Are you on the hunt for a new stove or heating appliance for your home? Hold on, don't go ahead and buy one just yet! It's crucial you first check what type of chimney flue you have in your home.Chimney flue types

Why is that? Not all appliances are compatible with all types of flues. Your chimney is the limiting factor. So it's essential to understand what the different types of chimney flue are.

We've been helping homeowners with their chimney needs for over a decade. This guide teaches you everything you need to know about chimney flues so you can confidently move forward. You can expect to learn:

  • What are the different types of chimney flue? (& other key terms)

  • How to identify your chimney type

  • And what type of fire or fireplace you can have

Chimney talk can get confusing, but we're here to get you sorted so you can move on to the fun stuff.


Let's Start With The Basics

Alright, before you learn about the different types of chimney flues, we need to agree on some key terms.

Chimney - the visible structure on the outside or roof of your house. It's the brick chimney or housing that runs from your fireplace up to the outside world.

Flue - the space or cavity inside the chimney. The flue is the passageway smoke and gases travel from your fire or heating appliance to vent out of your home.

If there is a pipe inside your chimney that connects to your stove, it's called a flue pipe. So, when we talk about a chimney flue, we refer to the inner part of the chimney that channels smoke and combustion gases out of your home.


Smooth And Round

Smoke moves up the flue in a swirling pattern. Round flues are more efficient than square or rectangular ones because they offer less obstruction to the natural flow of smoke. For best performance, the inner surface of your flue should be as smooth as possible.


What Are The Different Types of Flue? 

Let's get into specifics, so you can determine what you're working with.


Class 1 Chimneys

Also known as a traditional chimney stack. You likely know them as a brick chimney, and they are found on older properties built before 1960. 

What type of fires can you have?

Class 1 chimneys are built for open fires (wood and coal) and use natural circulation to remove waste gases from your fireplace. You can also have electric, gas, and gel fires - provided your fireplace chamber can accommodate them. For example, an inset electric fire will need a different setup than a solid fuel fire.

How do you tell if you have a class 1 chimney? 

This one is pretty easy. You can recognize these old ones by the brick chimney stack on the roof.Class1 chimney

Note: Not all Class 1 chimneys are the same. What really matters is how the inside of the chimney, or flue, is lined. A Class 1 chimney falls into one of two categories. 

Masonry Parged Flues

These are typically found on pre-1960 homes, and the brick chimney flue is usually lined with lime mortar. Masonry parged flues can accommodate almost any fire, stove or heating appliance. However, because they are poorly insulated, masonry parged flues can suffer from chimney draw issues. Lime mortar also comes with a few baked-in challenges.

What's the issue with lime mortar?

The main knock against lime mortar is that it's soft. So it's more susceptible to damage from wind, rain, and other weather conditions.

Why does mortar damage matter?

Damage to your mortar causes two main issues: 

  1. Any cracks or holes in the mortar reduce the insulation in your chimney. And the smoke from your fire needs to be hot to sustain a strong upwards draw and properly vent from your home.

  2. Combustibles like soot and creosote love to cling to rough surfaces. Any damage gives them extra nooks and crannies to claim as their new home. Don't let any freeloaders hang about.

These factors combine to reduce the efficiency and safety of your fireplace.

What can you do about it?

You should hire a chimney sweep to regularly inspect and clean your chimney. Also, if you have a class 1 chimney but aren't sure if it has lime mortar, your chimney sweep can tell you.

If you still have lime mortar in your brick chimney flue, we recommend you upgrade to a concrete, pumice, or clay lining. Or you can install a flue liner. Flue liners are usually made of stainless steel and are cheap, durable, and effective. 

Concrete, Pumice & Clay-Lined Flues

You find concrete, pumice and clay-lined chimney flues in traditional (class 1) chimneys where the house was built after 1960. They are also used to reline old masonry parged chimneys. All three materials have superior insulation properties compared to lime mortar, which means better fire performance and safety.

Class 2 ChimneysClass 2 chimney

The second most common flue type in Britain is the Class 2 chimney. Or, if you prefer, you can call them prefabricated flues. Prefabricated flues are built with interlocking steel flue pipes. They are commonly found built into modern houses. Although for older homes, you can often "convert" your class 1 chimney to a class 2.

How do you do that? You just run a prefabricated flue (metal interlocking pipes) down the cavity of your existing chimney.

How do you spot a class 2 chimney?

Easy - do you see a thin metal chimney on top of your roof? Or perhaps there is a large metal chimney structure attached to the exterior of your house? Ya - you have a class 2 chimney.

Pre Fabricated Flue Fire Types

If you have a pre-fabricated flue, you can use an electric fire, a gel-fueled fire, or a gas fireplace. As a result, they are a popular option for newer homes with modern heating appliances. Which gas fire you can use generally depends on your flue size. If your flue has a minimum diameter of 7 inches, you can have a class 1 or 2 gas fire. For 5-inch flues, you can only have a class 2 gas fire.

Pre Cast Chimney

Another flue type you might have are precast flues. They use a series of hollow concrete blocks inside your walls to create the chimney flue. A stainless steel pipe connects the flue to a small ridge vent on your roof.

If you can see a small vent or ridge tile on your roof, there's your sign you have a pre-cast flue.

Due to being much shallower than traditional chimneys and prefabricated flues, pre-cast flues are only compatible with class 2 gas fires. Of course, you can also have an electric fire, as they don't require a chimney at all. 

What If Your Home Doesn't Have a Chimney?

Even if your home doesn't have a chimney, you still have options. Once again, you can always install an electric fireplace - you just need a power outlet to plug into.
You also have three options for gas fireplaces. They need to be installed on an external wall, and you need to be able to connect your fireplace to a gas terminal. Consult a gas-safe registered engineer before you proceed.

Flueless Gas Fires

Flueless fires use a built-in catalytic converter to remove dangerous carbon monoxide. The remaining by-products are water vapour and carbon dioxide, which are vented outside. Flueless fires are 100% efficient.

However, there are some restrictions when it comes to room size (25-30 cubic meters). And you need to install a ventilation brick (similar to a cavity wall) on an external wall to allow fresh air to circulate.

Balanced Flue Gas Fires

Balanced flues are also known as closed-combustion gas fires. A balanced flue can only be installed in rooms with an external wall, as they require two pipes to run outside. One to supply air for combustion and one to exhaust gases away. Balanced flues are usually 80-90% efficient.  

Power Flue Gas Fires

These use a fan to extract fumes and require a power source and an air vent on an external wall for proper installation. 

A Handy Quick Summary

Chimney terminology can get confusing, and there's much to learn when you first dive in. But it doesn't need to be complicated. Here's a simple recap of the different chimney types and how to spot them.

Class 1 - a solid chimney on your roof. Usually brick, sometimes concrete. 2 sub-categories. Get a pro to inspect your class 1 before you proceed. 

  1. Masonry forged - has a lime mortar mix. Do you see brick and mortar inside your chimney?

  2. Upgraded lining - usually concrete, pumice or clay. Is there a liner on top of the bricks on the inside walls of your chimney?

Class 2 - Prefabricated. Think metal pipes. Is there a thin vertical metal chimney poking above your roof? Or can you see the whole structure bolted to the side of your house?

Precast - built into the walls in modern homes. Can you see a small ridge or tile vent on your roof?

No Chimney - none of the above visual clues.

And that covers the basics. But remember to... 

Always Follow The Regulations

You should always get your chimney flues inspected by an approved chimney sweep before you purchase a new stove.
For solid fuel fires, hop on over to HETAS to find a list of approved contractors.

For gas fires, you can find an approved Gas Safe Registered Engineer.

Did We Miss Anything? 

We did our best to go deep on this one, but you never know... If you want to know about any other topic, then please check out our FAQ page.

If you have something more specific, or you need to get your purchase sorted out quickly, then take advantage of our years of experience and head to our contact page.