Furnace display shows E10, unit may lock out after several failed ignition attempts, sometimes with delayed or rough ignition.

Safety first: Turn off power and gas. Allow the furnace to cool. Delayed ignition can cause small explosions in the burner compartment—do not ignore this fault.

• 1. Inspect burners: Remove the burner assembly from the Thermia furnace following the service manual. Check for rust, debris, or spider webs in the burner ports and crossover channels.
• 2. Clean burners: Use compressed air and a soft brush to clean burner ports and crossover slots. Do not enlarge the ports. Ensure all burners are aligned properly when reinstalled.
• 3. Check igniter position: Verify that the hot surface igniter is correctly positioned relative to the burner it lights. Adjust mounting if necessary so the igniter is fully in the gas stream.
• 4. Observe ignition sequence: After reassembly, restore power and gas. Watch through the sight glass: the burner should light smoothly across all sections. If ignition is still rough or delayed, a technician should check gas pressure and combustion.
• 5. Replace weak igniter if needed: If the igniter glows dimly or slowly, replace it with a Thermia-compatible igniter.