The boiler locks out more often during very cold weather, affecting external pipework.

Safety first: Turn off the boiler before working on the condensate pipe. Take care on icy ground.

• Locate external condensate pipe: Usually a small plastic pipe (21.5mm or similar) running to a drain.
• Thaw the pipe: Pour warm (not boiling) water along the length of the pipe, especially at bends and the outlet. Do not use naked flames or boiling water.
• Reset boiler: Once thawed, reset the boiler and check its operation.
• Engineer improvements:
  • Increase pipe diameter and improve insulation.
  • Reroute to minimize external exposure.

Frozen condensate is a very common cause of winter lockouts and can often be resolved without internal work.