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.
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