Furnace Ignitor Types: Hot Surface vs. Spark — What's the Difference?

If your furnace tries to start but the burners never light, the ignitor is one of the first things to investigate. Modern furnaces use two types of ignition systems — hot surface ignitors and direct spark ignitors — and understanding how each works helps you diagnose problems and avoid unnecessary service calls.

The End of Pilot Lights

Standing pilot lights — the small continuously burning flame that lit furnaces throughout the 1970s–1990s — have been largely replaced by electronic ignition. Standing pilots waste gas constantly, can be blown out, and fail to meet modern efficiency requirements. Today's furnaces (including all Goodman models) use one of two electronic ignition systems.

Hot Surface Ignitor (HSI)

How It Works

A hot surface ignitor is a small silicon carbide or silicon nitride element that glows orange-hot (1,800–2,500°F) when electrical current passes through it. When the furnace calls for heat, the control board energizes the ignitor for a warm-up period (typically 15–45 seconds), then opens the gas valve. Gas contacts the glowing element and ignites.

Types of Hot Surface Ignitors

• Silicon carbide (SiC): Older type, more fragile, typically white or gray in color. Warm-up time ~45 seconds. Resistance typically 40–70 ohms. Common on furnaces from the 1990s–early 2000s.
• Silicon nitride (Si3N4): Modern standard. More durable, faster warm-up (~15–20 seconds), longer lifespan. Common on all current Goodman furnaces.

HSI Pros and Cons

• Pro: Reliable ignition, no moving parts, works in any venting orientation
• Pro: Silicon nitride versions are quite durable (5–10+ year lifespan)
• Con: Silicon carbide versions are very fragile — touching them with bare hands (skin oils) causes premature failure
• Con: Fails completely when it breaks — no gradual degradation warning

Direct Spark Ignitor (DSI)

How It Works

Direct spark ignition works like a spark plug — a high-voltage spark jumps across an electrode gap near the burner, igniting the gas-air mixture directly. No warm-up period is needed. Some systems use a separate flame sensor to verify ignition; others use the same electrode for both spark and sensing.

DSI Pros and Cons

• Pro: Instant ignition — no 15–45 second warm-up delay
• Pro: No fragile element to break
• Pro: Can operate in outdoor or challenging environments
• Con: Spark noise (clicking) can be louder
• Con: Electrode gap must be maintained at correct specification
• Con: Less common on residential furnaces — primarily found in rooftop units and some older designs

What Goodman Furnaces Use

All current Goodman residential furnaces use silicon nitride hot surface ignitors. This is the most reliable, durable design available for residential applications. The silicon nitride element in a modern Goodman should last 5–10 years under normal conditions.

How to Tell If Your Ignitor Has Failed

Symptoms of a failed hot surface ignitor:

• Furnace starts inducer motor, waits, then shuts off without igniting
• You hear the gas valve click open but no burner flame follows
• Diagnostic LED shows ignition failure code (3 flashes on most Goodman models — see our LED code guide)
• Visual inspection shows broken, cracked, or burned-out element

Testing a Hot Surface Ignitor

You can test an HSI with a multimeter (with power OFF):

1. Disconnect the ignitor electrical connector
2. Set multimeter to resistance (ohms)
3. Measure resistance across the ignitor terminals
4. Silicon carbide: 40–70 ohms = good; open circuit = failed
5. Silicon nitride: typically 15–50 ohms = good; open circuit = failed

An open circuit (infinite resistance) means the element is broken. Note: an ignitor can test in-spec but still fail at operating temperature — resistance testing catches most failures but not all.

Replacing a Hot Surface Ignitor

HSI replacement is one of the easier furnace DIY repairs for mechanically inclined homeowners:

• Turn off power and gas to the furnace
• Locate the ignitor (usually visible next to the burner assembly)
• Disconnect the wire harness
• Remove the mounting screw (usually one Phillips head)
• Install new ignitor — do not touch the element with bare hands
• Reconnect wiring and test

Replacement ignitors are $20–$60 depending on model. Always verify the correct part number for your specific furnace model using the data label.

Flame Sensor: The Related Component

The flame sensor works alongside the ignitor — it verifies that a flame actually exists after ignition. A dirty or failed flame sensor causes the furnace to light briefly then shut off. This is a different problem from an ignitor failure. A failed ignitor means no flame ever lights. A failed flame sensor means the flame lights but the furnace shuts it off within 2–5 seconds because it can't confirm the flame is there.

Flame sensor cleaning (with fine steel wool or emery cloth) is a common DIY fix for this problem.

When to Call a Pro

If you've replaced the ignitor and confirmed the flame sensor is clean but the furnace still won't ignite reliably, the issue may be with the gas valve, control board, or gas supply pressure — all of which require a licensed technician to properly diagnose.

Related: Control Board Failure Guide | Gas Valve Guide | Furnace Noise Guide