Written on: August 3, 2025 by Timmie McElwain
As we begin another series of controls, this time looking at boiler controls, we will start with the older controls and then progress to the latest controls available.
The S9301A Integrated Boiler Control provides control of a high efficiency gas boiler with a combustion air blower and a combination hot surface igniter/flame sensor. The S9301A provides:
The S9301 is intended for residential boilers only. It is not intended for use with a vent damper. Other applications require Honeywell Residential Design Engineering review and written approval. Contact your Honeywell Sales Representative for assistance.
INPUT VOLTAGE:
Line: 120 VAC, 50/60 Hz.
Low: 24 VAC
CURRENT DRAW:
Line: 12 A max.
Low (Thermostat Anticipator Setting): 0.2 A plus gas control current.
OUTPUT RELAY LOAD RATINGS:
Hot surface igniter: 120 VAC, 5 A resistive.
Combustion blower motor: 120 VAC, 1.75 amps full load, and 3.0 amps locked rotor.
Circulator pump motor: 120 VAC, 2.7 amps full load, and 24 amps locked rotor.
Gas control: 24 VAC, 1 A.
TIMINGS:
Pre-purge: 15 seconds
Warm-up: 20 seconds (Trial 1); 30 seconds (Trials 2 and 3).
Trial for Ignition: Six seconds. First two seconds is Ignition Activation Period with igniter powered. Igniter switches to flame sensor for approximately the last second of trial period.
Between Trials Purge: 30 seconds.
Post-purge: 30 seconds.
Flame Failure Response: 0.8 seconds maximum.
GAS CONTROL: Any 24 VAC redundant direct ignition gas control rated at 1A or less. Honeywell VR8205 recommended.
HOT SURFACE IGNITER: Norton Model 271 or equivalent.
NOTE: Igniter must meet the following minimum specifications throughout its life:
The common ground required between the S9301A and the burner is supplied through the plug connected to the CN6 receptacle on the board. A good ground connection is essential for proper operation.
Make sure the transformer has adequate volt-amperes (VA). To determine the total VA requirement, add the maximum current draw in amps of all devices in the control circuit, including the boiler control and the gas control, and multiply times 24. The result is the minimum transformer VA rating. Use a class II transformer if replacement is required.
S9301A Integrated Boiler Control: Functional Description
General
The S9301A is a product designed to integrate and control all of the electrical components of a gas-fired boiler. For convenience, the control module has been designed as a central wiring point for all of the electrical components in the boiler.
The basic purpose of the S9301A is to monitor the thermostat and aquastat for heat demand, properly start and stop gas ignition and run the circulator and combustion blower motors. An air flow-proving switch is connected to the control to integrate surveillance of combustion air flow, flue and inlet blockage. Temperature limit switches are connected to the control to disable power to the gas valve.
The ignition means is provided by a hot surface ignition element. This element is also used as the flame detect probe, providing a single rod system. Three LEDs are used to indicate the operation of specific interconnected components, while a fourth shows proper power and microprocessor operation or “health.” Flashing of these indicators is used to aid in troubleshooting the boiler when an improper condition is detected.
Electrical connections have been segmented into seven connectors for ease of service. Separate connectors exist for system power, transformer primary, transformer secondary, igniter element, blower and circulator. The seventh connector ties in the thermostat, aquastat, pressure switch, burner ground and gas valve.
Safety Features
Starting with the control housing, the electronics are completely protected from general handling around all sides. The wiring connector system meets the latest ANSI requirements for mis-wiring prevention, through the use of non-interchangeable plugs and sockets. These are also color coded for assembly ease.
Special attention has been paid to the safety aspect of the internal control design.
Among these, two series connected (redundant) relays are used to control the gas valve. In addition, a separate gas valve relay power supply monitors the microprocessor for the presence of a specific “watch dog” signal to determine that the processor is still functioning correctly. Lacking such a signal, indicating a “crashed” or otherwise misbehaving microprocessor, the power supply will drop out, forcing the gas valve relays to de-energize.
Additionally, the gas valve is powered through a traditional 24 VAC series circuit consisting of the gas valve relays, the pressure switch, aquastat and auxiliary limits and finally, the thermostat. Thus, should the thermostat; aquastat or auxiliary limits; or pressure switch open a circuit, the gas valve will de-energize without intervention from the control. Numerous other internal design features are employed to assure a fail-safe operation, including a shut-down if the control should become wet.
The S9301A Integrated Boiler Control is fully design certified by the American Gas Association (AGA) and the Canadian Gas Association (CGA) as an Automatic Gas Ignition Device.
Reliability Features
Numerous factors in the control hardware and software design combine to build in long-term reliability. Component stress levels for voltages, current and temperature have been generously derated then analyzed through a design reliability prediction procedure.
Testing of the product design confirms operation through the specified range of voltages, temperatures, humidity and cycles of operation. 100% functional testing of production units and burn-in, plus a full conformal coating process, adds repeatability to the outgoing product.
Electrical transient and static protection built into the hardware combine with a two-tiered recovery method in the software functions. This approach helps assure a safe restart and resumption of operation, rather than the possibility of a nuisance lockout.
Sequence of Operation
1. Power up/standby.
2. Upon applying power, the control will reset, going through a two-second warm-up, and enter its thermostat scan state.
3. Initiate call for heat.
4. Once a valid call for heat is received from the thermostat, the circulator will be turned on. If the aquastat and any auxiliary limits are also closed, indicating that boiler water is cool enough, the control will go through a reset and enter a 15 second pre-purge, during which the combustion blower runs.
5. The red purge LED will be on during this period. The air flow pressure switch contacts are checked prior to startup of the blower, and thereafter monitor the combustion flow.
6. Next, the igniter and red igniter LED will be turned on and the igniter is allowed to heat to ignition temperature for a 20 second period. The purge LED is turned off during this period, although the combustion blower continues to run.
7. Gas ignition.
8. When the igniter warm-up period is completed, the gas valve is powered and a six-second Trial for Ignition (TFI) begins. The first two seconds of the TFI is the Ignition Activation Period (IAP).
9. During IAP, the igniter remains powered. At the end of the two-second IAP, the igniter and the igniter LED are turned off, while the gas valve remains powered. The igniter is now used as a flame rod to detect the main burner flame.
10. If the main burner did not light during the IAP, it will now light during the remainder of the TFI. While the burner may light at any time during the TFI, the igniter is enabled as a flame sensor only for the last second or so of the TFI.
11. Normal run.
12. If a flame is detected during the TFI, the circulator, inducer and the main burner operate normally to provide heat to the conditioned space.
13. The thermostat input, pressure switch, limit/aquastat circuit and the main burner flame are continually monitored to assure that the system is operating properly. When the call for heat ends, the main valve and circulator are shut off immediately. The inducer continues to run for a 30 second post-purge. The purge LED will turn on during this period.
Figure 3 is a ladder diagram that shows the relationship of the relays to each component, for example, on the call for heat relays, 1K and 2K are energized and the circulator and combustion air blower are turned on. ICM
