Written on: June 3, 2024 by Timmie McElwain
The majority of new equipment today involves some form of electronics, as well as the principal of electronic flame detection and the use of flame rectification as a safety and flame proving system. This is the case with both Forced Warm Air systems and Forced Hot Water systems.
There are, however, different ways it is applied—from intermittent pilot application to direct spark ignition and including hot surface ignition. Each of these systems has its own distinct advantages as well as some disadvantages. It can be easy to jump to conclusions and just keep changing parts until perhaps you fix the problem, but that can be time-consuming and expensive.
The controls that are used on equipment today are unique to a particular facet of operation. Understanding their operation and troubleshooting them is covered in this article on the Honeywell ST 9141A control, which is used on Gas Forced Warm Air systems.
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Honeywell ST 9141A control
For use on gas forced warm air systems
The ST9141A-1002 Electronic Fan Timer integrates control of all combustion blower and circulating fan operations in a gas warm air furnace. This control is the central wiring point for most of the electrical components in the furnace. The basic purposes of the ST9141A-1002 are to monitor the thermostat for heat, cool and fan demands, and run the induced draft blower motor up to a two-speed circulating fan as required. The ST9141A-1002 monitors an SPDT pressure switch, burner limit and primary limit.
A Honeywell SV9500 Hot Surface Pilot Ignition System Control manages the burner, which is energized through the pressure switch. A light-emitting diode (LED) indicates system status.
In addition, this model includes electronic air cleaner and humidifier convenience terminal connections.
Note: Use this control to replace an identical ST9141A-1002 Electronic Fan Timer only. This control is not for use in general applications.
The difference between the ST9120 series and the ST9141 is that the LED status indicator was added to the left of the two dipswitches.
ST9141 is similar to ST9120:
• Heat fan
Ø ON delay 30 seconds
Ø OFF delay adjustable (60, 100, 140, 180 seconds)
• Cool fan
Ø ON delay six seconds
Ø OFF delay 60 seconds
Light emitting diode
• Pulsing dim/bright—normal operation
• Off with control powered—defective control
• No pulsing dim/bright when powered—defective control
• Flashing
Ø One flash: burner limit open
Ø Two flashes: primary limit open
Ø Three flashes: pressure switch did not recycle during off cycle
Ø Four flashes: control in lockout due to overheating (limit open for more than 150 seconds on each of three consecutive cycles)
Figure 2 shows the two dipswitches for Fan Off time setting. The Fan On time is fixed at 30 seconds. Figure 3 expands on the LED status light diagnostics. This along with the operating sequence and the diagrams to follow will make troubleshooting much easier.
Figures 4, 5 and 6 show both the internal and wiring connections for the ST9141. Figure 6 shows an expanded view of the nine pin CN1 connector. That connector is also identified in Figure 4 and Figure 5. ICM
