Frozen Condensate Pipe: Getting Your Boiler Going Again
This is the most common reason a boiler stops working on a cold morning, and it is one of the very few boiler faults you can often deal with yourself in ten minutes.
Every winter we take calls that turn out to be this, and we would rather tell you how to fix it than charge you to come and pour warm water on a pipe.
- Gas Safe registered 981362
- Written from jobs we have actually done
- Milton Keynes and 20 miles around
- 8am – 8pm, Mon to Sat
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What is actually happening
Modern condensing boilers extract so much heat from the flue gases that water vapour condenses out of them. That water is mildly acidic and it has to drain away, which it does through a plastic pipe, usually white or grey and around 22mm across.
If that pipe runs outside, and the temperature drops far enough, the water in it freezes. The blockage backs up into the boiler, the boiler detects it, and it shuts itself down to protect itself. That is the safety system working correctly rather than a failure.
How to tell this is your problem
- It happened overnight or in the early morning, in freezing weather
- The boiler shows a fault code. Many display a specific condensate or blockage code, and your manual will identify it
- You may hear a gurgling or bubbling from the boiler when it tries to fire
- Everything else was fine yesterday
- You can see a plastic pipe leaving the boiler and going out through the wall
If it is mild outside, this is not your problem and something else is going on.
Thawing it, safely
- 1
Find the external section.
Follow the plastic pipe from the boiler to where it exits the wall. The frozen part is usually the exposed run outside, and most often at a bend, at the end, or wherever it is most exposed.
- 2
Use warm water, not boiling.
Warm from the tap is right. Boiling water can crack the pipe or its fittings, and then you have a genuine repair rather than a frozen pipe. A jug or a watering can is ideal.
- 3
Pour it along the pipe
working from the lowest end back up towards the wall. Wrapping a warm cloth or a hot water bottle around the pipe also works and is gentler.
- 4
Reset the boiler.
Once thawed, use the reset button. It may take a couple of minutes to fire.
Never use a blowtorch or any naked flame, do not use boiling water, and do not go up a ladder in icy conditions to reach a pipe at height
If the pipe is not safely reachable from the ground, call someone rather than risking it.
If it will not restart
If the boiler has thawed and still will not run, or if it locks out again immediately, the problem is something else and it needs looking at. See our boiler repair page.
Stopping it happening again
This is the important part, because if it froze once it will freeze again, and thawing a pipe every morning is not a fix.
- Lag the external run properly. Waterproof pipe insulation designed for outdoor use, not indoor foam that soaks and freezes solid.
- Have it re-routed internally which is the permanent answer where it is achievable. A condensate pipe that runs inside to an internal drain cannot freeze.
- Increase the pipe diameter on the external run. A wider pipe is far less prone to freezing, and current guidance for external condensate runs reflects that.
- Improve the fall. Water sitting in a dip freezes. A continuous fall to the discharge point leaves less standing water.
- Shorten the external run. The less pipe outside, the less exposure.
- Do it in October, not January. This is a straightforward job in mild weather and a miserable one in a cold snap.
While you are at it
The condensate pipe is one of three that freeze in a typical house. The other two are loft runs sitting above the insulation line and the outside tap supply. All three are worth dealing with in the same autumn afternoon. See our burst pipe page and outdoor tap installation.
Frequently asked questions
Is a frozen condensate pipe dangerous?
No. The boiler shuts down deliberately to protect itself, which is the safety system working as intended.
Can I use boiling water?
No. It can crack the pipe or its fittings. Warm water from the tap is what you want.
How long does it take to thaw?
Usually a few minutes with warm water. If it takes much longer, the blockage may be further along or somewhere you cannot reach.
It refroze the next morning.
Then it needs a permanent fix: lagging, re-routing internally, a wider pipe, or a better fall. Thawing it daily is not a solution.
The pipe is high up and I cannot reach it.
Do not go up a ladder in icy conditions. Call someone.
Why do modern boilers even have this pipe?
Because they are condensing boilers, which extract more heat from the flue gases than older ones did. That efficiency is exactly what produces the condensate.
What to do about it
If it will not restart
Call 07514 312117 or send a WhatsApp with a photo of the fault code. And if you want it re-routed or lagged before next winter, that is an autumn job worth booking. Gas Safe registered 981362, CIPHE member, fully insured.