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Winter Weather Advice - How to thaw a frozen condensate pipe

During winter months and extended periods of low temperatures, it's possible that your condensate pipe may freeze, especially if the pipe exits outside of your property. If your Vaillant boiler is flashing with either a F28 or an F29 code or you fear your condensate pipe is frozen, you can follow these simple steps.

Where to find it

The condensate pipe is usually a white overflow pipe, similar to one you may find on a washing machine or sink waste pipe. It allows your boiler to dispose of condensation to an outside drain via this pipe.

If the liquid from the expelled condensation freezes over, it can cause blockages and build-up of pressure. Not only can this reduce the performance of your central heating system, but you can damage your boiler as a result of the blockage. The pipe will usually be located on the external wall outside from where your boiler is. We’ve included some steps you can take at your home to help solve this issue.

How does it work?

Once the water vapour has been condensed, it turns into water. A condensing boiler, on average, creates around 2 litres of condensate water per hour, and it needs somewhere to go! By having a condensate pipe, the water is safely disposed of with the rest of the waste water, just like a sink or washing machine.

How to protect it

Once you have successfully thawed out the condensate pipe, you can make sure it doesn’t happen again by insulating the condensate pipe. By insulating the pipe, you are reducing the risk of it freezing and blocking again in the future. All good DIY stores should sell pipe insulation, sometimes called pipe lagging and this can be easily fitted with little effort

To insulate your pipes, you should make sure that the water is flowing correctly out of the pipe. Once this is done, you should then measure out the length of the pipe so then you can make sure that you have a sufficient enough amount of insulation. Cut the length you need and fit it around the exposed pipe. For different types of insulation for your condensate pipe, please see further information here.

Impact of a frozen condensate pipe

During cold weather, it’s possible for your condensate pipe to freeze if the pipe is outside of your property as water is passing through the pipe. If this happens, it is most likely that your boiler will fail to start, meaning no heating or hot water.

If a frozen condensate pipe is to blame for your boiler failing to start, it means that you have to defrost the water that has built up within the pipe before it will work again.

How do I know if my condensate pipe is frozen?

Before attempting to thaw a frozen condensate pipe, it’s important to make sure that’s the cause of the problem. The first warning sign that your condensate pipe has frozen may be an error code on your boiler. On Vaillant boilers, you’ll get an F28 or F29 code. If there is another fault code on your boiler, please contact you local Vaillant installer.

Error codes on other makes of boiler may vary, but a tell-tale sign is a gurgling noise coming from the boiler.

If the boiler was working fine up until a bit of a cold snap, there’s a strong chance your issue is a frozen condensate pipe.

How to defrost your frozen condensate pipe

Luckily, you can solve this issue yourself, without incurring call out fees from the manufacturer or a heating engineer.

For those who have a Vaillant boiler, look for fault codes (F28 or F29) on the boiler’s display screen as this will indicate a blocked pipe. If you have a third-party boiler, different codes may be used or you may hear a bubbling and gurgling sound coming from your boiler whilst in use. This could be a sign that your condensate pipe has frozen, which will require immediate attention to ensure continued performance.

Note, pipes are more likely to freeze if your boiler and heating system have not been in consistent use during a period of freezing temperatures. This often happens when you leave your home for holiday and do not use your boiler. We advise to keep your boiler active on a scheduled timer to avoid pipes freezing again.

The condensate drainage pipe is a plastic pipe, typically grey or white, connected to the bottom of your boiler. It runs through the wall and outside the property for part of its length. Usually your condensate pipe will be frozen on the most exposed external point, which will typically be at the end of the pipe, just below the drain or around the bend where the insulation is at its thinnest.

If possible, check the end of the pipe for any signs of frost to confirm it is frozen internally.

To defrost a condensate pipe, pour warm water along the affected area of the pipe to melt and remove the ice blockage. Use a suitable container, like a watering can or pitcher and continue the process until either the ice falls from the pipe or until the contents of the pipe are fully drained.

It is important not to use boiling water from a kettle as this can damage or crack the pipe, causing damage that will be costly to repair and render your boiler non-functioning for the duration.

Instead, you can use a hot water bottle and place it against the pipe to assist with the process or you can wrap a blanket around the external pipe to help insulate and prevent the issue from occurring again throughout the winter months.


You should always check your boiler manual for instructions on how to reset your boiler correctly before undertaking this operation, then you should restart your boiler.

You can avoid having to repeat this process in future by insulating your condensate pipe. By insulating your condensate pipe you are reducing the risk of it freezing and blocking again. All good DIY stores should sell pipe insulation, this can easily be fitted with little effort.

If you want more advice on defrosting your condensate pipe, please call a gas safe registered engineer and they will be able to help you. You can find your local Vaillant installer here.

For an easy step-by-step breakdown of the process, you can download our handy guide for resolving a frozen condensate pipe.