What are the different pipes that combi boilers use? Here we explain each and everyone of them and the important roles they play.
Combi or combination boilers are the most popular type of boiler in the UK with approximately 4 in 5 households owning one.
But how exactly do combi boilers work and what role do their various pipes play?
In this guide, we’ll discuss the pipework associated with a combi boiler and the function of each.
Let’s begin!
What is a Combi Boiler?
A combi or combinational boiler provides both central heating and hot water on demand from a single unit.
Therefore, unlike a conventional or system boiler, there is no need for additional space to store the hot water heater.
In the case of a conventional or system boiler, households will often use their loft to store additional components of the heating system.
Combi boilers also tend to be highly efficient.
In fact, the vast majority of combi boilers on the market have an A-rated efficiency (i.e., 90% or greater).
Explaining the Pipework of a Combi Boiler
Now that you have an idea of how a combi boiler works, we can look at the various pipes involved in a combi boiler central heating system.
Combi Boiler Pipework Quick Summary:
Before discussing each component of a combi boiler’s pipework separately, it’s helpful to see a full list of the pipes generally used.
Here are the main pipes featured with a combi boiler heating system:
Mains water pipe
Gas supply pipe
Condensate pipe
Boiler flue
Flow pipe
Return pipe
Pressure relief valve
Image showing a common pipework layout within a UK property using a high-pressure boiler:
Image by Heatable
As you may suspect, the first two pipes travel into the boiler from an external source.
The flow pipe, however, connects a combi boiler with the radiators (or/and underfloor heating) and various hot water outlets of the property.
The return pipe carries water back from the radiators to the boiler.
All in all, each pipe serves a unique purpose.
But how do they work in detail?
Combi Boiler Pipework
Mains Water Pipe
Firstly, the mains water pipe is designed to carry cold water in from the mains water supply.
This is the water that your combi boiler will use to heat up before providing hot water to heat your home and for direct hot water use via various outlets (e.g., showers or hot water taps).
Aside from the fact that the water will be cold upon entering your property, the exact type of water (i.e., hard or soft) will depend on where you live.
Hard water refers to water with a higher level of minerals (more than 200 milligrams per litre to be precise), whereas soft water features significantly less.
Hard water is more likely to cause issues such as a build-up of limescale, therefore, if you live in a hard water area you may want to have a water softener or descaler added.
Gas Supply Pipe
Assuming you have a gas boiler, this pipe provides your boiler with the gas needed as fuel to produce hot water.
The natural gas transported via this pipe to the boiler is initially taken from the main supply.
Of course, things will be different if you have an LPG boiler. In that case, the LPG will be sourced onsite from a storage tank.
The storage tank will need to be topped up every so often.
Condensate Pipe
The purpose of a condensate pipe is to expel wastewater from your combi boiler system.
In a modern combi gas boiler, latent heat is recycled from the waste exhaust gases.
Since this energy is harnessed, it meets the definition of a condensing boiler.
In the case of old non-condensing boilers, heat is only extracted from the burning of gases.
While the ability to extract heat from waste gases makes condensing combi boilers more efficient than their non-condensing counterparts, they do result in condensate (i.e., water vapour).
This condensate must be expelled and this is where the condensate pipe comes in.
This acidic wastewater is deposited into the sewer via the condensate pipe which is generally made of plastic or PVC.
The condensate pipe will deposit around 2-3 litres of condensate water per hour when your boiler is operating.
Moreover, your combi boiler will likely release the water in 300ml increments.
Boiler Flue
Just as the condensate pipe is designed to expel harmful water, a boiler flue serves the purpose of depositing dangerous gases (e.g., carbon monoxide).
The flue sits at the side of the property and will expel this gas into the atmosphere.
Boiler flues come either in a round or square shape. Beyond that, they can be installed either vertically or horizontally.
Flow Pipes
Once water and gas enter your boiler, it has everything it needs to generate hot water.
Burning gas fuels the heating process to warm up water which can then be used to meet your heating and/or hot water needs.
Your combi boiler will have two flow pipes.
One to transport hot water to your radiators for heating rooms and another to carry hot water to outlets (e.g., hot water taps and showers).
The diverter valve within your combi boiler is used to determine which of these flow pipes hot water will enter.
For example, if you have the heating on but also wish to use a hot water tap, the diverter valve will close itself allowing the water to be directed to the hot water outlet flow pipe.
Alternatively, when the diverter valve is open, the hot water will travel straight to the radiators via the first flow pipe.
Due to this setup, it’s not possible for a combi boiler to provide hot water for heating and hot water outlets simultaneously.
Return Pipes
Naturally hot water that travels to outlets such as a hot tap is non-returnable.
However, the water in your radiators can be returned to the boiler for reuse via the return pipes.
Two or more return pipes may be necessary to carry water from various sources (e.g., downstairs radiators and upstairs radiators) before connecting into a single return pipe that reaches your boiler.
Considering underfloor heating? Read our complete guide on underfloor heating with a combi boiler.
The return pipes help to rid your radiators of water that has cooled down.
However, these pipes also offer a range of additional benefits.
For example, the water that return pipes send back to your boiler will not be as cold as the new water entering from the mains.
Therefore, not quite as much energy will be needed to heat this water up.
Ultimately, the return pipes can help maximise the efficiency of your combi boiler.
This is possible by using up the remaining heat energy of this used water.
Pressure Relief Pipe
Lastly, there is the pressure relief pipe or pressure relief valve.
The purpose of this device is to ensure that pressurised fluid can safely leave your heating system. It will do so in the form of a gas.
If too much water builds up in your system, it will need an exit point.
Therefore, the use of a spring-loaded relief valve will open when there is too much pressure exerted upon it in order to protect you and your heating system.
The gas exhausts then exit through a pipe in the side of the property.
Considering the complex pipework and various moving parts of a combi gas boiler, it’s important to have it serviced annually.
Moreover, when issues arise, you should have them addressed as soon as possible.
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