How to unfreeze your condensate pipe Re February 6, 2023

How to unfreeze your condensate pipe

Frozen condensate pipe 2

As the temperature is dropping, your boiler will need a little TLC to work as it should. 

In the winter, you might start noticing more issues with your boiler.

One of the most common problems with your boiler during cold weather is a frozen condensate pipe. While a qualified heating engineer is usually the first port of call, you will be left without the use of your boiler and will have to pay costly call-out fees. 

Luckily, you can do this job yourself with a little guidance, and we’re here to walk you through it step-by-step, so put down your wallet! 

What is a condensate pipe?

A condensate pipe is essentially a waste pipe that carries residual water safely away into a drain. This water is acidic in nature and is produced when the water in the boiler comes into contact with flue gases. The water can terminate inside to a waste pipe or directly outside to a drain. The process is called condensing.

Condensing increases your boiler’s efficiency and ensures that you get your desired heat supply, that’s why we always say that a condensate pipe is essential for a healthy boiler.

Tip – to find the condensate pipe, look for either a 21.5mm white, plastic pipe or 32mm white, plastic pipe. In some cases, you will find it in 40mm white, plastic pipe. The pipe will be connected to your boiler, and it will be white, we might have mentioned that…

Why does a condensate pipe freeze?

During winter, the water in the condensate pipe may freeze when exposed to very cold temperatures. When this happens, the boiler will continue to dispose of this wastewater but as it now has nowhere to go – due to the water freezing, and essentially blocking the route to the drain, it backs up into the boiler.

To prevent any damage, your boiler will then lock itself out and will let you know via a fault code or a sequence of fault lights, depending on what boiler you have. 

You might also be able to hear a gurgling noise inside the boiler. Tip: It’s not your boiler freshening it’s breath for a tinder date.

So, how does it work?

Your boiler operates by burning gas – when this happens, carbon dioxide and water vapour are given off as by-products which are then expelled externally through your boiler’s flue system. There is a lot of heat loss with this process. What a condensing boiler does is recover some of this heat loss from the by-products. They will pass through the heat exchanger where they cool and condense back into liquid form. The heat which is recovered in this process is used to warm the returning water from your heating system.

The good news is that most, if not all condensing boilers are now rated over 90% for efficiency compared to the older boilers which can fall as low as 60%. That’s a substantial difference which makes it obvious to see the logic behind buying one.

Tip – To easily understand what this means, think of 100% efficiency as £1. If your boiler is only 60% efficient, you lose 40p for every £1 you spend on your heating.

How do you safely defrost your condensate pipe?

There are multiple ways to unfreeze your condensate pipe. Here is our step-by-step guide: 

1. Confirm the condensate pipe is frozen

Depending on your boiler, a frozen condensate pipe can be identified by a fault code or warning light on the boiler’s display. You may also want to listen out for a gurgling or bubbling sound coming out of your boiler.

The error code varies with every boiler brand. Here are a few brands’ error codes:


Fault code


E133 or E28


L2, LF, or F2


F1, F4, F28 or F29


E133, E28, E1 or E33

Worcester Bosch

EA229 or D5


F28 or F29


F28 or F29



If your boiler manufacturer isn’t listed above, take a look in the boiler manual, or ask Dr Google. If you still can’t find the fault code, get an engineer to check things over.

2. Find where the blockage is

Try to locate the blockage so you know exactly where to pour your hot water. If you have a very long condensate pipe and are struggling to find the blockage, you might want to pay special attention to any bends and end of the pipe as you are more likely to find a blockage there. The pipe is probably frozen at its most exposed point.

Tip – running your hand over the pipe until you can feel the area that is relatively colder than the rest can help you spot the blockage faster. 

These methods are very simple to carry out and are completely safe but if you’re not confident in attempting these or have a concern over having damaged your condensate pipe, it’s recommended to contact a qualified heating engineer. 

3. Pour warm water to defrost the condensate pipe

Time for action! Pour warm water along the pipe and repeat the process until the condensate pipe is defrosted. Alternatively, use a hot water bottle or a heat pack to defrost the pipe. 

Tip – Don’t use boiling water as it could cause the plastic pipe to melt. Instead, boil a kettle and let it cool for 15 minutes.

These methods are very simple to carry out and are completely safe but if you’re not confident in attempting these or have a concern over having damaged your condensate pipe, it’s recommended to contact a qualified heating engineer.

4. Restart your boiler

Once you’ve defrosted the condensate pipe, it’s time to give your boiler a quick restart. Just check your boilers manual for instructions on how to reset the boiler. 

5. Still frozen?

You might have to repeat the process several times to get the required result, however, if it’s still not working, we recommend calling a qualified heating engineer.

Is your boiler is a condensing boiler?

Check underneath your boiler to see if you have a white plastic pipe connected to it. Condensing boilers were introduced in April of 2005, so if your boiler was installed after this period, it is almost definitely a condensing boiler.

Benefits of a condensing boiler

There are multiple benefits to a condensing boiler, some of these include:

  • Lower emissions – Condensing boilers have substantially lower emissions than non-condensing boilers.
  • High efficiency – This means that a lot less gas needs to be burned to operate your heating/hot water. Less gas burned means lower gas bills.

In some cases, it won’t work on the first attempt so it might be that you need to repeat the process. It is possible that your condensate pipe can be frozen all the way back into the boiler. If this is the case, do not attempt to unfreeze the inside of the boiler as you run the risk of damaging components inside. Additionally, the front cover of a boiler should only be removed by a qualified heating engineer.

If you are concerned about your boiler and you aren’t sure whether your condensate pipe is to blame, contact us – we’re always happy to offer boiler advice.

How to prevent your condensate pipe from freezing?

You now know how a frozen condensate pipe can affect your boiler and heating system. You also know that it can, in most cases, be easily rectified. But is there something you can do to prevent it from getting to that stage at all?

Ways to prevent a frozen condensate pipe

  • Insulate the external pipework – You can buy pipe lagging from any builders or plumbers’ merchant relatively cheaply. All you have to do is cut it to size and pop it on your condensate pipe.
  • Increase the temperature of your heating – Although not as effective as the pipe lagging, increasing the temperature of your heating will reduce the amount of condensate waste produced.
  • Install a drain cover – add a leaf guard over the open end of your drain to get rid of any wind chill. 
  • You can have trace heating installed, which is a cable that runs along the inside or outside of your condensate pipe to keep it warm. While this sounds like a good idea, it can be costly.

Condensate pipe regulations

There are also some condensate pipe regulations that have to be followed when they’re fitted outside
  • The pipe must take the most direct and “most vertical” route 
  • The diameter of the pipe must be 32mm
  • Insulate with external grade PVC coated insulation
  • The end of the pipe is cut at 45 degrees as opposed to a straight cut and must be as close to the drain as possible


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