Step by step
Confirm the likely cause
A frozen external condensate pipe is one possible cause of a winter lockout; use the boiler manual to check.
Warm the outside pipe
Pour warm, not boiling, water over the accessible frozen section. A warm cloth can help too.
Repeat patiently
Continue gently until the ice clears. Do not strike the pipe or use a naked flame.
Restart as instructed
Follow the boiler manual's reset steps and check that condensate drains normally.
Plan prevention
Ask an installer about suitable insulation or routing if the pipe freezes repeatedly.
Know when to stop
When to stop and call a professional
Stop if the pipe is damaged, the boiler still fails, or the frozen section is unsafe to reach. Never open the boiler casing or work on gas components.
General information for a UK home; always follow your product instructions and current local rules. Images are illustrative, not technical diagrams.