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How to defrost a condensate pipe

Frozen Pipes

Even the most well-maintained boiler will struggle to function if its condensate pipe freezes. Thankfully, defrosting or thawing your condensate pipe isn’t a difficult job to complete. This step-by-step guide and how-to video will help you get your boiler up and running again in no time!

Jump to:

What is a condensate pipe?
Signs that your condensate pipe is frozen
Step-by-step: Safely thawing your frozen condensate pipe
How to prevent your condensate pipe from freezing
Should you pour boiling or hot water on a frozen condensate pipe?

What is a condensate pipe?

A condensate pipe transfers waste water from your boiler to your outside drain.

It’s typically a white or grey plastic pipe that travels from your boiler through the wall directly outside where your boiler is found.

During chilly weather, this pipe may freeze, resulting in blockages that cause condensation to back-up into your boiler. If left untreated, this build-up can cause your boiler to shut down as a safety feature.

Signs that your boiler condensate pipe is frozen

There are many different brands and models of boiler, so they don’t all react in the same way if the condensate pipe is frozen, but there are some common signs to look out for. These include:

  • The heating not coming on when expected
  • The boiler not firing up when you switch on the heating or run a hot tap
  • The boiler making an unusual gurgling noise
  • The boiler’s display panel (if it has one) displaying a fault code or a warning light

Step-by-step: Safely thawing your frozen condensate pipe

Take a look at our video below, which shows you how to thaw a frozen condensate pipe.

If you suspect your condensate pipe has frozen, the following steps should help to get things back to normal in no time.

Remember, if you don’t feel confident following these steps, or if your frozen condensate pipe can't be reached safely, then you should contact a Gas Safe registered engineer to help.

1. Check the condensate pipe is frozen

Your condensate pipe can only freeze in low temperatures, so if it hasn’t fallen below freezing overnight or during the daytime, it’s unlikely that the problem is a frozen boiler condensate pipe.

Depending on your boiler’s make and model, a frozen condensate pipe may be detectable by a “fault code” or warning light on the display. Gurgling or bubbling sounds coming from the boiler or condensate pipe itself can also be signs that your pipe has frozen.

Please do not use sharp objects, like a screwdriver, to investigate or try to unblock your pipe, as this may cause damage to the condensate pipe.

Checking Thermostat

2. Find the blockage

Condensate pipes typically freeze at their most exposed point: this might be the open end of the pipe, or at the bend or elbow.

When running your hand over the pipe, you will find a section that’s colder than the rest - this is where the blockage has developed.

If your boiler is located upstairs, meaning that your condensate pipe can’t safely be reached by you because it’s high up on an outside wall, you might not be able to do this. You should contact a Gas Safe registered engineer instead.

Frozen Pipes

3. Use hot water to thaw the pipe

Using a jug or a watering can, carefully pour hot water along the length of the pipe, repeating the process until the pipe thaws.

Don’t use boiling water, as it can crack or damage the pipe. If you’re using a kettle, make sure to let it stand for at least 15 minutes once it’s boiled. The ideal temperature for the water to thaw your frozen condensate pipe is between 60°C and 70°C.

Alternatively, you can use a hot water bottle or a heat pack to add longer-lasting heat that will defrost the pipe. This can be more effective than pouring hot water over the condensate pipe if the frozen section is large.

Kettle And Heat Pack

4. Restart your boiler

Once the frozen section has been thawed and cleared, check your boiler manual for instructions on how to reset the boiler correctly.

If you follow the directions correctly, your boiler should restart.

Restart Boiler

5. Repeat to avoid defeat

It may take several attempts before your boiler resets, but if it doesn’t, you may need to call a Gas Safe registered engineer for further help.

Reset Boiler

How to prevent your condensate pipe from freezing

If your condensate pipe isn’t already insulated, once it has thawed out, wrap it in some old towels to help prevent it freezing again while the temperatures are still low.

Condensate pipe insulation

As soon as possible, pop to your local DIY store and buy foam pipe insulation for the condensate pipe – just make sure to check it’s for external use. This insulation comes in a range of sizes, so measure the pipe’s diameter before you buy.

You can fit condensate pipe insulation yourself, as long as you can safely reach the pipe. You will need to cut it to the right length, and it has a slit on one side so that you can wrap it around the pipe. Once the condensate pipe insulation is in place, you can twist it so that the slit faces the wall and use some zip ties in a couple of places to make it even more secure. 

Other ways to avoid a frozen condensate pipe

Some people find that running their heating on a low temperature overnight is often enough to prevent the condensate pipe, along with other pipework, from freezing. This does mean using more energy, but it also means that the property takes less time to heat up in the morning because it’s already warmer, so it doesn’t always mean higher bills. 

During extreme weather conditions, even quality insulation may not be enough to prevent the condensate pipe from freezing. It may help to temporarily run your boiler with the boiler thermostat set as high as possible for as long as the cold spell lasts. This reduces the amount of condensate that forms and so will lower the chances of the water freezing in the pipe. However, it will cost more to run the heating at a higher temperature, radiators will get very hot and, if you do this, it’s important to remember to change the boiler thermostat settings back to normal as soon as the very cold weather passes.

If you continue to experience problems, it’s best to call a Gas Safe registered engineer to help.

Should you pour boiling or hot water on a frozen condensate pipe?

Two Hot Water Bottles

Never pour boiling water onto your condensate pipe, as it might cause it to crack, warp or melt. We recommend only pouring hot or warm water onto your problem pipe once it has had time to cool.

If you’re using a kettle, make sure you’ve let it cool down for at least 15 minutes after boiling before using it. It’s always a good idea to test the temperature before pouring the water onto the pipe but make sure that you don't scald yourself when doing so. The ideal temperate for water to pour over a frozen condensate pipe is somewhere between 60°C and 70°C. 

You may want to follow this up by using a hot water bottle or a heat pack against the pipe to help it thaw out thoroughly over a longer period of time. 

Be careful when pouring the water – it may freeze on the ground and become a slip hazard.