Furnace display shows E11, unit may start but shuts down with a condensate-related alarm, especially on high-efficiency Thermia models.

Safety first: Turn off power. Be cautious of water around electrical components.

• 1. Locate condensate system: On condensing Thermia furnaces, find the condensate trap and drain tubing connected to the secondary heat exchanger and inducer housing.
• 2. Inspect for clogs: Disconnect the drain tubing and trap. Flush with warm water and a small amount of mild detergent. Remove any slime or debris. Ensure the trap is reinstalled in the correct orientation.
• 3. Check drain routing: Verify that the drain line slopes continuously downward to the floor drain or condensate pump. Eliminate sags where water can collect.
• 4. Test condensate pump (if used): Pour water into the pump reservoir and confirm it activates and discharges water. Replace the pump if it fails to run or pump out.
• 5. Inspect condensate collector box seals: Ensure gaskets and seals on the Thermia condensate collector are intact and not leaking into the furnace cabinet.
• 6. Reset and test: After clearing and reassembling, restore power and run the furnace through a full heating cycle. If E11 returns, a technician should inspect for internal heat exchanger or collector issues.