What's the difference between a Class 1 and a Class 2 chimney?

What's the difference between a Class 1 and a Class 2 chimney?

A wood burning or multi fuel stove can only be installed by using a Class 1 Chimney. A Class 1 Chimney is probably how most people imagine a chimney to look. It is usually stone or brick built or if built after the mid 60s it will generally have an additional clay or concrete lining. It will also usually still have its traditional fireplace opening at the base (although not always) as well as feature a chimney stack on or near the roof (if this has not been removed). The internal diameter of a traditional chimney is usually a minimum of 180mm (approximately 7”) or more. Older Class 1 chimneys and those with a diameter of over 200mm tend to be problematic and so for safety and efficiency, they are typically relined with double skinned stainless steel flexible liner when a wood burning or multi fuel stove is being installed. Whether you are fitting a flue liner or not you must ensure that the existing chimney is structurally sound before you proceed with the stove installation.

You may see elsewhere on the internet the term ‘Class 1 Chimney Liner’. It’s easy to believe that this flexible chimney liner can somehow be used as a substitute for a Class 1 Chimney – it can’t. It simply means that it can be used within a Class 1 Chimney for the purposes of installing a stove. It must never be used on its own as an alternative to a Class 1 Chimney.

CLASS 1 TWIN WALL CHIMNEY

If your home does not feature a Class 1 Chimney then in most cases you can very easily and safely create a new one with a Twin Wall Insulated Flue System. These come with internal diameters of 125mm (5”) for Defra Smoke Exempt Appliances of 5kW and below or 150mm (6”) for all non-Defra Exempt stoves whatever their output. In some rare cases you may need to specify an internal diameter of 175mm. The insulation layer, sandwiched between two stainless steel skins, significantly reduces the external temperature of the pipe to provide greater installation safety and flexibility. At the same time the insulation keeps the flue gases as hot as possible to ensure a more consistent and reliable flue draught that will help the performance of the stove. With the appropriate adaptor the twin wall system can be connected straight to the top of the stove without the need for a connecting single skin vitreous enamel flue pipe.

CLASS 2 CHIMNEY

A Class 2 Chimney normally refers to a prefabricated single skin metal chimney (usually interlocking stainless steel pipes) or precast concrete block flue system with a 125mm (5”) diameter which is built into a home’s interior walls and is specifically designed for use with gas appliances. It must not be used to install a wood burner or multi fuel stove. So, if you are thinking of replacing an old gas fire with a new wood burning or multi fuel stove then you need to establish whether you have a Class 2 Chimney before you proceed. Typically a Class 2 Chimney can be recognised from a metal cowl or short pot on the roof, usually without a chimney stack, or on many modern homes, a raised ridge vent on the roof line made from metal, brick or concrete. Only a Class 2 gas fire or Class 2 gas stove will be suitable for this kind of chimney. Alternatively there’s now a great choice of electric fires with realistic fuel beds and LED flame patterns.

BUILDING CONTROL AND CHIMNEYS

Any work carried out during the installation of a flue liner and wood burning or multi fuel stove using an existing Class 1 Chimney or creating a new Twin Wall Chimney must be notified to your local authority Building Control officer to ensure that it complies with building regulations.