Witten by Bernard Morgan (Engineer). First Published: October 8, 2019. Updated: December 7, 2023
Understanding the role of a condensate pipe in your heating system is crucial for optimal performance and safety.
A condensate pipe’s primary function is to safely channel the acidic waste water produced during your boiler’s condensing process to an external drain.
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.
By the end of this guide, you’ll have a comprehensive understanding of what a condensate pipe is and why it’s essential for your heating system.
You can learn more about condensing boilers and the various components that are used in some of our other help guides:
A condensate pipe’s primary function is to safely transfer acidic waste water produced from the boiler’s condensing process from the property via an external drain.
This is the basic definition, but if you need to familiarise yourself with the different terms, components and procedures that come with having a boiler installed, things can often get a bit overwhelming. It makes sense to break things down so that you can build your knowledge from the ground up.
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°C in some areas. When this happened, many older boilers started to shut down, unable to cope with the cold.
At its peak, Boilerhut would typically field hundreds of phone calls in one day during this time, most of which were because of frozen condensate pipes.
What is condensate?
Condensate 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 from the condensate pipe.
An hour-long boiler operation may produce around 2 or 3 litres of condensate water waste. This waste gets released in small increments, which is often why you might hear water travelling through your pipes during everyday use.
The Viessmann Vitodens 050 is an excellent example of a modern condensing boiler.
What Are Condensing Boilers?
Because of regulations initially 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.
Condensing boilers can extract excess heat produced from waste gases. Doing so is much more efficient because the heat that would otherwise be lost due to waste is repurposed and used to heat your home.
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.
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 (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:
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 plastic condensate pipe that does this. It’s also because of the way condensing boilers are designed internally.
Using a high-quality internal boiler design, condensing boilers can recover more heat. 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 but 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.
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.
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.
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.
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.
I don’t have an outside drain for my condensate pipe
Most soakaways will look something like this, along 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’.
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 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.
As winter approaches, 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?
Thankfully, taking appropriate measures for a frozen condensate pipe is easy. 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.
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.