Boiler not firing up in cold weather

In temperatures below freezing, the most common cause of a boiler failing to start is a frozen condensate pipe. Modern condensing boilers produce acidic condensate water that drains via a white plastic pipe (typically 21.5mm or 32mm) to an external drain or soakaway. This pipe freezes in cold snaps.

Check your boiler's fault code — most modern boilers display a code for a condensate blockage. Common codes: Vaillant F.28/F.29, Worcester Bosch EA 229, Ideal F1, Baxi E119. Consult your boiler manual.

Find the condensate pipe. It exits the boiler (usually at the bottom) and typically runs along an external wall or through it to the outside. It will be cold to the touch at or near where it exits the building.

Thaw the pipe: apply a warm (not boiling) hot water bottle to the frozen section, or pour warm water along the pipe. Do this repeatedly until the blockage clears. You may hear the boiler gurgle as condensate drains away.

Once clear, reset the boiler (usually a button or hold the power button for 3 seconds). The boiler should fire up within a minute.

To prevent recurrence: lag the external section of pipe with waterproof foam lagging (£3–£5 per metre from any hardware store). If the pipe runs a long horizontal distance outside, it may need rerouting to a shorter, steeper drop by a plumber.