Gas Furnaces and the Silicon Carbide Hot Surface Igniter

An orchestra of events are necessary to ignite your gas furnace’s furnace flame and reach its target location – sparks from an silicon carbide hot surface ignitor (HSI) are essential in making this happen.

HSIs are resistance heaters that glow red hot when voltage is applied. They typically last five to ten years before needing replacement periodically.

Low Voltage

Silicon carbide hot surface ignitors (HSIs), are responsible for heating up the fuel/air mixture in your furnace so it can ignite. While pilot lights often consume more energy than their furnace counterparts, an HSI will only activate when sent a signal from its control board to do so. With its ability to burn hot before cooling back off again after each cycle, an HSI typically lasts five or ten years before needing replacing.

If a HSI starts to burn up frequently, this could be caused by numerous reasons. One common culprit could be too much power; if an HSI receiving more than 120 volts may burn out faster. If this appears to be an issue for your device, contact your power company and see if they can reduce how much voltage is being sent out to it.

Another possibility is that something is wrong with the ignitor itself. If it was made from cheaper silicon carbide material, such as silicon nitride ignitors would likely last longer before burning out quickly compared to their silicon carbide counterparts. If this is indeed the case for you, OEM replacement ignitors from your gas furnace manufacturer may need to be installed as quickly.

As soon as an HSI has become unusable due to overuse or neglect, its element may show signs of being cracked – often identifiable by white spots or lines across its element. Although functional, cracked elements will have shorter lifespan than their non-cracked counterparts.

Sometimes HSIs can be saved simply by replacing their ignitor in an appropriate location. When installed correctly, an ignitor should be installed so that its tip is in close proximity to the gas burner rather than too far away; otherwise, its temperature might never reach its ideal setting and won’t ignite its fuel/air mixture properly.

Dirt

As with any component in a gas furnace, silicon carbide hot surface ignitors (HSIs) eventually wear down and need replacing, potentially leading to start-up issues or the furnace shutting off prematurely or repeatedly. If you suspect an HSI may be failing, first attempt some troubleshooting steps before replacing it entirely.

Recent models of gas furnaces typically utilize an electronic ignition system with an infrared hot surface ignitor to ignite their burners when heat is required. Also referred to as glow plugs or glow sticks, this method is an easy and cost-effective solution that gets red hot when applied voltage to it, and allows gas to begin burning more effectively.

An ignitor can be damaged by any number of things, from dusty air and grease buildup, to dirt clogging up its chamber. When this occurs, overheating occurs or no glow appears from it at all. To clean an ignitor that has become dirty, disconnect and wipe down with cloth or sponge using some water; or spraying directly. Carbon deposits could break off and block its ability to produce sparks if spraying directly hits it directly.

Most ignitors are made from silicon carbide or silicone nitride and typically come in the shape of either a 1.5″ long flat stick or 2-inch diameter cylindrical shape with wires connected to a ceramic base, insulating them from voltage. Once voltage is applied, carbide or nitride elements within begin glowing orange when voltage is applied – an indicator for replacement by both homeowners and professional HVAC technicians alike.

Silicon carbide ignitors may still be found in many furnaces, but more durable silicon nitride igniters are becoming the go-to choice. Nitride igniters can better withstand high temperatures and rapid heating/cooling cycles without cracking. Opting for these igniters instead will save on energy costs while increasing furnace lifespan.

Grease

An air furnace equipped with a silicon carbide hot surface ignitor requires an adequate amount of grease for proper functioning. Without enough lubricant to prevent its ignition system from burning out too quickly and overheating quickly, overheat and breakdown become common causes of igniter failure. Luckily, replacement igniter modules from manufacturers like White Rodgers can easily be installed as direct replacement modules and offer an affordable alternative to replacing its original unit.

When a thermostat calls for heat, a 24-volt signal is sent to the module which activates pre-purge if installed and waits 15 to 30 seconds before lighting the main burners – depending on its manufacturer some modules also activate blower relays or blowers at this time.

Pre-purge delays are designed to give the ignitor time to reach its proper temperature before lighting main burners, giving 17 or 34 seconds (in certain models 20-40) for reaching that optimal level.

Once an ignitor reaches its desired temperature, it detects flame rectification or, on certain models, separate flame sensors to monitor flame output. Any contamination such as dust, drywall dust, fiberglass insulation or sealants that interferes with this signal could reduce igniter life considerably; should too many such obstructions come too close, the igniter may even burn out before it has completed this process of rectification.

Insulators on flame sensors may become compromised and cause short circuiting in humid environments where water drips onto them, especially if their ceramic insulator cracks. Check your sensor regularly for cracks in its ceramic insulator and replace if necessary; cracked ceramic insulators will likely become hot to the touch, and any cracks can allow wires to contact metal on your ignitor or furnace frame and cause a short to ground. If it is located somewhere other than dry conditions, move it to an environment more suitable to reduce short circuiting risks.

Overfired Gas Valve

Hot surface ignitors are one of the primary heat sources used to ignite combustion processes in gas furnaces and many appliances that use natural or propane gas, including boilers, water heaters and fireplaces. Although other forms of ignition such as sparks or pilot lights exist, they require constant attention in order to sustain flames; with an ignitor only needing heating during initialization phase. Common examples include boilers, water heaters and fireplaces containing these devices.

Silicon carbide hot surface ignitors typically sit inside of a ceramic holder and are about the size of a quarter, identifiable by white silica dust that forms when operating. They should last between five and ten years before needing replacing; when their time comes it is important to inspect an igniter closely for any cracks or breaks in its insulation sleeve and connections, and use a voltmeter test to confirm open circuit or continuity when voltage is applied – either of these conditions indicates replacement is required.

As soon as thermostat contacts close and an inquiry for heat is made (thermostat contacts closed), the control board sends a 24-volt signal to the igniter module. Once heated up, an igniter begins its work and will activate after an indefinite period (typically 15-30 seconds) of time delay specific to each unit.

As soon as an ignitor has heated sufficiently, it will signal to its igniter module that there is flame present and open the main gas valve based on unit type and installer settings. Once flame verification has occurred, gas valve can then be shut off again before finally switching off ignitor.

If a silicon carbide hot surface ignitor doesn’t respond during its ON time, testing with a multimeter for continuity or by looking at its room temperature resistance (RTR) is recommended. Body oils can sometimes cause the element to break down and show higher resistance at certain temperatures than it should, making this an excellent test before replacing an ineffective HSI. There are upgrade kits available from most manufacturers to allow users to replace old style silicon carbide ignitors with modern silicon nitride igniters; usually including replacement igniters as well as wire connection adapters for ease of installation of new igniters.