The Difference Between Single Wall and Twin Wall Flue
Single wall flue pipe may also be referred to as single skin, vitreous enamel or connecting flue pipe.
Twin wall flue pipe may also be referred to as prefabricated flue pipe, factory made metal chimneys and double skin flue pipe.
Single wall flue pipe is generally manufactured from steel and coated with enamel to provide a smooth internal and external finish. However it can also be manufactured using stainless steel with no enamel coating. The term “single wall” refers to the fact that there is only one layer of steel and no insulation.
Twin wall flue pipe is manufactured from stainless steel, usually of 316 grade quality. It is made up of two steel walls, which are kept separate by a layer of high grade insulation.
Single wall flue pipe is usually provided in either a matt black or gloss black finish, but can be sprayed to suit using high temperature paint.
Twin wall flue pipe is usually provided in a stainless steel finish, but can be powder coated in black or ivory to suit your preference. Powder coating is used rather than spray paint as it is more durable; twin wall flue achieves higher temperatures due to the layer of insulation.
Generally both single and twin wall flue can be used with gas, solid fuel and oil – but do check the specific product you intend to purchase.
Single wall flue pipe is designed to connect to the top or rear outlet of a stove and essentially act as the connecting piece between the stove and the main flue system. This flue system may be flexible flue liner that is installed into a masonry chimney, or it may be a twin wall flue system.
A twin wall flue system is used when a stove is fitted into a property where no masonry chimney exists. It can run internally or externally and therefore doesn’t just comprise of lengths of flue pipe that we have been discussing in this article.
You cannot use single wall flue pipe in place of twin wall flue pipe. Single wall flue pipe is not insulated and therefore loses a lot of heat. The layer of insulation in twin wall flue keeps the flue gases warmer, allowing the flue to perform correctly and efficiently, whilst also reducing the build up of condensates within the flue system. Single wall flue pipe is also not designed to be fitted through walls, ceilings and roofs as with twin wall flue pipe. The insulation in twin wall flue allows a clearance of only 50-65mm (depending on the specific flue system) to combustible materials, whereas single wall flue needs to be three times the diameter of the pipe away from combustible materials.