Error 20; heater will not start and indicates a condensate or flue-related fault.

Safety: Isolate power. Condensate is mildly acidic; wear gloves and eye protection.

Steps:

• Check external condensate pipe: Outside, inspect the plastic condensate discharge pipe for freezing or blockage. Thaw gently with warm (not boiling) water if frozen.
• Clean condensate trap: Inside the heater, locate the condensate trap under the heat exchanger. Remove the cap or trap body, empty contents, and flush with clean water. Reassemble securely, ensuring seals are in place.
• Verify pipe slope: Ensure the condensate pipe runs downhill continuously with no sags where water can collect and block flow.

Note: After clearing, reset the heater and observe for leaks around the trap and pipe connections.