What Is a Hot Surface Igniter?
A hot surface igniter (HSI) is the electric element that replaces the standing pilot in modern gas furnaces. When the thermostat calls for heat, the control board energizes the igniter, which heats to 1,800-2,500°F within 15-30 seconds, then ignites the gas from the burners. It is the single most frequently replaced furnace component.
Two Types: Silicon Carbide vs Silicon Nitride
| Property | Silicon Carbide (SiC) | Silicon Nitride (Si?N?) |
|---|---|---|
| Color | Dark gray / charcoal | Light gray / beige |
| Resistance (cold) | 40-90 ? | 15-40 ? |
| Operating voltage | 120V | 80-120V (universal) |
| Warm-up time | 30-45 seconds | 15-20 seconds |
| Fragility | Extremely fragile - do not touch | More durable, handles vibration |
| Service life | 3-7 years | 7-15 years |
| Typical brands | Older Carrier, Lennox, Goodman | Carrier, Trane, Rheem (post-2010) |
Never touch either type with bare hands. Skin oils cause hot spots that crack the element. Handle only by the lead wires or ceramic base.
How to Test a Hot Surface Igniter
Step 1: Turn Off Power
Switch the furnace disconnect to OFF. Verify with a non-contact voltage tester before proceeding. Do not rely on the thermostat alone.
Step 2: Locate the Igniter
The igniter mounts directly above or adjacent to the burner assembly. It is connected to the control board via two low-voltage wires (or a plug connector on newer units). You may need to remove the lower furnace door panel.
Step 3: Resistance Test with Multimeter
Set your multimeter to ohms (?). Disconnect the igniter connector. Place probes on each igniter terminal.
- Silicon Carbide: 40-90 ? = good. Below 40 ? or above 90 ? = replace.
- Silicon Nitride: 15-40 ? = good. OL (overload/open circuit) = failed.
- Any reading of OL (open): Igniter is cracked internally - replace immediately.
Step 4: Visual Inspection
Look for hairline cracks across the heating element. Even a crack that is not yet open will fail under thermal stress within days. If you see any cracking, replace regardless of resistance reading.
Step 5: Live Voltage Test (Advanced)
With power restored and a call for heat initiated, measure voltage across the igniter terminals during the warm-up period. You should read 120VAC (or 80-100VAC for silicon nitride universal igniters). Zero voltage means the control board is not energizing the igniter - check the board, not the igniter.
Common Failure Symptoms
- Furnace attempts to light but no ignition - you hear the gas valve open but no flame
- Error code: "Ignition failure" or "3-blink fault" on Carrier/Bryant boards
- Igniter glows briefly then goes out before gas lights (contaminated or weak igniter)
- Furnace completely locks out after 3 attempts
Replacement Specifications
Always match the replacement igniter to your furnace model number - not just the physical shape. Key specifications:
- Voltage rating (120V vs 80V universal)
- Resistance range (SiC vs Si?N?)
- Mounting configuration (flat, 90°, or angled bracket)
- Lead length and connector type
Universal igniters like the Supco SIG1000 or Honeywell Q3400A are compatible with most furnaces but always verify against your model number before ordering.
Pro Tips
- Always carry a replacement igniter on any furnace service call in winter - they fail at the worst times
- A weak or sluggish igniter may still test in-spec on ohms but fail under load - if the furnace has 3+ ignition failures, replace the igniter
- After installation, confirm the igniter sits within 1/8 inch of the burner surface for reliable ignition
- Silicon carbide igniters cannot be cross-wired with silicon nitride replacements without verifying voltage compatibility