How do I thaw a frozen condensate pipe?

This is the single most common reason a boiler stops on the coldest morning of the year, and it is one of the few boiler faults a householder can safely resolve. It is also frequently confused with a frozen water pipe, where the correct first step — turning the water off — is exactly wrong here.

Safety

Step by step

1. Recognise the symptoms.The boiler stops during or just after a cold snap, often with a fault code, and there may be a gurgling or bubbling noise as it tries to fire. Heating and hot water both go. It is very characteristic of a hard frost.
2. Understand what the pipe is.A condensing boiler produces acidic waste water as it extracts extra heat from the flue gases. That drains away through a plastic pipe, usually 22mm, often run outside to a drain. It carries a trickle, which is why it freezes.
3. Do not turn the water off.This is not a supply pipe. Advice for frozen water pipes — isolate at the stopcock — does not apply and achieves nothing here.
4. Find the pipe outside.White or grey plastic, usually 22mm, emerging from the wall near the boiler and running down to a drain, a gully or a soakaway. It is the only plastic pipe coming out at that point.
5. Find where the ice is.Usually at the most exposed point — an outside bend, the open end, or where it enters a drain. Feel along it: the frozen section is noticeably colder and may be visibly iced at the tip.
6. Use warm water, not boiling.Comfortably hot from the tap — around the temperature of washing-up water. Boiling water can crack the plastic and, when it runs off, freezes into ice underfoot exactly where you are working.
7. Pour it along the outside of the pipe.Over the pipe from the top of the frozen section downwards, repeatedly. A watering can or a jug. You are warming the pipe from outside, not pouring anything into it.
8. Work from the end back if needed.If the blockage is at the open end, warming that first gives the melting water somewhere to go. Otherwise you clear ice above a plug that is still frozen.
9. Use a hot water bottle or cloths for an awkward spot.A hot water bottle held against the pipe, or hot cloths wrapped round and replaced as they cool, work well where pouring is impractical. Slower and just as effective.
10. Reset the boiler.Once water flows freely again, press the reset button. Most boilers restart within a few minutes. If it locks out again immediately, there is still ice in the pipe or a separate fault.
11. Insulate the pipe before the next frost.Weatherproof foam lagging on the whole external run. It costs very little and is the difference between this happening every winter and not at all.
12. Get it rerouted if it recurs.A pipe with a long external run, a low fall, or a dip holding water will keep freezing. An engineer can increase the diameter, improve the fall, or route it internally to a waste pipe, which is the permanent fix.
13. Call an engineer if it will not restart.If water flows and the boiler still will not fire, or it shuts down again with no ice present, that is a different fault. Do not open the boiler.

Tips

• Insulate the external condensate run in autumn along with the outside tap. It takes ten minutes and prevents the most common cold-morning boiler failure.
• Know where the pipe is before you need to find it in the dark at six in the morning. Look for it now.
• If the boiler shows a fault code, note it before resetting. It tells an engineer a great deal if the problem turns out not to be ice.

Common mistakes

• Pouring boiling water on the pipe — It can crack the plastic, and the run-off freezes into ice on the ground where you are standing. Comfortably hot tap water works perfectly well.
• Turning the water off at the stopcock — That is the procedure for a frozen supply pipe. The condensate pipe carries waste from the boiler, and isolating the water supply does nothing for it.

If it doesn't work

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