Written on: December 16, 2024 by Timmie McElwain
Part 1 from the May/Jun 2024 Indoor Comfort
Part 2 from the Jul/Aug 2024 Indoor Comfort
Part 3 from the Sep/Oct 2024 Indoor Comfort
As we continue to look at modern systems found on equipment and the controls associated with them, we start to see a shift toward Universal Replacement Controls. This is a first step toward simplifying technician truck stock requirements, where one control will potentially replace several previously used on Forced Warm Air Systems.
Today’s modern electronic equipment incorporates the principles of Electronic Flame Detection. There are different applications determined by the equipment manufacturers’ design applications. Each of these systems has its own distinct set of advantages and problems. The ST9120U-1003 is the forerunner to the ST9120U-1011.
Honeywell is not alone in this trend of moving toward Universal Replacement Controls. White Rodgers has also taken this direction with their controls.
As Industry moves toward the modulation condensing class of equipment (often abbreviated to mod-cons), this is the last step in replacement for some of the older systems still out it in the field today. Those systems do not require complete replacement; often just a control replacement will suffice.
ST9120U Universal Electronic Fan Timers
The first step toward universal replacement Electronic Fan Timers was the ST9120U-1003 (there is also a newer version, the ST9120U-1011). Carrying this product means a reduction in truck stock and, once the technician gets accustomed to using it, a reduction in replacement time. The ST9120U Universal Electronic Fan Timers integrate controls 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. The basic purposes of the ST9120U 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 electronic fan timers also monitor limit switch strings and energize separate ignition control systems through SPST pressure switches.
The ST9101 to ST9120U wiring conversion is illustrated in Table 1. The ST9120U can replace any ST9101 or ST9120 listed in Table 2, which also gives the switch settings for the unit being replaced to ensure compatibility and lists the particular original equipment manufacturer (OEM). The electronic fan timers feature a field-adjustable heat “Fan-On” delay, a field-adjustable heat “Fan-Off” delay, a field-adjustable cool “Fan-On” delay and a fixed cool “Fan-Off” delay.
Electronic air cleaner (EAC) and humidifier (HUM) convenience terminal connections are provided as an option. Continuous low speed indoor air circulation is also provided.
See Figure 2 to adjust timings. Figure 3 shows the wiring adapter to be used to connect the ST9120U to the existing harness on the board being replaced (there will be further instructions on this). P1 is the nine-pin connector, which will have the connector off the board being replaced, plugged into it.
Figure 4 shows the harness connected to the ST9120U board; to save time, the technician can pre-wire this. The portion of the harness at the top of Figure 4 is what connects together with the harness being removed from the old board.
ST9120 to ST9120U Wiring Conversion Instructions
1. Turn off power to appliance. Carefully remove each wire and connect directly to the corresponding location on the new ST9120U control board. Be careful to directly connect to the new terminal with the same-labeled identity or label each wire prior to removing from the original board. If original thermostat connection was a quick connect, cut off quick connect, strip wire and insert into terminal block.
2. Identify the model number of the board being replaced and set the dip switches based on Table 2.
ST9101 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. Use Table 1 to determine the proper ST9120U terminal for each corresponding ST9101 connection. Be careful to directly connect to the new terminal or label each wire prior to removal from original board.
2. Unplug the nine-pin connector and plug existing wiring harness directly into the nine-pin connector found on the ST9101 wiring adapter.
3. Plug the six-pin connector found on the ST9101 wiring adapter directly to the open six-pin connector on the new ST9120U control board.
4. Take the two remaining white wires on the ST9101 wiring adapter and connect to “Neutral” quick connect terminals three and four on the ST9120U.
5. Take the black wire on the ST9101 wiring adapter and connect to the S1 quick connect terminal on the ST9120U.
6. Take the blue wire on the ST9101 wiring adapter and connect to the D1 quick connect terminal on the ST9120U
7. Identify the model number of the board being replaced and set the dip switches based on Table 2.
Table 3 gives the wire adapter connections and the wire adapter schematic. This can be compared with Figure 5, the wiring diagram showing the connections from the six-pin connector and the nine-pin connector along with the color of the wires.
Figure 6 is a breakdown of the ST9120U; on the left hand side is the twinning connections “Z1” and “Z2”. There is also a five-amp fuse located next to six-pin connector CN1. Relay K1 is for the Circulating Fan, K2 is for Heat/Cool and K3 is for the Combustion Air Blower.
Setting Adjustable Heat Fan DIP Switches
Set the heat “Fan-On” delay and cool “Fan-On”delay DIP switches as shown in Figure 2. The “Heat-On”delay time switch #4 (30 or 60 seconds) starts when the main gas valve is energized at the start of a thermostat call for heat; the “Heat-Off”delay time switches #3 and #4 (60, 100, 140 and 180 seconds) starts when the main gas valve is de-energized at the end of a thermostat call for heat. The cool “Fan-On”delay time switch #1 (four or 30 seconds) begins when the compressor is energized at the start of a thermostat call for cool.
Table 4 is the operating sequence for the ST9120U. ICM
Timmie M. McElwain is President of Gas Appliance Service, which provides training for those servicing gas combustion equipment. He is a certified instructor and test proctor for the Propane Gas Association in their CETP program.
