Ignition Module | Heater Service & Troubleshooting

Hot-Surface Ignition Module

The principal components of a hot-surface ignition system are the hot-surface ignition module, a line voltage silicon carbide igniter (also sometimes called a glow stick or glow plug), a remote flame sensor, a 24-volt AC ignition-detection control, and a 24-volt (AC) redundant gas valve (see Figure 5-69). The flame sensor is designed to detect the presence of a flame. It can be mounted remotely on multiple burners or next to the gas burner.

The hot-surface ignition module, similar to the one shown in Figure 5-70, is a microprocessor-based gas ignition control designed for direct ignition gas-fired appliances. It provides direct main gas burner ignition, remote sensing, and prepurging. It will retry for ignition and has a fixed time for flame lockout.

Some hot-surface ignition modules have self-diagnostic capabilities. A diagnostic light on the HSI module provides the following information:

• If the diagnostic light on the module flashes on and off one time at initial startup, the unit is functioning properly.
• If the diagnostic light is lit continuously, there is most likely an internal problem with the module. Check for an internal problem by interrupting the line power or 25-volt thermostatic power for a few seconds and then restore it. If the burner still fails to ignite, replace the module.
• If the diagnostic light continues to flash, the problem is in the external components or wiring.

For HIS modules without self-diagnostic capabilities, a qualified HVAC technician or electrician should troubleshoot the system with the appropriate test equipment. The test equipment should include the following:

• A volt-ohm meter for checking both the voltage and the resistance.
• A precision microammeter for checking the flame sensor output and location.
• A pressure gauge (low scale) for checking gas pressure.

Warning
Extreme caution must be taken when working on a hot-surface ignition system. Because of the high voltage present, there is always the potential for serious electrical shock.

If the unit is not equipped with a self-diagnostic light, closely follow the troubleshooting suggestions provided by the manufacturer. These will be specific to the make and model. Some simple things to look for include the following:

• Checking to make certain the manual knob on the gas valve is in the on position and gas is available at the inlet piping
• Checking the outlet gas pressure to make sure it matches the nameplate rating
• Checking the wire leads to the gas valve for proper connection or damage

Direct-Spark Ignition Module

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The direct-spark ignition (DSI) module illustrated in Figure 5-67 is a low-voltage, solid-state unit that controls the gas valve, monitors the burner flame, and generates a high-voltage spark for ignition. DSI modules are available with or without a purge timer and with separate or combined igniters and flame sensors. Typical wiring connections for a direct-spark ignition system are illustrated in Figure 5-68.

Intermittent Pilot Ignition Module

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An intermittent pilot ignition module is a solid-state ignition device used to automatically light a pilot burner and simultaneously energize (operate) the main gas valve of the heating system when the room thermostat calls for heat (see Figure 5-65). Figure 5-66 illustrates the wiring connections between an intermittent pilot module, the dualvalve
combination valve, and the combined pilot burner and igniter sensor unit.

The operating sequence for a gas burner operated by an intermittent pilot ignition control is as follows:

1. Room thermostat calls for heat, and the intermittent pilot ignition module simultaneously opens the pilot valve and supplies a continuous spark to the electrode in the pilot burner.
2. Pilot burner gas ignites and produces a flame.
3. Pilot flame sensor detects the pilot flame and signals the intermittent pilot ignition control to discontinue the spark and energize (open) the main gas valve.

The main gas valve will not be energized until the flame sensor detects the presence of the pilot flame. As long as the main gas valve remains closed, no gas from the supply line can flow through the burners. Should a loss of flame occur, the main gas valve closes and the spark recurs within 0.5 second.

4. Gas from the gas supply line flows through the now-open main gas valve to the burners and ignites. This is the burn or on cycle. When the heat has reached the level required by the thermostat setting, the main gas closes and the burner or burners shut down. This is the off cycle in an intermittent pilot ignition control system.

Some Useful Definitions
• Run—The period during which the main gas valve remains energized and the spark is turned off after the successful
ignition.
• Trial for ignition—The period during which the pilot valve and spark are activated, attempting to ignite gas at the main gas burner.
• Flameout—The loss of proven flame.
• Proven flame—A pilot flame detected by a flame sensory device.
• On cycle—Period of time during which the main gas valve is open and the burners are operating.
• Off cycle—Period of time during which the main gas valve is closed and the burners are not operating.

If the pilot flame is extinguished, even though the room thermostat is still calling for heat, the intermittent pilot ignition control immediately deenergizes the main gas valve, causing it to close its open supply port and stop the flow of gas to the burners. A spark at the pilot burner electrode will recur within 0.8 second.

As soon as the pilot flame is reignited and detected by the pilot flame sensor, the main gas valve is energized, the valve port is opened, and the spark is extinguished. The intermittent pilot ignition control then deenergizes the pilot gas and main burner gas valve when the thermostat stops calling for heat.

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