Boiler lockout occurs mainly in very cold weather, flue terminal or condensate pipe may be icy.

Safety first: Turn off the boiler before attempting to thaw pipes. Do not use open flames or blowtorches on plastic pipes.

Step-by-step checks:

• 1. Inspect condensate pipe: Locate the external section of the condensate pipe. If it feels solid and is very cold, it may be frozen.
• 2. Thaw safely: Use warm (not boiling) water poured over the pipe, or apply a warm cloth to thaw the ice. Do not damage the pipe or joints.
• 3. Insulate pipe: Once thawed, insulate the external condensate pipe with suitable weatherproof insulation to reduce future freezing.
• 4. Check flue terminal: Remove any ice or snow blocking the flue outlet.
• 5. Reset boiler: After ensuring free drainage and clear flue, reset the boiler and test operation.