How a Furnace Ignitor Works

A furnace ignitor is the component that safely starts the combustion process in a gas furnace by producing the heat or spark needed to ignite the burner. This article explains the main types of furnace ignitors, how they operate step-by-step, common failure modes, troubleshooting tips, maintenance practices, safety considerations, and replacement guidance. Understanding how a furnace ignitor works helps homeowners diagnose issues, communicate with HVAC technicians, and make informed decisions about repairs or replacements.

Ignitor Type	How It Ignites	Typical Lifespan
Hot Surface Ignitor (HSI)	Resistive heating element glows red-hot	3–10 years
Flame Sensor	Detects flame presence; not primary ignitor	5–15 years
Spark Ignitor (Pilotless)	Produces electrical spark across electrodes	5–15 years

Types Of Furnace Ignitors

There are two primary categories of modern furnace ignitors used in residential systems: Hot Surface Ignitors (HSI) and Spark Ignitors. Older furnaces used a standing pilot light, which is largely replaced by electronic ignitors for efficiency and safety.

Hot Surface Ignitors (HSI)

HSI elements are typically made from silicon carbide or silicon nitride and operate like an electric heater. When energized, the element reaches temperatures around 1,200°F and glows orange to ignite the gas at the burner ports.

Spark Ignitors

Spark ignitors, sometimes called intermittent pilot or direct spark systems, generate a high-voltage spark between two electrodes to ignite the gas. These are common in furnaces and gas appliances requiring immediate ignition without a glowing surface.

Standing Pilot

Older systems used a constantly burning pilot flame. While simple, this method wastes fuel and is largely phased out in modern residential furnaces due to efficiency and emission concerns.

Key Components Related To Ignition

A furnace ignition system includes several components that work together to ensure safe ignition and operation.

• Ignitor Element: HSI or spark electrode that creates heat or spark.
• Control Board: Sends signals to energize the ignitor and monitor safety switches and flame detection.
• Gas Valve: Opens to allow gas flow after the ignitor reaches operating state or when the spark sequence is active.
• Flame Sensor: Confirms presence of flame and tells the control board to keep the gas valve open.
• Limit Switches and Pressure Switches: Ensure safe conditions before ignition (e.g., proper venting and blower status).

How A Furnace Ignitor Works: Step-By-Step

The ignition sequence is coordinated by the furnace control board and several safety interlocks. The following steps describe a typical modern gas furnace ignition cycle with an HSI or spark ignitor.

1. Thermostat Call: Thermostat signals heat demand to the furnace control board.
2. Pre-Purge / Blower On: Inducer motor runs to clear combustion gases and create proper vent pressure. Pressure switch confirms airflow.
3. Ignitor Energized: For HSI, the control board sends current to the ignitor until it reaches glowing temperature. For spark systems, the board triggers high-voltage pulses to electrodes.
4. Gas Valve Opens: Once the ignitor indicates readiness (HSI glowing or spark sequence active), the control board opens the gas valve to allow gas to flow to burners.
5. Ignition: Gas meets the hot surface or spark and ignites, producing a flame at the burner ports.
6. Flame Detection: Flame sensor detects flame presence and signals the control board to keep the gas valve open.
7. Continuous Operation: Furnace runs until thermostat setpoint is met, then the control board shuts off the gas and begins post-purge before stopping the inducer and blower as required.

Why Ignitors Fail And Common Symptoms

Ignitor failure is common and typically caused by thermal cycling, mechanical stress, contamination, electrical issues, or improper installation. Recognizing symptoms helps determine if the ignitor or another component is at fault.

• HSI Failures: Visible cracks, broken element, or lack of glow. Furnace tries to light but fails repeatedly.
• Spark Ignitor Failures: No spark visible at electrodes, weak or intermittent sparks, corrosion on electrodes.
• Symptoms: Furnace blower runs but burners won’t ignite, repeated ignition trials, error codes on control board, or the furnace locks out after several failed attempts.

Troubleshooting A Faulty Ignitor

Basic troubleshooting can identify whether the ignitor is the issue, but electrical work and gas handling should be left to qualified technicians.

1. Check error codes on the furnace’s LED diagnostic and consult the manual for specifics.
2. Visually inspect the ignitor for cracks or discoloration. Do not touch HSI elements; oils from skin can damage them.
3. Test for continuity with a multimeter: most HSI elements show a low resistance; an open circuit indicates failure.
4. For spark systems, look for visible spark during ignition and inspect electrode gap and corrosion.
5. Verify related safety interlocks (pressure switch, limit switch) and ensure the inducer motor runs properly.

Maintenance To Extend Ignitor Life

Regular maintenance reduces ignition problems and improves furnace longevity. Annual professional tune-ups are recommended before the heating season.

• Keep Components Clean: Dust and soot buildup can insulate HSI elements and contaminate flame sensors.
• Inspect Flame Sensor: A dirty flame sensor causes false flame failure; cleaning with emery cloth helps.
• Ensure Proper Electrical Connections: Loose or corroded connections cause voltage drops and stress ignitors.
• Maintain Proper Venting And Airflow: Restrictive venting can stress components and trigger repeated ignition cycles.

Replacement And Cost Considerations

When replacement is necessary, several factors influence choice and cost: ignitor type, furnace model, accessibility, and labor rates. Homeowners should consider efficiency and compatibility.

Item	Typical Cost Range (Parts + Labor)
Hot Surface Ignitor Replacement	$150–$400
Spark Ignitor Replacement	$100–$350
Flame Sensor Cleaning / Replacement	$50–$200

Costs vary by region and furnace design. OEM parts may cost more than aftermarket equivalents but often match original performance. Always verify compatibility with the furnace model number.

Safety And Best Practices

Working with gas appliances involves risks. Follow these safety principles when dealing with furnace ignitors.

• Turn Off Power And Gas: Before inspecting or replacing ignitors, shut off electrical power at the breaker and the gas supply.
• Do Not Touch HSI Elements: Oils from skin cause hot spots and premature failure.
• Hire Licensed Technicians For Gas Work: Complex diagnostics, gas valve replacement, and venting issues require professional service.
• Watch For Carbon Monoxide Signs: If the furnace exhibits incomplete combustion or malfunctioning venting, evacuate and call a professional.

How Technology Is Changing Furnace Ignition

Advances aim to improve reliability, efficiency, and diagnostics. Modern control boards provide precise sequencing, self-diagnostics, and connectivity for smart thermostats.

• Adaptive Ignition Timing: Boards can optimize ignition timing to reduce stress on ignitors.
• Improved Materials: Silicon nitride ignitors last longer than older silicon carbide designs.
• Smart Diagnostics: Error codes and wirelessly connected HVAC systems help technicians pinpoint failures quickly.

Frequently Asked Questions

How Long Does A Furnace Ignitor Last?

Hot surface ignitors commonly last 3–10 years depending on cycles and duty. Spark ignitors can last 5–15 years. Frequent on/off cycles and poor maintenance shorten lifespan.

Can An Ignitor Be Repaired?

Ignitors are typically replaced rather than repaired because they are delicate and inexpensive relative to labor. Flame sensors can often be cleaned instead of replaced.

Why Does The Furnace Click But Not Light?

A clicking sound may be the control board attempting to ignite. Causes include a failed ignitor, clogged burners, defective gas valve, or failed safety interlocks like the pressure switch.

Is It Safe To Run A Furnace With A Faulty Ignitor?

Running a furnace with ignition problems can lead to repeated gas valve attempts, increased wear, and potential safety lockouts. It is best to stop using the furnace and seek service.

Practical Tips For Homeowners

• Schedule annual professional maintenance before winter to check the ignitor, flame sensor, gas pressure, and venting.
• Keep the area around the furnace clean to prevent dust accumulation that can affect ignition components.
• Note and report error codes displayed by the furnace; they speed up diagnosis.
• When replacing an ignitor, insist on a compatible part and a technician who follows manufacturer torque and handling guidelines.

Understanding how a furnace ignitor works helps homeowners identify problems early and communicate effectively with HVAC professionals. Regular maintenance, timely replacement, and safe practices minimize downtime and extend furnace life while ensuring safe, efficient operation.