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

If your boiler has stopped during a cold snap and it is showing a fault code, there is a good chance nothing is broken. A frozen condensate pipe is the most common winter boiler fault in the country, and it is one of the few you can usually fix yourself in about ten minutes.

How to tell

Three things together point at it:

• It stopped overnight or during a hard frost, not gradually.
• The boiler is showing a fault code and may be making a gurgling or bubbling noise.
• There is a plastic pipe, usually white or grey, about 22mm, running from the boiler through the wall to outside.

That outside pipe is the condensate. It carries acidic water away from the boiler, and in a hard frost the water in it can freeze and block it. The boiler detects the blockage and shuts itself down, which is exactly what it should do.

How to thaw it

1. Find the blockage. It is usually at the most exposed point, an outside bend, the open end, or where the pipe runs along a cold wall.
2. Warm water, not boiling. Pour warm water along the pipe, concentrating on the bends. A hot water bottle or a microwaved wheat bag held against it works well.
3. Reset the boiler. Once the pipe is clear, press the reset button. Most boilers restart within a minute or two.

What not to do

Never use boiling water. It can crack the pipe, and you have turned a ten-minute fix into a repair.

Do not use a naked flame or a heat gun. On a plastic pipe this is exactly as bad as it sounds.

Do not keep hitting reset. If it will not restart after two attempts, something else is wrong and repeated resets will not help.

Stopping it happening again

A pipe that freezes once will freeze again. The permanent fixes are worth doing before next winter: lagging the external run with proper waterproof insulation, increasing the pipe diameter outside, or re-routing it internally to a soil stack so it never sees the cold at all.

If it is not the condensate

If the pipe is clear and the boiler still will not fire, stop there and ring someone. And if you can smell gas at any point, leave the house, do not touch light switches, and ring the National Gas Emergency Service on 0800 111 999 before anyone else.