How to Replace a Hot Surface Ignitor: Step-by-Step Guide

A failed hot surface ignitor is one of the most common reasons a furnace won't ignite. The ignitor is a fragile ceramic element that glows red-hot to light the gas burner — and after years of thermal cycling, it eventually cracks or burns out. The good news: replacement is straightforward and takes less than 30 minutes with the right part.

What Is a Hot Surface Ignitor?

A hot surface ignitor (HSI) is an electric heating element made of silicon nitride or silicon carbide that reaches temperatures above 1,800°F to ignite the gas burner in a furnace. When the thermostat calls for heat, the control board sends 24V or 120V to the ignitor — it glows, the gas valve opens, and the burner lights. If the ignitor fails, the furnace goes through ignition attempts and locks out without firing.

Signs of a Failed Hot Surface Ignitor

• Furnace runs but no heat is produced
• Ignition attempts cycle 2-3 times then lockout
• Visible crack or break on the ignitor element
• Ignitor glows but burner doesn't light (may indicate gas valve issue instead)
• Multimeter reads OL (open circuit) across ignitor terminals

Tools You'll Need

• Multimeter
• Flathead and Phillips screwdrivers
• Needle-nose pliers
• Replacement hot surface ignitor (matching part number)
• Gloves (handle ignitor body only — never touch the ceramic element)

Safety Warning

Always turn off power to the furnace at the breaker AND shut off the gas supply before opening the unit. Never touch the ceramic element of the ignitor with bare hands — skin oils cause hot spots that dramatically shorten the ignitor's lifespan. Handle only by the mounting bracket or wiring harness.

Step-by-Step Replacement

Step 1 — Confirm the Ignitor Has Failed

Set your multimeter to resistance (Ohms) mode. Disconnect the ignitor wiring harness and test across the two terminals. A working silicon nitride ignitor reads between 15–75 ohms. An OL (open circuit) reading confirms the ignitor is failed.

Step 2 — Turn Off Power and Gas

Shut off the furnace at the breaker. Turn off the gas supply valve. Wait 5 minutes before opening the furnace.

Step 3 — Access the Ignitor

Remove the furnace access panel. The hot surface ignitor is located near the burner assembly, mounted on a bracket with a 2-wire harness connector. It sits directly in the path of the gas flame.

Step 4 — Document and Disconnect

Take a photo of the ignitor position and wiring before removing anything. Disconnect the wiring harness connector — do not pull on the wires themselves, only on the connector body.

Step 5 — Remove the Old Ignitor

Remove the mounting screw(s) holding the ignitor bracket in place. Carefully slide the ignitor out of its position. If it's cracked, handle with care to avoid ceramic fragments.

Step 6 — Install the Replacement

Handle the new ignitor only by its mounting bracket — never touch the ceramic element. Slide it into position, secure the mounting screw(s), and reconnect the wiring harness.

Step 7 — Restore Power and Test

Restore gas supply and power at the breaker. Set the thermostat to call for heat. Watch through the sight glass (if available) — the ignitor should glow orange/red within 15-30 seconds, followed by the burner lighting.

How to Choose the Right Replacement Ignitor

Match these specs from the original ignitor or equipment documentation:

1. Part number — most reliable match method
2. Voltage (24V or 120V — not interchangeable)
3. Lead length and connector type
4. Mounting bracket configuration

Universal ignitors (like the Honeywell Q3400A series) are designed to replace multiple manufacturer part numbers across common SmartValve systems — always verify compatibility against your specific model before ordering.