Combi Boiler Pipework Re March 17, 2023

Combi Boiler Pipework

Frozen condensate pipe 2

When it comes to installing a new combi boiler or upgrading your existing heating system, understanding the different types of pipework involved is essential. Whether you’re a homeowner or a landlord, having a clear understanding of combi boiler pipework can help you make informed decisions and ensure your system is running efficiently.

In this blog, we’ll be breaking down the different types of pipes you’ll find in a combi boiler system and explaining how they work. From flow and return pipes to condensate pipes, we’ve got you covered. So, grab a cuppa and let’s get started on this piping-hot topic!

What's a combi boiler?

Combi boilers, also known as combination boilers, are the most popular heating systems in the UK. In fact, 23 million homes rely on gas boilers for central heating and hot water (80% of those households use combi gas boilers). 

Combis have become increasingly popular in recent years due to their compact size and ability to provide hot water and heating on demand without needing a separate water tank.

How do combi boilers work?

Combi boilers heat water directly from the mains supply and then supply hot water to taps and showers as needed. Unlike traditional heating systems, there is no need for a separate water tank or cylinder to store hot water, which makes combi boilers much more space-efficient. 

When a hot water tap or shower is turned on, the combi boiler detects the demand and begins heating water as it flows through the heat exchanger. This means that hot water is available almost instantly, and there is no need to wait for a tank to heat up.

In addition to providing hot water, combi boilers also provide heating to radiators throughout the house – this is achieved using the same heat exchanger that heats the hot water, with the addition of a diverter valve that can switch the flow of water between the hot water tap and the radiators. 

In a nutshell, think of a combi boiler as the ultimate multitasker – it heats your home and provides hot water all in one, making your life easier and more comfortable! 

Pipework of a combi

Now on to the technical stuff – so let’s look closer at the different pipes that make up your central heating system. 

A combi boiler heating system includes 7 different types; mains water pipe, gas supply pipe, condensate pipe, boiler flue, flow pipe, return pipe and pressure relief valve. Will explain what they are and how they work together below;

Gas supply pipe

A gas supply pipe in a boiler is a pipe that carries natural gas from the gas meter into the boiler to fuel the combustion process. The gas supply pipe is usually made of copper or steel and is connected to the gas meter. 

Now let’s theoretically take a closer look inside the boiler to understand the importance of a gas supply pipe – The natural gas which was brought by the gas supply pipe gets burned inside the boiler to produce heat that is used to heat water or provide space heating. 

The gas supply pipe must be sized appropriately to ensure it can deliver enough gas to the boiler to meet its heating needs. The size of the gas supply pipe is determined by factors such as the size of the boiler, the heating load, and the distance from the gas meter.

It is crucial to ensure that the gas supply pipe is properly installed and maintained by a Gas Safe Engineer to ensure safe and efficient boiler operation, as gas leaks can be dangerous. 

Mains water pipe

The mains water pipe is the pipe that carries cold water from the water main supply to the combi boiler. The mains water pipe is a vital part of a combi boiler system as it provides a constant flow of water to the boiler, which then gets heated and distributed throughout your home to ensure that the system can quickly and efficiently provide hot water & heating whenever you need it.

Condensate pipe

A condensate pipe is a vital element of a condensing boiler system. Condensing boilers are highly efficient as they capture and reuse the heat from the water vapour produced during combustion. 

The condensate pipe carries away the acidic water (condensate) produced by this process and directs it into a drain or outside the property.

The condensate pipe is typically a white colour pipe made of plastic or PVC and runs from the boiler to a suitable drainage point.

The cold temperature in the winter month can cause the condensate pipe to freeze, preventing the boiler from working as it should. That’s why you should protect the condensate pipe from freezing to ensure that your boiler continues to work efficiently and effectively when you need it most.

Boiler flue

In simple terms, a boiler flue is a pipe or duct that connects your boiler to the outside of your house, extracting the waste gases and condensation from your heating system so they don’t get pumped around your home.

The flue’s primary purpose is to remove harmful gases, such as carbon monoxide, that are produced by the boiler during combustion.

How does a boiler flue work?

When gas is burned in the boiler’s combustion chamber, it creates heat. At the same time, waste gas fumes are also created. The boiler flue is designed to extract these fumes from your home, as they could become potentially dangerous.

It’s a legal requirement that all gas boilers are installed with a flue to ensure the safe removal of exhaust gases from the property. Boiler flues can be either horizontal or vertical and typically exit the property via a wall or roof.

It’s essential to ensure that the flue is installed correctly (by a Gas Safe Engineer) and meets all safety standards to prevent the risk of harmful gases from being released into the property. That’s why annual boiler service is non-negotiable. A yearly boiler service ensures that your boiler is functioning and the warranty is still valid. It also detects (and predicts) issues with your heating system before they arise.

Flow and return pipes

The flow and return pipes are the primary pipework in a combi boiler system. In a nutshell, the flow pipe is the pipe that carries hot water from the boiler to the radiators, underfloor heating system, or hot water outlets, i.e. showers and taps. 

The return pipe, on the other hand, carries the cooler water back from the radiators or hot water outlets to the boiler. 

Together, the flow and return pipes ensure that hot water or heating is distributed evenly throughout the property. The pump helps to maintain the flow of hot water through the system, ensuring that every radiator or hot water outlet receives the right amount of heat.

These pipes are typically made from copper, an excellent heat conductor and corrosion-resistant conductor, and they are often insulated to reduce heat loss and improve efficiency.

Pressure relief pipe (AKA pressure relief valve)

A boiler pressure relief valve, or PRV, as it’s known in the trade, is responsible for releasing water when the pressure starts getting too high in your boiler to protect your heating system from over-pressurising.

A combi boiler works by heating water directly from the mains and distributing it to the hot water taps and radiators. As the water heats up, it expands and can cause the pressure inside the boiler to rise. If the pressure inside the boiler becomes too high, it can cause damage to the boiler and potentially become a safety hazard. 

The pressure relief valve is designed to open automatically if the pressure inside the combi boiler becomes too high, releasing water and steam to relieve the pressure. This helps to prevent damage to the boiler and ensures that the system operates safely.

The importance of a boiler service

As you can see, your heating system contains various complex parts and pipework, so ensuring your boiler gets serviced annually will help prevent breakdowns and ensure that your home is kept warm and comfortable throughout the year.

During a boiler service, a qualified heating engineer will carry out a series of checks and tests on your boiler, including inspecting the boiler for any signs of damage or wear, checking the gas pressure, and testing the safety controls.

Regular boiler servicing can help to identify any potential issues with your boiler before they develop into more significant problems, preventing costly breakdowns and repairs. It can also help ensure that your boiler operates safely, reducing the risk of carbon monoxide leaks and other safety hazards.

In addition to improving the safety and efficiency of your boiler, regular servicing can also help to extend the lifespan of your boiler, ensuring that it continues to operate at its best for many years to come. So sign up for our boiler service plan for just £8.99 a month


A frozen boiler condensate pipe can cause your boiler to stop working, leaving you without heating and hot water. However, there are several steps you can take to unfreeze a frozen boiler condensate pipe:

    1. Locate the frozen section: You can usually identify the frozen section of the condensate pipe by feeling for a solid blockage in the pipe or by noticing a buildup of ice around the pipe.
    2. Warm the frozen section: You can use a hot water bottle, heat wrap, or a hairdryer to warm the frozen section of the condensate pipe gently. It is important to be careful not to overheat the pipe, as this can cause damage.
    3. Thaw the pipe: Once you have warmed the frozen section of the pipe, you can use warm water to thaw the ice. You can pour warm (not boiling) water over the frozen section of the pipe, or use a hot towel or cloth to help melt the ice.
    4. Restart your boiler: Once the pipe is thawed, you can restart your boiler. It may take a few minutes for the boiler to restart and heat up again.

If you are unsure about how to unfreeze a frozen boiler condensate pipe, read our blog. 

To prevent your boiler condensate pipe from freezing again, you can take the following steps:

    • Insulate the pipe with lagging or insulation to help keep it warm.
    • Keep the heating on at a low temperature, even when you are not at home, to help prevent the pipe from freezing.
    • Clear any debris or blockages from the end of the pipe to ensure that water can flow freely.

Bleeding your radiator is a simple process that can help to improve the efficiency of your central heating system and keep your home warm and comfortable. Here’s a step-by-step guide to help you bleed your rads: 

    • Turn off your heating system: Before you start bleeding your radiator, turn off your central heating system and wait for the radiator to cool down.
    • Locate the valve: Locate the valve at the top of the radiator, usually at one end. The valve is typically a small square shape and may have a small water release valve underneath.
    • Prepare for the job: Place a towel or container underneath the valve to catch any water that may leak out during the bleeding process. You may also want to wear gloves to protect your hands from hot water.
    • Open the valve: Using a radiator key or a flat-head screwdriver, turn the valve anti-clockwise (lefty-loosey) until you hear a hissing sound. This means that the air is being released from the radiator.
    • Close the valve: Once water begins to escape from the valve, tighten it back up by turning it clockwise (righty tighty).
    • Check the pressure: Check the pressure gauge on your boiler to make sure that the pressure hasn’t dropped too low. If it has, you may need to top up the pressure using the filling loop on your boiler.
    • Repeat if necessary: Repeat this process for any other radiators in your home that may need bleeding.

By bleeding your radiators, you can improve the efficiency of your central heating system, reduce your energy bills, and keep your home warm and comfortable throughout the winter months.