Hot Surface Igniter Operation and Troublehooting

March 1, 2001
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Some service technicians may be surprised to learn that the silicon carbide element of a hot surface igniter (HSI) can be handled without damage. However, it is better and safer to handle the igniter by the ceramic holder. The myth that the silicon carbide tip cannot be handled because body oils cause contamination is untrue.

On a typical heating system with HSI, a call for heat (thermostat contacts closed) will send a 24-V signal to the igniter module. When energized, the module will power up the igniter. If the module is a prepurge model, it will delay 15 or 30 sec before the igniter is activated. On prepurge models, the module will energize the combustion blower or other relays at the beginning of the cycle.

Once the prepurge timing is up (if so equipped), the silicon carbide igniter heats up to a proper ignition temperature (above 1,800°F) in either 17 or 34 sec, 20 or 40 sec for some models (depending on the manufacturer of the module).

Note: A 17- or 20-sec igniter can be used on a 34- or 40-sec application, but you could not use a 17- or 20-sec module with a 24- or 40-sec igniter.

At the end of the igniter warm-up period, the gas valve main opens. The igniter will remain on for a specific amount of time (seconds) depending on the specific ignition module being used. This “on” time or trial for ignition time can vary, depending on the specific ignition module being used. When main burner ignition occurs, the flame is sensed by the igniter (local sense) or by a remote sensor (remote sense).

With the main burner flame established, the igniter is turned off (12 V is shut off to the igniter).

Note: The burner flame must be detected within the timed trial for ignition. If no flame is detected, the gas main valve is de-energized, shutting off the gas flow. The system may go into lockout or, if it has a retry model, it will retry the number of times allocated.



Problem: Hot Surface Igniter Does Not Glow

Possible causes:
  • No main power;
  • Faulty transformer;
  • Faulty thermostat;
  • Faulty limit switch;
  • Faulty blower interlock switch (pressure switch, combustion blower proving switch);
  • Faulty hot surface igniter; or
  • Faulty ignition control or integrated control.
  • Solution: Perform normal system checks of main power, secondary or transformer, etc. With power on and the thermostat calling, check voltage at the 24 V or TH to 24 V ground at the module or integrated control.

    If 24 V does not check at the transformer, also check other controls in the circuit from transformer to module or integrated control. Check for 120 V from L1 to neutral (L2); check for 120 V from “IGN” to “IGN” or, on some modules, from the HSI terminals. If you have 24 V or 120 V to the module or integrated control, and no 120 or 24 V out of the module or integrated control, the module or control is faulty. Check the amp draw of the igniter; amperage should not exceed 4.75 A.

    Procedure:

    1. Do a visual check of the igniter for signs of damage or cracks. The sleeving over the wire should be examined for chafing, burned portions, or cuts in the wire. The connectors should be properly seated and free from oxidation and/or corrosion. Look for hot spots on the igniter. Observe the igniter during heat up. If a bright, white line across one of the igniter legs is detected, a crack may exist that could cause premature failure.

    Allow the igniter to cool and perform a resistance test. Addi-tional signs of a crack are an “open” igniter (shows no continuity when tested), or a buildup of white silica dust around the bright spot. Replace the igniter if you see these cracks.

    2. There are several possible causes for repeated igniter failures. One of the causes could be high supply voltage. A hot surface igniter can burn out at approximately 132 V. Even voltages in excess of 125 V may reduce igniter life. If high voltage is present, the power company should be requested to lower the power.

    3. Other causes for igniter failure include drywall dust, fiber glass insulation, sealants, or other contaminants that may accumulate on the igniter. In some cases, condensate dripping on the igniter causes it to fail.

    4. Furnace or boiler short cycling, delayed ignition, or an overgassed condition also contribute to shortened igniter life.



    How to Perform A Resistance Test

    One manufacturer (Norton) recommends performing a simple room temperature resistance (RTR) test after installing the igniter. Note: Remember to disconnect the leads to ensure that only the resistance of the igniter is measured.

    If the RTR is not to specification for igniter Model 201, which is a 34-sec warm-up time igniter of 45 to 400 ohms, or Model 271, a 17-sec warm-up time igniter of 40 to 75 ohms, then the silicone tip is damaged in some way and should be replaced.

    When troubleshooting an appliance where the igniter is suspect, the RTR will be higher on a used igniter, but the resistance should be no more than double the original resistance at installation.



    Igniter Glows But Main Burner Won’t Light

    Possible causes:
  • Improper igniter alignment;
  • Faulty ignition control;
  • Faulty gas valve;
  • High inlet pressure (lp gas);
  • Polarity reversed; or
  • No earth ground.
  • Solution: Make sure gas is available at the gas valve. Too high a pressure will lock up the gas valve. Make sure the igniter is in position (you cannot move the igniter from its designed position).

    Check for a good earth ground from L1 to the furnace chassis, you should read 120 V; if not, check and/or repair ignition ground wire or ignition control mounting screws. A jumper from ground to the gas line should give a good ground. Check for 24 V to the gas valve; if yes and the valve does not open, replace the valve; if no, replace the ignition module.

    Procedure: 1. If the igniter is going to be used as a sensor, make sure the flame is capable of providing a good rectification signal. Make sure that about 3¼ to 1 in. of the flame sensor or igniter sensor is continuously immersed in the flame for the best flame signal. Bend the bracket or the flame sensor, and/or relocate the sensor as necessary. Do not relocate an igniter or combination igniter-sensor.

    2. Check for excessive (more than 1,000°F/538°C) temperature at the ceramic insulator on the flame sensor. Excessive temperature can cause a short to ground; move the sensor to a cooler location or shield the insulator. Do not relocate an igniter or combination igniter-sensor.

    3. Check for a cracked ceramic insulator, which can cause a short to ground, and replace the sensor if necessary. Make sure that the electrical connections are clean and tight. Replace damaged wire with moisture-resistant No. 18 wire rated for continuous duty up to 221°F/105°C.



    Checking The Flame Signal

    It is important to realize that when the igniter is also being used as a sensor, there is some difficulty in being able to actually check the microamps.

    According to a “Norton Igniter Products Technical Bulletin,” “Sensing through flame rectification, be it direct (through the igniter) or remote (separate flame) involves certain components and variables. The object is to use the ionized particles in the flame (burning gas) to conduct a current and complete an electrical circuit.

    “The control module initiates an ac signal that is sent out to the igniter. The flame acts as a diode and converts the ac signal to a rectified dc signal. The strength of the signal required to prove the flame, and therefore to keep the gas valve open, is dependent on the control module and varies from one control manufacturer’s brand to another. Signal strength can be affected by the type of burner, position of the igniter in the flame, age of the igniter, type of gas, coating on the igniter, and any impurities that could have built up over time. It is imperative that the flame remains in contact with the burner, and that the burner and control module have the same common ground.

    “When using the igniter as the sense unit, it is important to remember that as an igniter ages, a thin silicon oxide (SiO2) layer is formed on the surface. This is part of the normal aging process of a silicon carbide (SiC) igniter. As this oxide layer is formed, it actually helps seal the underlying SiC grains and inhibits further rapid oxidation. The SiO2 that has formed is a glass which is an insulator, and will diminish the strength of the flame signal that is being sent out. Whether the signal will still be strong enough to keep the valve open as the igniter ages is application dependent.”



    Main Burner Shuts Off Before T-stat Is Satisfied

    Possible causes:
  • Improper igniter alignment;
  • Faulty ignition control;
  • Contaminated igniter and/or sensor (remote sensing);
  • Bad burner ground.
  • Solution: Check for proper polarity. Check for proper igniter position; make sure there is proper ignition control grounding.

    Check for foreign matter on the igniter or sensor. Clean or replace. Check the main burner ground by checking continuity between ground and burner. If previous checks are OK, you may need to check the microamps on the system.

    The procedure for checking the flame signal with HSI when the igniter is also being used as a sensor is outlined in the two following procedures.

    Procedure one: Note: Do not attempt this measurement unless you are familiar with line voltage measurements. The HSI, line, and meter leads will be live during this measurement; touching them may result in electrical shock.

    1. Remove all power.

    2. Remove lower HSI wire from the electronic module. Place a single-pole, single-throw (spst) switch (rated for at least 5 A @120 vac) between the lower HSI terminal and the disconnected lead. Make sure the switch is in the closed position.

    3. Attach the red (positive, +) lead of W136 meter (or equivalent) through a 1-meg resistor to the electronic module side of the switch. Attach the black (negative, -) lead to the W136 to the igniter side of the switch.

    4. Make sure the meter is set on the µA scale.

    5. Reapply power.

    6. After the burner is lit and the hot surface igniter turns off, open the switch and read the µA. Never open the switch while the hot surface igniter is energized. This may damage the W136 meter.

    7. After the reading is taken, remove all power and reconnect the system. Minimum readings for proper operation should be 0.8 µA.

    Procedure two:

    (See Figure 2 for reference.)

    1. With the power off, remove the lead from the module that feeds 120 vac to the igniter.

    2. The lead removed from the module should be connected to the alligator clip, and the alligator clip to the HSI terminal on the module.

    3. Plug the two banana plugs into the multimeter; observe the polarity (red into red, black into black).

    4. Turn the power on and call for heat. The igniter should glow and burner ignition should commence.

    5. At anytime after ignition, push the momentary switch (NC); it will open and you should read microamps on your meter.

    McElwain, president of Gas Appliance Service Training and Consulting, teaches service and troubleshooting skills. The company has troubleshooting guides covering 38 topics, and teaches a seminar series on the Fundamentals of Gas, Circuitry and Troubleshooting, Hydronic Controls, Electric Ignition Systems, Advanced Electric Ignition Systems, Powerpile Systems, and Conversion Burners. For more information, contact McElwain at Gas Appliance Service Training and Consulting, 22 Griffith Dr., Riverside, RI 02915; 401-437-0557.

    Publication date: 03/05/2001

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