and dip switch settings. ICM
