Boiler stopped in the cold? It is probably a frozen condensate pipe

Every winter it happens on the first proper frost. The heating was fine last night, and this morning the boiler is showing a fault code, gurgling, or just refusing to fire. Before you assume the boiler has died, check the condensate pipe. On a modern condensing boiler it is the most common cold-weather fault there is, and it is often a ten minute fix you can do yourself.

What the condensate pipe does

Modern boilers are condensing boilers. They squeeze extra heat out of the exhaust gases, and that process produces water, which drains away through a plastic pipe, usually around 22mm wide. If that pipe runs outside to a drain, the water sitting in it can freeze in a cold snap. The ice blocks the pipe, water backs up into the boiler, and the boiler shuts itself down to protect itself.

Signs it is frozen

• It is below freezing, or has been overnight.
• The boiler shows a fault or lockout code and will not relight.
• You can hear gurgling coming from the boiler.
• There is a plastic pipe running from the boiler through an outside wall, and it is exposed.

How to thaw it safely

1. 1Find the pipe. Follow the plastic pipe from the bottom of the boiler to where it goes outside. The frozen part is usually the most exposed stretch, or the open end near the drain.
2. 2Warm it with warm water, not boiling. Pour warm water from a kettle that has been left to cool a little, or use a hot water bottle or a heat wrap, along the outside section. Boiling water can crack plastic pipe and is dangerous on an icy path.
3. 3Work towards the end of the pipe. Keep going until you hear or see water draining out.
4. 4Reset the boiler. Once it is draining, use the reset button as described in your boiler manual. It may take a minute or two to relight.

If the pipe is high up, awkward to reach, or on a wall above a path that is iced over, do not climb a ladder to get to it. It is not worth the risk for a job someone can come and do.

When to call someone instead

• You have thawed the pipe and reset the boiler, and it still will not fire.
• The pipe is cracked, or water is leaking from the boiler itself.
• It freezes again every cold night.
• You cannot find or safely reach the pipe.

Stopping it happening every winter

A condensate pipe that freezes regularly usually has a fixable cause. The outside section can often be shortened, rerouted to drain inside, insulated with the right lagging, or replaced with a wider pipe that is much less likely to block with ice. These are small jobs, and far better done in autumn than on a freezing morning with no heating.

If yours froze last winter, it is worth having it looked at during the annual boiler service. I cover Preston, Penwortham, Chorley, Leyland, Garstang and Kirkham, and I would rather sort the pipe in October than get the call in January.