TYPES OF FLUE

The Ultimate Guide To Chimneys And Flue Systems

10 August 2022

Choosing the right solid fuel or gas fireplace appliance that is fit for your property can be difficult as there are many different chimneys and flue systems that may obstruct or change your final buying decisions.

Before searching for your new fireplace, you must first of all determine what chimney you do or don’t have. This will give you a good starting point for eliminating certain types of fires as it is not a one size fits all approach.

A HETAS or Gas Safe registered engineer will be able to advise on the most suitable fireplace for your current chimney situation.

In this guide, we will provide an overview of knowledge, advice and best practices for chimneys and flue systems to get you on track to finding your new fireplace appliance for your home.

Estimated reading time: 13 minutes

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Chimney Types

The style, size and the condition of a chimney will be the deciding factor for what fireplace will be possible within your property.

Whether you decide to go for a solid fuel or gas fire appliance you will need to know what system will be best, the cost implications and possibilities that are available before making any purchases.

The last thing you want to do is spend lots of money on a fireplace to then be told that it cannot be installed and that it will require lots of work at a very expensive price.

Let’s get started…

Class 1 Chimneys

For use with solid fuel, electric and gas fires

A Class 1 is a traditional chimney type built from stone or brick and sometimes lined with clay or concrete, if built after the mid 60s.

They are often found in older properties sometimes with a 7″ (175mm) diameter clay pipe flue lining that can be used to burn solid fuel.

Old or damaged Class 1 chimneys are re-lined and converted to a Class 2 flue via a 5″ (125mm) or more diameter stainless steel flexible flue liner. (see below)

Flexible flue liner for a Class 1 chimney

This improves efficiency greatly and ensures that the chimney is safe from leaking or blockages. It may not have even been sealed in the first place, depending on the age of the property.

Moreover,iIt can also make it easier to sweep overtime but make sure that your flue liner does not get punctured.

You can test your chimneys condition with a smoke test. Use smoke pellets to see how the chimney draws and if it leaks.

Easily recognisable by a chimney stack that runs from floor to roof level, a chimney pot on the stack or an old open fireplace at the base of the stack in your home.

Key Features

Suitable for solid fuel fires.
Generally suitable for all types of gas fires.
Usually has an internal chimney breast of up to 15″ (375mm) deep.
Opening Size is normally a 36″ x 36″ (375mm x 375mm) opening with a lintel above and can be closed down to a 16″ x 22″ (400 x 550mm) opening.
Chairbrick (or Milner Brick) – A tapered firebrick fitted to the rear of the chimney fire opening to create the recess for a solid fuel fire, taper tray DGF gas fire. (this is very often removed for the installation of a full depth inset gas fire).

Typical Construction Types

A square hole approx. 9″ x 9″ (225x225mm) formed of bricks
Or a 7″ (175mm) diameter of clay pipes (usually found in homes built since 1970).

The purpose of a Class 1 chimney is to draw the waste fumes up and out of the chimney using natural circulation throughout the room.

This worked well for open fires but meant that they weren’t that efficient. Open fires are gradually being phased out as they do not conform to new eco design 2022 regulations.

Class 2 Chimneys

A Class 2 chimney is a pre-cast flue constructed of concrete flue blocks or a pre-fabricated metal box and pipe system.

Furthermore, newer homes may also have a Class 2 with a 5″ minimum diameter that terminates with a
cowl. The most common is a gas cowl as illustrated.

Pre-fabricated Metal Box Flues

For use with solid fuel, gas and electric fires

Usually found in a property where no chimney or flue system was installed at the time of construction.

Gas Cowl

They can be identified by a metal flue out of the roof with a metal cowl on top of a chimney stack which can be found in new and old properties. They can be fitted to the exterior or interior of a property.

This kind of system may be found on an old property in circumstances where the chimney has been fitted with a new stainless steel liner.

Pre-fabricated flues are suitable for only semi inset living flame; outset living flame and radiant gas fires.

Pre-fabricated flues are therefore not suitable for solid fuel appliances; DGF taper tray or basket type gas fires and most fully inset gas fires.

A pre-fab flue could also be found on a newer home which has a built-in steel flue.

Pre-cast Concrete Flues

For use with electric or gas fires

Often referred to as letter box flue, modern houses generally later than 1960s often have a pre-cast concrete flue system in the UK.

Usually identifiable from the starter block or letter-box opening that terminates with a ridge tile, as illustrated below.

It is a shallow concrete structure that is built within the interior house cavity walls.

Popular with house builders since the early 1970’s as this is considerably less expensive to build than a Class One chimney.
Usually concrete built into the internal skin of a cavity walled house during construction.

They vary from 5″ (125mm) to 8″ (200mm) from the rear face of the flue to the plaster finish. The lack of depth mean that many inset gas fires and all solid fuel appliances are not suitable.

Example of a pre-cast flue system (letter box flue)

Generally speaking properties constructed before 1986 will have a narrower flue block system than those constructed after this.

Pre-cast flues are suitable for class 2 fire such as semi inset living flame, outset living flame, outset radiant gas fires, slim-line and multi-flue fires. Electric fires are also accepted.

Pre-cast flues are not suitable for solid fuel appliances; DGF taper trays or basket type gas fires or most fully inset living flame gas fireplaces.

Gas fires should always be clarified withe the relevant manufacturer’s instructions before installation.

Twin Wall Flue System

For use with solid fuel (wood burners and multi fuel stoves)

Twin wall chimney flue is a double layered insulated stainless steel metal flue pipe that transports gases from wood burning or multi fuel stoves into the atmosphere.

Twin wall flue system by Bonfire.

This system is used when there is no chimney breast or a direct route up and out the fireplace appliance. Doubly insulated as it will be on the exterior of your property.

The downside to this flue means that it may take longer to warm up during the colder months leaving a blockage of cold air in the flue system.

No Chimney

A property might not always have a chimney that can be utilised for a solid fuel or gas fireplace appliance but this is not a problem.

Wood burning and multi fuel stoves will always require a Class 1 or Class 2 chimney to vent waste gases.

However, electric fires require no ventilation for fumes or gases and so a false chimney breast or media wall can be built to house the appliance. This can be constructed from wood and plasterboard as electric fireplaces have only a 1-2 kW heat output.

Although, you do have a few options for gas fires:

Balanced Flue

A closed combustion gas fireplace that works by taking air for combustion from outside the property through the outer flue tube, mixing this with the ignited gas at the burner and exiting the fire through the inner flue tube to the outside of the property.

Balanced flue gas fires are available in inset or outset versions and are glass fronted. They require installation on a suitable outside wall of a property.

They use two pipes. Balanced flue gas fires are usually 80-90% efficient.

Conventional and Balanced Flue Systems

Conventional Flues

Conventional Flues are the most common type of chimney installed, mainly traditional brick. They work by pulling the warm air rising up out of the chimney away considering that air is passing over the top of it. This pulls air from the room through the fire/stove and up into the chimney. Conventional flue chimneys terminate in clear air 2.4m away from anything or 600mm above the ridge of the roof.

Different types:

Class 1 – Brick built or clay/ concrete lined traditional chimneys opening to a fireplace at the bottom.
Class 2 – Prefabricated metal flue systems added to a property that has no chimney. Connected directly to a stove/fire than opening up into a fireplace.

Wood Burning, Multifuel and Pellet Stoves are conventional flues. Gas Stoves/ fires can work on both flue types but are normally bespoke made. Conventional Flues might need lining if the property of a certain age.

Balanced Flues

Balanced Flues only apply to gas stove/fires. They are constructed of one pipe inside the other. The inside pipe removes waste gasses and the other brings air for combustion. Balanced flues are commonly known as concentric flues and are not twinwall flues. Balanced flues do not require any additional air from the room and so all stoves/ fires will be glass fronted or have a closed door that cannot be opened. They are room sealed.

Advantages

Heating Efficiency – They bring in fresh air from outside, more oxygen is available to mix with the gas for complete burning.
Not restricted to a particular chimney size – They can work with or without a chimney.
Can be fitted to a ceiling or wall – Depending on preferences or room requirements.

Disadvantages

Fire behind glass – Some people do not like the aesthetic visual appeal of this.
Glass can become scorched – With not very good upkeep or servicing.
Fire surround consideration – High temperatures generated by the fire.

These flues do not get as hot as conventional flues and they do not need to be run as high as conventional flues. Instead all you need is an external wall, where you can either come direct out the back of the appliance, through the wall and terminate, or rise up, bend 90° through the wall and terminate at a higher level.
So, if you have no chimney in your home, or if you are looking to place a fire in a room that either has no chimney or the chimney is not where you would like to place the fire, then a balanced flue could be the perfect option for you.
Flueless
Flueless is an alternative to a conventional or balanced flue. Electric, bio-ethanol and gas do not require a chimney to run. Gas stoves and fires feature a catalytic converter built in to them that burns off all the waste gases, meaning that it does not emit any fumes back into the room and as such requires no flue.

· Converting a Conventional Flue into a Balanced Flue

Chimney Renovation Kits allow you to convert conventional flues into a balanced flues. These special kits provide a concentric pipe off the top of the stove/fire into the chimney where they will seal into a special register plate.

Fan Powered Flue

Also known as a fanned flue, a powered flue gas fire works by means of a mechanical extraction system i.e a fan unit which removed the waste gases from the gas fire to the outside of the property.

A fan flue alternative is available for new homes built without a chimney or older properties
where the flue is no longer of sound integrity. All that is required is a suitable outside wall.

The fan unit should always be positioned on a suitable outside wall with up to 2 bends within the flue system.

The flue is concealed behind a purpose made skirting fitting. It is also important to note that powerflue gas fires also need an electricity supply to work.

Appliances can also be open fronted.

The flue is concealed behind a purpose made skirting fitting. It is also important to note that powerflue gas fires also need an electricity supply to work.

Appliances can also be open fronted.

Flue Terminals

Flue terminals are designed to prevent down draught and positioned so that fumes cannot enter the house.

There are a wide variety of flue terminals on the market that are and aren’t suitable for gas fireplaces:

Suitable flue terminals for gas fireplaces

These are often fitted to a Class One flue to ensure the following:

The flue’s performance is optimised and down draught avoided in certain locations.
No ingress of birds or birds’ nests that will impair the flues performance.

Unsuitable flue terminals for gas fireplaces.

Carbon Monoxide

Carbon Monoxide (CO) is a highly poisonous gas that is released when fuels are burnt. Faulty or badly maintained gas or solid fuel appliances, installations and blocked or faulty flues can lead to Carbon Monoxide poisoning and ultimately to death.

Example of Soot above fireplace

CO poisoning, dependent upon its severity, can vary from mild flue like symptoms to headaches and sleepiness to death when the CO concentrations in the air reach critical levels.

You can’t smell it
You can’t see it

It is quite normal to detect low levels of CO in an average living room (up to 20 ppm) particularly in a smoker’s room.

If you see a lot of soot above a fireplace opening this is a good indication of Carbon Monoxide build up.

It is now a regulation that every sold fuel installation has to be fitted with a Carbon Monoxide detector.

Placement

Your CO detector should be placed within 300 mm from any wall, between 1 and 3 metres horizontally from the appliance as high up as possible and not within 150mm of the ceiling.

Conclusion

There are basically two types of Flue Systems – Class One which is a traditional brick built chimney or a 7″ (175mm) diameter clay pipe flue that can be used to burn solid fuel.

Or Class Two which is either a pre-cast letterbox concrete flue built into the cavity of a wall or a 5″ (125mm) diameter pre-fabricated metal flue. These are normally built into newer properties or inserted into Class One chimneys that are not suitable for burning solid fuel.

It is also possible for there to be no flue at all – if this is the case a gas fire and fireplace can be fitted on a suitable outside wall using a balanced flue or power flue.

A fireplace fitted with an electric fire is of course a further option.

FAQ

What is a Class 1 chimney?

A Class 1 chimney is a traditional chimney type built from stone or brick and sometimes lined with clay or concrete, if built after the mid 60s. They are found in older properties.

Old Class 1 chimneys tend to be relined with a stainless steel flexible flue liner when a multi fuel or wood burning stove have been installed. This is for efficiency and safety as Class 1 chimneys can leak if not lined properly or if the chimney has weathered overtime.

A Class 1 chimney will normally feature a chimney stack, chimney pot or an old open fireplace at the bottom of the chimney. These are good ways of telling whether you have a Class 1 or Class 2 chimney in your property.

What is a Class 2 chimney?

A Class 2 chimney is a pre-cast concrete or pre-fabricated metal flue liner system which is made from interlocking metal flue pipes, held together by locking bands and wall brackets. They can be identified by a metal cowl on top of a chimney stack.

What fires can use a Class 2 chimney?

You can have electric, gas and gel fires with a pre-fabricated and pre-cast flue system.

How many fires can a flue serve?

The golden rule is: One Flue = One Fire. This is a legal requirement and may damage the flue system if you override it.