14 Feb 2021

If you’re looking into chimneys for any reason at all, you may have reached the inevitable point where you start to ask one very important question.

What is the difference between a flue and a chimney?

At first glance, they seem pretty similar, and, indeed, are regularly interchanged as ways to describe what most ‘normal’ people know as “the chimney on our house”.

However, if you’re one of those chimney owners, it really is a good idea to get your head around the difference between a flue and a chimney, because if you ever need to troubleshoot it yourself, you’ll at least have something to go on.

We’re not actually chimney sweepers…

Yep, you read that right – the SweepSmart team aren’t actually chimney sweepers – for the same reason the words ‘flue’ and ‘chimney’ are so easily (and incorrectly) interchangeable.

A more accurate way to describe the SweepSmart team is as “flue sweepers”. The reason for this is made clear when we look at the following simple definitions:

  • Chimney: This is the structure which contains one or several flues.
  • Flue: This is the space inside the chimney which allows the passage of flue gasses.

The flue is, therefore, one specific part of an overall chimney system, whereas the chimney is actually made up of several components. Those are the chimney breast you see in your home, the chimney structure which contains the flue, the chimney stack above the roof and, finally, the chimney pot everyone can see outside.

Parts of chimney

The different types of flues

Flues are available in several different types of material. This is why some flues can’t be used for solid fuel and are only suitable for gas.

That’s the reason you should always seek professional advice if you’re unsure about which flue you have installed in your home.

So, now we understand the difference between chimneys and flues, let’s take a look at the most common types of flue to get an idea of how they’re used and why they benefit certain types of property.

Masonry parged flues

These are typically found in houses built before 1966 and feature a square or rectangular flue which is lined with lime mortar mixture.

They’re suitable for most types of fuel, but, due to their age, usually need to be tested fully by a professional before they can be used.

For instance, if you own an older house, it doesn’t necessarily mean there’s a masonry flue present, and the original flue may have been relined, regardless.

Best get it checked out!

Stainless steel flexible liners

Some flues might need to be relined entirely, and stainless-steel flexible liners are a popular choice.

They come in two forms:

–       single skin liners for gas fires (referred to as ‘Class 2’); and

–       twin-skin liners which are manufactured with overlapping, high-grade stainless steel for multi-fuel and wood appliances.

Once again, if you’re unsure, it’s best to ask a professional to see if you have a stainless-steel flexible liner or need one fitting.

Pumice, Concrete, and Clay Liners

These types of flue are incredibly common in properties built during or after 1966.

However, it’s important to note that pumice, clay, and concrete can also be used to reline an older chimney. We certainly see a lot of these materials in action when sweeping.

Prefabricated stainless steel flue systems

Easily assembled and supported, these systems consist of an outer case and inner liner with insulation filling the space in between both materials.

Prefabricated stainless steel flues are typically fitted when a chimney isn’t present.

Special mention must also be made to gas flue blocks, which are specifically designed for gas effect fires. They’re usually built within the interior of the house walls and the eagle-eyed will spot their presence thanks to a small ridge vent on the property’s roof.

The difference between Class 1 and Class 2 chimneys

You may have reached this blog post while trying to answer the question “what is a Class 1 or 2 chimney?” – and it’s a great question.

Chimneys are typically categorised into classes, with the two main classes best described as follows:

  • Class 1 chimney:  These were most common in houses built up until around 1960. They’re brick-built and situated either on either an external or internal wall. They usually contain multiple flues, too, and can be used with all types of solid fuel fires.

  • Class 2 chimney: These were built from the 1960s onwards. They feature interlocking metal pipes which run through the house and can only be used with specific types of gas fire.

There are also balanced flues, which exit from the back of gas fires and fanned flues, which take air for burning from the room in which they’re situated.

Need to talk flues? Still not sure about the difference between a chimney and a flue? Or maybe you need your chimney flue swept? Just get in touch with the SweepSmart team.