What Is a Condensate Pipe?

A condensate pipe’s main function is to transfer acidic waste water produced from the boiler’s condensing process, safely away and out the property via an external drain.

Condensate Pipe with lagging

Condensate Pipe with lagging

This is the basic definition, but if you’re unfamiliar with the different terms, components and procedures that come with having a boiler installed, things can often get a bit overwhelming. It, therefore, makes sense to break things down so that you can build your knowledge from the ground up.

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Be as prepared as possible for the coming winter months. If you recall the Beast from the East back in 2018, the storm was making its way across the country with temperatures as low as -18 in some areas. When this happened, many older boilers started to shut down, unable to cope with the cold. At it’s peak, Boilerhut would typically field hundreds of phone calls in one day during this time, most of which were because of frozen condensate pipes.

This is exactly the sort of information that you’ll find in this latest post. By the time you’ve finished reading, you’ll have an understanding of the following;

  • A detailed definition of the condensate pipe and how it works
  • What the condensate pipe is made of and how this may improve your boiler’s efficiency
  • How a condensing boiler and condensate pipe saves you money compared to older systems
  • Basic history of condensing boilers being introduced in the UK and what it’s meant for improved boiler performance
  • Guidance on different size of condensate pipe and how this can affect your central heating system

As well as this, you can learn more about condensing boilers and the various components that are used in some of our other blogs:

So without further ado, let’s explore ‘What is a Condensate Pipe’ together!

What does a condensate pipe do?

Condensate pipe exiting into drain

Let’s start with a very basic definition. The main job of the condensate pipe is to transfer the wastewater that gets produced from the boiler, into the sewer. As simple as this may appear to be however, it’s important to know why this happens.

What is condensate?

Condensate is what gets produced when your boiler is being used to heat your home or produce hot water. This is because when your boiler burns fuel, a chemical reaction causes produces carbon dioxide and water vapour by-products. Once enough heat is produced from this vapour, it’s condensed back into water form. After this water has been collected, it gets expelled out the condensate pipe.

A typical hour-long operation of your boiler may produce around 2 or 3 litres of the condensate water waste. This waste generally gets released in 300ml increments, and is often the reason you might hear water travelling through your pipes during intervals.

What are condensing boilers?

Because of regulations originally introduced in 2005, every single boiler in the UK needs to be a condensing boiler. This is regardless of whether it’s a combi, open vent or a sealed system boiler.

This means that even if you buy a Viessmann, Worcester Bosch, Baxi or Vaillant – it will still be a condensing boiler.

Viessmann Vitodens 050-W Combi Boiler

The Viessmann Vitodens 050 is an excellent example of a modern condensing boiler.

Condensing boilers are able to extract excess heat produced from waste gases. In doing so, they’re lot more efficient because the heat that would otherwise be lost due to waste is repurposed and used to heat your home.

Condensing boilers are designed as a huge improvement of efficiency compared to non-condensing.

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How do condensing boilers work?

When a condensing boiler is operational, the two heat exchangers housed inside are able to recycle any excess heat produced and return it back to the system. No modern boilers (condensing boilers included) need a pilot light, as they use an electric spark when hot water is required.

For further help, the diagram below offers a visual explanation of how a condensing boiler works:

How a condensing boiler works

How can a condensate pipe save me money?

How to save yourself money

Stricly speaking, it’s far more than just your plastic condensate pipe that does this. It’s also because of the way condensing boilers are designed internally.

By utilising a high quality internal boiler design, condensing boilers are able to recover more heat. They do this by recycling gases normally expelled through the flue. Before they’re released, they pass through the boiler’s internal heat exchanger, which then recycles the heat from these gases that’s otherwise lost.

What size condensate pipe do I need?

When it comes to having a condensate pipe installed, there are certain restrictions in terms of size and position that must be considered. The minimum size for the overflow pipe is 21.5mm, and needs to be made from a non-metallic material for allowing a reasonable flow of water waste. However, this 21.5mm only refers to the size for internal terminations. External condensate pipes need to be a minimum of 32mm and insulated with Type-O insulation.

As well as this, the material needs to be plastic, because the condensate water waste that gets expelled from the system has slight acidic content. The pipe needs to be made from a material different to copper or steel that won’t be corroded.

Do they need to be positioned a certain way?

Condensate pipes need to be positioned so that they have a gradual fall, with a minimum drop of 44mm per metre. Below is a visual comparison of what is required and what must be avoided for a high-quality condensate pipe installation.

How your condensing flue needs to be positioned

On many occasions, the boiler will be installed in the middle of the property. As a result, you won’t have the access or fall required for an internal condensate pipe.

Because many bathrooms are nicely tiled and decorated, it means you’re unable to bring your floorboards up to arrange a solution. In times such as these, you would use a condensate pump, a down pipe or soil stack.

Soil stack and downpipe diagram

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Condensate Pumps

Condensate pumps collect and disperse the dangerous waste water that’s produced by the boiler during its operation. As mentioned, they’re used when gravity is otherwise unable to move this water. A good example of this is if your boiler were to be installed in a basement flat. Because it’s below ground level, it’s unable to move this waste water and you’ll need a condensate pump for easy access.

I don’t have an outside drain for my condensate pipe

On some occasions, there won’t be an outside drain for the waste water to exit out of the condensate pipe. If this applies to you, then don’t panic. You can also get what’s called a ‘Soakaway’.


While their name is somewhat self-explanatory, Soakaways offer an excellent alternative if you don’t have an external drain for the condensate pipe. The basic purpose is to control the release of condensate water waste safely into the ground. It catches this water waste and through the use of limescale chippings, is able to neutralise the acidic content of the escaping water.

Limescale chipping neutralise acidity in the water waste

Most soakaways will look something like this, along with limescale chippings

By using a soakaway you can also reduce the probability of any acidic condensate to remain in the system itself. This will extend the lifespan of the boiler, as well as improve the overall quality of your central heating system.

NOTE: If you use a Cesspit in your garden, be aware that you will be unable to position the condensate pipe near this.

My condensate pipe is frozen

Firstly, this is one of the more common combi boiler problems that comes up when talking about your heating system’s condensate pipe.

As winter approaches, it’s often more likely for condensate pipes to become frozen. This is especially true if the pipe in question runs outside without proper protection or insulation.

When this occurs, the water travelling down the pipe freezes, which automatically prevents your boiler from being able to fire up. The result? No heating or hot water during the winter…right when you need it the most!

How do I unfreeze my condensate pipe?

Thankfully, it’s quite easy to take appropriate measures for a frozen condensate pipe. When this occurs, there are usually some pretty good signs that you can look out for. This includes keeping an eye on your boiler to see if it displays particular error codes. Another telltale sign that this might be the problem, is that you can hear a ‘slurping’ sound when your boiler gets switched on.

Use warm water

This is the easiest way for you to deal with the issue. You can simply pour warm water over the affected area of the pipe which will eventually thaw. One thing to remember is not to let the water pour over any nearby paths or walkways. While it seems obvious, this can sometimes go unnoticed and as a result, can cause a major slip hazard.

how to unfreeze your condensate pipe

Use a hot water bottle

If you don’t have the right kind of access for pouring warm water directly over your frozen condensate pipe, then there are other ways for you to deal with this issue. You can also place a hot water bottle over the affected area. The benefit of this method is that you can avoid any splash damage from hot water, and you’ll likely avoid creating another slip hazard.


Your condensate pipe can terminate above or below ground. If above ground however, you will have to have it lagged. This will add further insulation to the pipe itself, and ensure that it can continue to function through the colder winter months. Any professional installer should include lagging as standard as a preventative measure.

What if it doesn’t work?

You’ll be able to tell pretty quickly if these options work or not. If you find that the boiler still isn’t working after ice has been melted down or removed, the next step should be to have someone come in to inspect the boiler. We must stress as a national installer ourselves, you need to make sure you have a fully-qualified, highly-trained professional carry this procedure out for you. They will need to be confident in what they’re doing, and have an understand of how this can be fed to the general public.


As you can see from this article, while there are a few things to consider when asking ‘What is a condensate pipe?’, the answer is relatively simple. By expelling the water waste from your system down an external drain, the dangerous fumes that are produced can escape in a safe manner away from the property. At the same time, because of the nature of condensing boilers, you’re able to save a lot of money since your boiler is able to recycle any lost heat to continue to heat your home.