Written on: February 17, 2025 by Timmie McElwain
This article will be the first of two parts covering the ST9120U-1011 Universal Electronic Fan Timers from Honeywell. Keep in mind that there are other companies that produce similar controls, leading to many similarities among the controls. This allows for some interchangeability among manufacturers, such as White Rodgers.
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.
Figure 4 demonstrates the replacement of the ST9120 basic model with the ST9120U. The system shown is with a Honeywell S8600 Series intermittent pilot ignition control. Note that the J3 six-wire plug-in is not used with this setup. On the J1 pin connector, the upper right-hand pin 4 is not used, nor is the left-hand middle pin 2, as they have 24-volt potential, which is not needed in this application.
Figure 5 illustrates wiring hookup with the ST9120U replacing a ST9120 EFT with a SV9500/SV9501 SmartValve. Note that the J1 six-pin connector upper right-hand pin 1 provides the uninterrupted 24 VAC (HOT) to the SmartValve, and the left-hand middle pin connector 2 is the 24-volt common. The lower left-hand pin 3 is the interrupted 24 VAC through the pressure switch to the SmartValve. The EFT output signal (DATA) for starting the “Fan-On” sequence is fed from the right-hand middle pin connector 5 of the J1 connector. Note once again that the J3 connector is not used.
We will continue discussing this control system in the March/April 2025 issue of Indoor Comfort Marketing. ICM
