a wall-fitted gas boiler

Keeping your house boiler in top shape is something every savvy homeowner like yourself understands perfectly. But when considering oil or gas boiler maintenance, many tend to overlook the flues.

These unsung heroes of your heating system don’t usually receive the same attention as boilers or radiators. And that’s a mistake because your boiler flue plays a crucial role in keeping your heating system operating safely.

Today, we’ll uncover the ins and outs of boiler flues, ensuring you get the complete picture of your heating system to keep it in the best shape possible at all times. But let’s start with the basics…

What Is a Boiler Flue?

Fuel-burning boilers, either oil or gas, get the heat from combustion. During this process, they create waste gas, which, in larger quantities, can be harmful to your health. To prevent it from building up, boilers come equipped with flues.

A boiler flue can be defined as your home’s exhaust pipe. It’s attached to the boiler, running from the combustion chamber to, usually, the nearest external wall, enabling gas to escape safely from the boiler to the outside and releasing waste products, such as gas, air, and condensation, into the atmosphere.

Without a flue, these by-products would build up, eventually becoming a health hazard, such as carbon monoxide poisoning or mould. With the flue working efficiently, you can protect your house from any harmful heating system-related waste products, keeping it safe and warm.

Where Is a Boiler Flue Located?

Usually, a boiler flue passes horizontally through the nearest external wall. In some instances, however, the flue must be placed vertically and exit through the roof. Be sure to read the boiler manual to determine which type of boiler flue you require.

The type of flue required usually depends on the shape of your house and the location of your boiler. It will also depend on the type of oil or gas boiler you have, which leads us to the next question…

Do All Boilers Have Flues?

Although most gas and oil boilers require flues, there are instances where there’s no flue to be found. This is usually the case with back boilers.

These are traditional units that have an open system, which means they don’t come equipped with flues. Such boilers are, however, tightly regulated and can be inefficient and even harmful if not handled properly.

For these reasons, most UK households move away from back boilers, switching to more sustainable options, such as combi boilers or condensing boilers.

What Is a Boiler Flue Liner?

It’s impossible to discuss boiler flues without mentioning flue liners. These are the flexible metal liners sitting within flues, connecting the boiler’s exhaust to the outlet. You can say they provide a ‘tunnel’ for the boiler gases to go through.

Vertical vs Horizontal Boiler Flues

As mentioned, the boiler flue duct can either be horizontal or vertical. The first option is the most common and easiest to install.

Flues are installed horizontally if the boiler is located against the external wall. The flue pipe will then go through that wall, providing the shortest possible way for the boiler’s waste products to escape outside.

Vertical flues are more complex to install. These flues are required when the boiler isn’t fitted against the external wall, for instance, in a basement. In such instances, the flue must be installed vertically, going upward and through the roof.

A vertical flue will look similar to the chimney and can be installed through the house or externally using a proper twin-wall flue system.

As said, though, installing such a flue can be troublesome, which often affects the costs. There might also be issues with accessing the roof.

What Are Boiler Flue Regulations?

How and where you install your boiler flue is important for many reasons. The most important, however, is safety, governed by strict regulations one must adhere to when installing their boiler flue.

Here’s a rundown of the most important boiler flue regulations in the UK:

  • The flue should be located at least 300 mm away from a vent or window - either beside, above, or below;
  • The boiler flue must be 75 mm below the guttering and drain pipes;
  • The flue must be a minimum of 2.1 metres above the ground level if placed through the external wall;
  • The flue should be 200 mm below the eaves and roof overhangs;
  • Vertical flues must be at least 2 metres below roof windows or skylights;
  • Vertical flues must also be a minimum of 300 mm above the flat roof or roof pitch.

What Is the Best Oil or Gas Boiler Flue & Flue Liner?

The flue and flue liner for your oil or gas boiler need to tick two primary boxes:

  • it needs to be able to withstand high temperatures and;
  • it must be sturdy and highly durable.

For this reason, the best option for boilers is the 316-grade stainless steel flue, similar to the 316-flue kit liner we offer here at Flue Pipes. Flues with liners made of this material are incredibly durable and can withstand high temperatures produced by oil and gas boilers.

As for the remaining requirements, the shape and length of your boiler flue will depend on your individual case, including the type of your gas or oil boiler, where it’s located, and whether it needs to be installed horizontally or vertically.

Key Takeaways

Finding the right flue for your gas or oil boiler is vital for its safe and efficient performance. Your boiler flue makes sure that potentially harmful by-products don’t escape the combustion chamber but are securely taken outside and released into the atmosphere.

However, as covered, selecting the appropriate boiler flue can be tricky, as its type will depend on the type of boiler you have, where it’s located, and the size and shape of your house. It must also be installed according to the UK’s boiler flue regulations.

All this can be overwhelming to deal with on your own. Luckily, we’re here to help. At Flue Pipes, we know everything there is about flue systems. Contact us today or head to our ‘tips’ section, where you can find the most important guides on flue regulations and installation.

And remember, whenever you’re in need of a high-quality flue system, our store has everything you require.


How much is an oil or gas boiler flue?

The price of a boiler flue depends on several factors, including its shape, length, material, and whether the flue needs to be installed vertically or horizontally.

The longer the flue, the more it costs. The same goes for horizontal and vertical flues, where the first are usually cheaper than the latter. To give you an estimate, you should expect to pay around £50-£150 for a horizontal flue. Vertical flues typically cost more, ranging from £100-£250. That’s for the flue itself. You also need to add installation costs, which will vary depending on the complexity of the task.

What happens if a boiler flue is blocked?

Your boiler flue can be blocked by leaves, insects, bird nests, or other debris. If that happens, don’t try to unblock it yourself. Turn off your boiler and call a Gas Safe Registered (GSR) engineer to inspect it.

It’s also best to open the windows and doors to ventilate your house. Furthermore, if you suspect that the flue has been blocked for some time, it’s advised to seek medical assistance in case you might’ve carbon monoxide poisoning.

Must a flue for a gas or oil boiler be straight?

For maximised efficiency, a boiler flue should be as short and straight as possible. However, it’s not always possible, especially if the flue must be installed vertically. Plus, since the flue needs to be at least 2.1 metres above the ground level, you might need to go upward first before turning outside. In such instances, it’s okay to include bends. Their number will depend on the length of the flue, but it’s still recommended to minimise their number.

Can I install a flue for my boiler myself?

No, oil or gas boiler flue installation must be performed by a Gas Safe registered engineer. Doing it without proper training and experience could be disastrous, leading to a gas leak or an explosion. Due to these risks, it’s required by law for all boiler-related installations to be conducted by qualified professionals.

Can I extend my boiler flue?

If you are extending your house or suspect the boiler flue isn't fitted adequately away from a window, vent, or door, you might need to extend it. To do that, it’s best to first check the boiler manual to find recommended specifications for the boiler flue. Once you learn that, contact a qualified engineer to install the correct flue extension.