How to Thaw a Frozen Condensate Pipe and Prevent It From Freezing

Frozen condensate pipes are a frequent issue with newer condensing boilers. Discover how you can quickly fix the issue

Frozen condensate pipes are a frequent issue with newer condensing boilers. When the condensate pipe freezes, the boiler should shut down automatically. 

Not all boilers are condensing boilers, so this article won’t apply to everyone. 

Every boiler installed in a UK home past April 1st 2005, should be a condensing boiler by law. 

Condensing boilers were also fitted before then, so don’t rule out having a condensing boiler just because it was installed pre-2005.  

To find out if you have one, simply locate the typically white PVC pipe mounted on the external wall at the rear of the boiler. This is the condensate pipe, designed to transport wastewater from the boiler into the drain. It may or may not be covered in foam insulation. 


If this pipe freezes, the boiler will stop working. Here’s how to thaw a frozen condensate pipe. 

What is a condensate pipe?

The condensate pipe transports acidic wastewater from the boiler into the drain. Wastewater accumulates as the boiler recycles additional heat from the flue gases, improving overall energy efficiency. Improved efficiency is why condensing boilers become obligatory for new installations post-2005. 

There is one small downside to condensing boilers. In the winter, it’s not too uncommon for the condensate pipe to freeze. If it does, your boiler will stop working. 

Luckily, this is a fairly benign and easy-to-solve boiler issue.

Why do condensate pipes freeze?

Condensate pipes carry water which is liable to freeze in cold temperatures. If the pipe freezes, your boiler should shut down automatically. 

If applicable, your boiler may display one of the following error codes to indicate the condensate pipe is frozen: 

ATAG - 133

Baxi - E133 or E28

Glow-worm - F1, F4, F28 or F29

Main - E133

Potterton - E133

Vaillant - F28 or F29

Viessmann - F4EE

Worcester Bosch - EA229 D5

Uninsulated or poorly-sized condensate pipes are most vulnerable. Thinner, white PVC pipes without insulation are most liable to freeze. 

There are regulations advising installers on fitting condensate pipes to avoid freezing, but installers don’t always follow these, and the pipe may still freeze anyway. 

Finding the condensate pipe

Locating the condensate pipe should be straightforward. Find your boiler and locate a PVC pipe headed diagonally across the external wall behind it. The condensate pipe should connect to your vertical drain. 

White Frozen condensate pipe on brick wall
Above: Frozen condensate pipe


The condensate pipe should be quite thin - 32mm is the most common. It may or may not be insulated. It’s not uncommon for insulation to wear and erode over time, so many of these pipes start freezing in winter. 

How do I thaw a condensate pipe?

Fortunately, thawing a frozen condensate pipe is relatively straightforward. 


First, feel the length of the pipe to get an idea of where the blockage is - you’ll feel some sort of resistance where the water has frozen. Most blockages occur at the bends. Don’t worry too much if you can’t find it. 


Then, you have two options for thawing the pipe:

  1. Smother the pipe with a towel soaked in hot water. You can also use a microwavable heating pack or hot water bottle. You may need to repeat the process multiple times. 
  2. Fill a jug, watering can or another vessel with hot water (not boiling). Gently pour the water across the length of the pipe. You might need to repeat this a couple of times. 

Steaming hot water coming out of tap
Above: Water should be hot, not boiling!

Be careful not to slip on any spilt water that starts to freeze on the ground! That’s pretty much it - either of these techniques should be pretty effective in thawing the pipe. 

Once thawed, you may need to reset your boiler or turn it off and on again. Some models will detect the blockage has disappeared and fire up again within a few minutes. If your boiler displayed an error code, this should disappear. 

How to prevent a frozen condensate pipe?

Frozen condensate pipes are relatively easy to fix, but it’s best to prevent them from freezing in the first place. 

First and foremost, installers should fit condensate pipes in accordance with specific regulations and the manufacturer’s guidance. This isn’t always the case in practice, and many condensate pipes are left exposed to the elements. 


Check your pipe to ensure it meets the following criteria: 

  • All condensate pipes should be insulated with weatherproof external PVC-coated insulation.
  • The diameter of the pipe must be 32mm.
  • The pipe must terminate as close to the drain as possible.
  • The pipe should take the shortest route to the drain available, minimising exposure to cold temperatures. 

If your pipe fails to meet these criteria, you can try and address the issue yourself or ask a trained heating engineer to help. Don’t dismantle the pipe without shutting off your boiler. If major alterations are needed, it’s best to ask for advice. 

Boiler condensate pipe insulation

Condensate pipes are relatively easy to insulate yourself. You can buy insulation kits specifically made for condensate pipes from plumbing and DIY stores. Otherwise, installing basic piping insulation is better than nothing. 

To install, simply wrap the insulation around the width of the pipe and secure it with cable ties. 

Black pipe insulation around white pipe
Above: Pipe Insulation

Some boilers are fitted with a simple device called a condensate syphon, which adjusts the flow of water down the condensate pipe, ensuring that any water discharged is warmer, preventing freezing. 

Worcester Bosch provides their own condensate syphon, which prevents freezing at temperatures down to minus 15C for up to 48-hours. This is a cost-effective solution for chronically freezing pipes. 

Summary: How to Thaw a Frozen Condensate Pipe and Prevent It From Freezing

Condensate pipes are installed at the back of newer condensing boilers. They’re easy to identify - look for a diagonally-angled pipe protruding from the external wall near your boiler. 

While they should be insulated, condensate pipes can freeze in winter. This will stop your boiler from functioning properly. 


Thawing the pipe with warm water is straightforward. Then, consider properly insulating the pipe to prevent future issues. 


As always, contact our heating engineers if you’re unsure of what to do. 

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