What Size Twin Wall Flues Are There?

For many people looking to add a flue to their home, a twin wall flue is an excellent option. But, in order to select the right twin wall flue, it’s vital that you know what size you require. Keep reading and the Trade Price Flues team will take you through the different twin wall flue sizing options that are available to you…

Why does twin wall flue size matter?

Twin wall flues are typically used to create a chimney in a home which doesn’t currently have one.

For example, you may live in a newly built home which doesn’t have a traditional chimney. Yet, you want to install a wood-burning stove. The solution is to add a twin wall flue to your home.

The twin wall flue will connect to your stove, and then run either through the inside of your home exiting out of the roofline - or run through an external wall and up the outside of your home.

Getting this right requires you to have a good understanding of the size of the twin wall flue you require. Get this wrong and your twin wall flue will not only fail to function correctly, but will also violate the Building Regulations.

Understanding twin wall flue sizing

To understand how twin wall flue sizing works, you first need to understand the construction of this type of flue.

As the name suggests, a twin wall flue is a flue which is made up of two flues. One flue sits within the other. These are the ‘twin walls’ that the flue’s name refers to.

The space between these two flues or walls is typically filled with high-grade insulation.

When it comes to understanding which size twin wall flue you need for your appliance, it is the inner diameter of the twin wall flue that you need to pay attention to.

It’s best practice to ensure that the inner diameter of the twin wall flue is the same as the stove-pipe or chimney gather that you’ll be connecting it to (although it is possible to buy adapters).

Twin wall flue diameters

Now that you know that it’s the internal diameter of a twin wall flue that you need to pay attention to, let’s take a look at the most common diameters available.

Twin wall flues are available in four common diameters:

> 5 inch (125 mm).

> 6 inch (150 mm).

> 7 inch (180 mm).

> 8 inch (200 mm).

When installing a twin wall flue, it’s important to also factor in the external diameter of the flue - especially if you’ll be routing the flue through the inside of your building (e.g. through floors and ceilings).

This is because you need to create a sufficient gap between the outside of the flue and any materials that it is passing through.

Here at Trade Price Flues, our twin wall flues require a 70 mm gap between any materials near the flue.

To provide you with a tangible example, imagine that you’ve bought a 6-inch twin wall flue system.

The 6-inch diameter (150 mm) refers to the internal diameter of the flue. The flue system in question has an external diameter of 8.26 inches (210 mm).

This means that if you’re routing the flue through a floor or ceiling, you’ll need to cut a hole that is at least 280 mm wide. This ensures that you are creating enough distance between the outside of the flue and the floor/ceiling that it is passing through.

This is important because, although twin wall flues are insulated - and thus don’t heat up too much - they still shouldn’t be placed directly against combustible materials.

Twin wall flue lengths

As well as being aware of the correct diameter of your flue, you’ll also need to be aware of the length of your twin wall flue.

This is a significant point, as you’re going to need your twin wall flue to stretch from your stove or fireplace all the way up the roofline of your home.

As we’ve written previously, the exact length and overall set up of your twin wall flue is going to be determined by the Building Regulations.

For example, where your twin wall flue exits your roofline, the regulations stipulate that there must be a horizontal gap of at least 2.3 metres between the flue’s chimney cowl (the top of the flue), and the roof’s surface.

As twin wall flue kits are generally made up of sections which push together (like the ones we stock here at Trade Price Flues), you’ll need to measure the total distance from your stove or fireplace and the exit from the roofline (bearing in mind the 2.3 metre gap mentioned above).

Here at Trade Price Flues, you’ll find twin wall flue sections in a variety of lengths, including:

> 250 mm.

> 500 mm.

> 1,000 mm.

We also stock adjustable length twin wall flue sections, such as:

> 380 - 585 mm.

> 590 - 1,005 mm.

To make things even easier, we also stock cuttable lengths of twin wall flue, so you can create bespoke lengths when needed.

Twin wall flue bends

Whilst the ideal twin wall flue set up is a perfectly straight vertical one, we know that is not always possible.

This is particularly true if you intend to route your twin wall flue through an external wall and up outside your home to the roofline.

That’s why it’s possible to buy sections of twin wall flue in various bends.

Shop Now - Twin Wall Flue Bends

These ‘bend’ sections of flue pipe are available in a series of degrees. At Trade Price Flues, we stock the following degrees of bend:

> 15 degrees.

> 30 degrees.

> 45 degrees.

> 90 degrees.

It’s also possible to buy twin wall flue sections in different shapes, known as ‘tees’. These are sections of flue pipe that can help you to achieve particularly complex flue constructions - particularly where a flue is being routed through an external wall.

Note - a twin wall flue system cannot have more than four 45 degree bends. Please refer to the UK Building Regulations for further information, or read our guide here.

How to find the dimensions of twin wall flue sections

So, if you’ve made the decision to buy one of our twin wall flues, you’re going to want to make sure you’re buying the right size sections.

At Trade Price Flues, we make it easy to select the correct size sections for your twin wall flue.

As the image below shows, every one of our twin wall flue sections features a diagram which shows the section’s internal diameter, external diameter, and length.

Simply click on the thumbnail, and you’ll be presented with a large schematic of the flue section that includes dimensions.

Where to buy twin wall flues UK

If you’re looking for a twin wall flue solution for your home, then you’ve got a number of buying options.

Which option is right for you will largely be determined by your home and the type of installation you’ll be undertaking.

Twin wall flue kits

The easiest buying solution is to purchase a twin wall flue kit. We list many twin wall flue kits on our website that cover the most common installation scenarios.

These kits are suitable for a variety of installation scenarios, such as:

> Single storey.

> Double storey.

> Straight up (vertical).

> Offset.

> Internal routing.

> External routing.

Note that our kits are based on variations of the above.

For example, if you’re installing a stove (with a 6-inch stove-pipe) in a single story bungalow, and you’re routing the flue straight up the inside of your property, then you can buy a 6-inch stainless steel single storey straight up internal system.

Shop Now - Complete Twin Wall Flue Kits

Likewise, if you’re installing a stove (with a 6-inch stove-pipe) in a two-storey home, and you’re routing the flue vertically up the external wall of your property, then you can buy a 6-inch stainless steel external flue system kit with offset.

Note - we’ve endeavoured to create twin wall flue kits for most installation scenarios. However, if you can’t find a kit that’s suitable for your home, then contact us. We can help you put together an appropriate twin wall flue kit.

Individual twin wall flue sections

If you have a particularly complex installation scenario, or you’re looking to make adjustments to an existing twin wall flue, then it’s possible to buy individual twin wall flue sections.

Our range of twin wall flue sections can be searched based on the finish of the section. You can search for stainless steel twin wall flue sections, or for black twin wall flue sections.

Twin wall flue installation advice

Are you about to embark on a twin wall flue installation project? Then the Trade Price Flues team can help.

We have put together an extensive resource database which answers many of the most common questions people have regarding twin wall flue installation.

The best prices on twin wall flues

Ready to buy a twin wall flue? Whether you need a complete kit, or a handful of sections, you’ll find what you’re looking for at the best prices here at Trade Price Flues.

We offer trade prices to the public, with nationwide delivery available. We also endeavour to keep as much stock as possible - meaning you’ll usually be able to get what you require quickly and easily.

Shop twin wall flue kits and sections now

For more chimney and flue advice, read the Trade Price Flues blog…

What Is a Twin Wall Flue System? | How to Install a Twin Wall Flue Through a Ceiling or Wall | How to Connect Your Flue Liner to Your Stove Pipe