Safety: Take care when working outside in icy conditions. Condensate is mildly acidic.

Locate frozen section: Feel along the external condensate pipe for hard, icy sections.
Thaw pipe: Pour warm (not boiling) water over the frozen section or use a hot water bottle to gently thaw.
Reset boiler: Once thawed, reset the boiler and confirm normal operation.
Prevent recurrence: Insulate the external pipe, increase its diameter where possible, and improve the fall to reduce standing water.

Boiler lockout occurs mainly in very cold weather, with external condensate pipe present

Possible Causes