What the condensate pipe does
Modern condensing boilers produce a small amount of slightly acidic water as they run. It leaves the boiler through a plastic condensate pipe, which runs to an internal waste pipe or outside the house. In a hard frost the water in the pipe can freeze and block it, and the boiler shuts itself down to protect itself.
Signs it is frozen
The boiler stops heating or making hot water during freezing weather, and may show a fault code or lock out. You might hear gurgling or bubbling from the boiler or the pipe. It often starts on the coldest mornings and may work again once it warms up. Check your manual for the code, or see our guide to common boiler fault codes.
How to thaw it safely
Find the plastic pipe leaving the bottom of the boiler and follow it to the outside. The frozen section is usually the exposed part, often at a bend or the end. Pour warm water over it, not boiling water, which can crack the plastic. Then reset the boiler as your manual describes. Do not pour water over the boiler itself, do not open its casing, and do not take risks on a ladder or icy ground.
Stopping it happening again
Frozen condensate pipes tend to repeat. Insulating the exposed pipe helps, and an engineer can shorten the outside run, re-route it inside or use a larger pipe, all of which reduce the chance of freezing. Our guide to preventing boiler breakdown in winter covers other cold-weather checks.
When to call an engineer
Call a Gas Safe engineer if it keeps freezing, you cannot find the blockage, the pipe looks damaged or the boiler will not restart. Low pressure can also lock a boiler out, so see why a boiler loses pressure if the pipe is not frozen.
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