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What is a Condensate Pipe?

What is a condensate pipe, and why do you need one.
Read our guide today.

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What is a Condensate Pipe?

Witten by Bernard Morgan (Engineer) & Adam Apperley (Marketing). Published: October 8, 2019. Updated: May 13, 2024

A condensate pipe’s primary function is to safely channel acidic waste water produced during your boiler‘s condensing process to an external drain. All modern condensing LPG, oil and gas boilers have these pipes whether you have a system boiler or a storage combi boiler.

This guide aims to break down the complexities surrounding condensate pipes, from their construction and material to their impact on boiler efficiency. You’ll also learn about the regulations governing these pipes and how to deal with common issues like freezing during winter.

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What is condensate?

Condensate is acidic water. It is produced when your boiler is used to heat your home or produce hot water.

This is because when your boiler burns fuel the chemical reaction produces carbon dioxide, water vapour and other by-products. Once enough heat is extracted from this vapour, it’s condensed into liquid form. After collecting this water, it gets expelled through the condensate pipe.

An hour-long boiler operation may produce around 2 or 3 litres of condensate waste. This waste gets released in small increments, which is often why you might hear water travelling through your pipes during everyday use.

Why Do We Need Condensate Pipes?

As condensate is acidic it cannot be expelled onto the ground. It must go into a waste water pipe, like a sink or washing machine drain.

If you don’t have a drain nearby a condensate pump can be used to move the liquid from the boiler to a drain. Another option is a soak away. This is a specific hole in the ground with alkaline chips to neutralise the acid.

What Are Condensing Boilers?

Regulations initially introduced in 2005 mean 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.

Condensing boilers can extract excess heat produced from waste gases. Doing so is more efficient because the heat that would otherwise be lost into the atmosphere.

Condensing boilers are a considerable step forward compared to non-condensing boilers of the past. However, they are more complex and require yearly servicing to ensure ongoing operation.

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How do condensing boilers work?

When a condensing boiler is operational, the two heat exchangers inside can recycle any excess heat and return it to the system. No modern boilers need a pilot light, as they use an electric spark when required.

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

How a condensing boiler works

A simple diagram of the workings of a heating system with a condensing boiler.

How Can A Condensate Pipe Save Me Money?

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

Well designed condensing boilers can recover more heat than others. They do this by recycling gases usually expelled through the flue. Before they’re released, they pass through the boiler’s internal heat exchanger, which then recycles the heat from the otherwise lost gases.

Condensing boilers are highly efficient, which means you’re spending less on your gas, LPG or oil bills.

For example, the latest Viessmann Vitodens 100-W is 94% efficient – so for every £1 you spend on gas for heating, 94p goes to the house, and 6p is lost through waste.

Compare that to a newly installed non-condensing boiler from before 2005, which would have been 78-80% efficient, and will continue to lose efficiency over time. Even more shocking is that an old non-condensing boiler today will be at least 17 or 18 years old, meaning it is probably only 50-60% efficient now. And back boilers are likely even less efficient than that.

The heating costs for a 50% efficient non-condensing boiler are double what a non-condensing boiler would be. Remember, 50% efficient means 50p out of every £1 goes out of the flue into the air.

This is why it’s often economical to look at replacing a non-condensing boiler as soon as possible. It might have a significant upfront cost, but it could pay you back that money in 3-5 years. You can get a new boiler quote instantly online and it will probably be less than you think.

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What Size Condensate Pipe Do I Need?

When it comes to having a condensate pipe installed, certain restrictions in terms of size and position must be considered. The minimum size for the overflow pipe is 21.5mm to allow 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.

In addition, the material needs to be plastic because the condensate water waste expelled from the system is slightly acidic. The pipe must be made from a material that can’t corrode, meaning copper or steel cannot be used.

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.

If your boiler is in an airing cupboard, the condensate pipe can run into a bathroom and terminate in the waste pipe. However, many bathrooms are decorated with tiles, meaning you may be unable to run the pipe under the floor. In these cases, you would use a condensate pump, a down-pipe or a soil stack.

Soil stack and downpipe diagram

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 cannot move this water. A good example 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.

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

Limescale chipping neutralise acidity in the water waste

Most soakaways will look something like this. They are then filled with limescale chippings.

Occasionally, there won’t be an outside drain for the waste water to exit the condensate pipe. If this applies to you, then please don’t worry. You can also get what’s called a ‘Soakaway’.


Soakaways offer an alternative if you don’t have an external drain for the condensate pipe. The primary purpose is to safely control condensate water waste’s release into the ground.

The soakaway is filled with lime chippings that neutralise the acidic content of the escaping water.

By using a soakaway, you can also reduce the probability of any acidic condensate remaining in the system itself. This can extend the boiler’s lifespan and improve the overall quality of your central heating system.

NOTE: You cannot place a soakaway near to a cesspit.

My condensate pipe is frozen

Freezing is one of the more common combi boiler problems that come up when talking about your heating system’s condensate pipe. When cold, condensate pipes are more likely to become frozen. This is especially true if the pipe runs outside without proper protection or insulation.

When this occurs, the water travelling down the pipe freezes, preventing 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?

When this occurs, there are usually some pretty good signs that you can look out for. This includes checking 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.

how to unfreeze your condensate pipe

Use warm water

This is the easiest way for you to deal with the issue. You can pour warm water over the affected pipe, which will eventually thaw. One thing to remember is to keep the water from flowing over any nearby paths or walkways. While it seems obvious, this can sometimes go unnoticed and, as a result, can cause a significant slip hazard.

Use a hot water bottle

If you don’t have the proper access for pouring warm water directly over your frozen condensate pipe, then there are other ways 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 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 it can continue functioning through the colder winter months. Any professional installer should include lagging as a standard as a preventative measure.

What if it doesn’t work?

You’ll be able to tell quickly whether these options work. If you find that the boiler still isn’t working after the ice has been melted down or removed, the next step should be to have someone inspect the boiler.

You must ensure a fully qualified, highly-trained professional carries out this procedure for you. They must be confident in what they’re doing and understand manufacturers’ guidelines.


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 produced can escape safely 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 can recycle any lost heat to continue to heat your home.

What do I do next?

Are you still having issues? Tried defrosting it, looking for blockages or places it could become disconnected?

The next step would be to contact your local heating engineer to discuss them coming to have a look at it. They should be able to come over and let you know what they think is wrong.

Unless we installed your boiler, we don’t repair boilers or condensate pipes.

Once you have a diagnosis from an engineer, hopefully, you’ll know what needs to be done. If you can get it fixed, great. If it’s fixable but expensive you’ll need to think about if the repair is worth the money. Sometimes a new boiler, although more money, could be the better option. Especially if it has a 10 or 12-year warranty.

If the condensate pipe issue was just the main symptom of a broken boiler, get a quote, and we’ll see if we can get you a new boiler in a day or two.

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