An igniter produces the spark for direct ignition of the main gas burner in various heating applications (gas furnaces, gas boilers,gas water heaters, etc.). The Honeywell Q347 igniter shown in Figure 5-71 consists of an internal electrode with a ceramic insulator, bracket, and ground strap.

The flame-sensing rod is separate from the hot-surface igniter in most hot-surface ignition systems.

An igniter is used to provide the spark to ignite the main burner flame. Some igniters have an integral flame sensor. When this is the case, the igniter both ignites and senses (proves) the main gas burner flame.

The operating sequence of a system in which an igniter is used may be summarized as follows:

1. Room thermostat calls for heat.
2. Gas valve opens and gas flows to the burner(s).
3. Burner ignites when the gas reaches the main burner.
4. Spark igniter shuts off.

The duration of the spark operation must be within the igniter manufacturer’ s specified lockout timing period. The igniter manufacturer will provide a chart of the ignition control lockout times in the service literature for the igniter. The example shown in Table 5-3 is for Honeywell’ s Q347 igniter.

The electrode spark gap in the igniter must be within the specified maximum (see Figure 5-72). If the gap is not within specifications, it will have to be adjusted for optimum performance.

The flame rod of a combined igniter–flame sensor unit must be immersed 1 inch in the burner flame to produce the best flame signal (see Figure 5-73). Examples of poor flame conditions and their probable causes are illustrated in Figure 5-74.

The flame signal can also be adversely affected by a bent bracket, bent rod, or cracked ceramic insulator. Sometimes the bracket can be bent back into shape. If the rod is bent or the ceramic insulator is cracked, the igniter should be replaced.

Always check the specifications of the replacement hot-surface igniter before installing it. Not all igniters have the same voltage or warm-up time as the original design.

The igniter used in a hot-surface ignition operating system differs in design from the type used in intermittent pilot or direct-spark ignition systems described in the preceding paragraphs (see Figure 5-75).

Be careful when replacing a hot-surface igniter because they are fragile and easily damaged. Sometimes a crack in the igniter surface is so small that it is not visible. A cracked hot-surface igniter may still work, but it will have a much shorter service life. After it is installed, check the hot-surface igniter for any inconsistencies in its glow pattern.