The present invention relates to insulated chimney pipes of the type that are commonly used to conduct hot waste gases from a domestic sold fuel burner, oil or gas central heating boiler or the like to the atmosphere in place of traditional brick chimneys.
Such chimney pipes must satisfy stringent British Standards tests administered by the Agrement Board to ensure that they satisfy requirements as to ease of installation and access, adequate service life and low thermal conductivity. Chimney pipes frequently pass through floors and lofts and may thus pass very close to combustible material. It is therefore important that the external temperature of chimney pipes can not rise above a predetermined level, and in order to satisfy the relevant British Standard on this criterion most insulated chimney pipes are of a double wall construction. The inner wall is invariably of stainless steel whilst the outer wall may be of galvanised or vinyl coated steel and spaced from the inner wall by a layer, typically about 40 mm thick, of thermally insulating material such as mineral wool, fire-clay or rock granules. However, for chimneys of a larger diameter even a double wall construction may not be adequate. Thus a 200 mm diameter chimney for use with open fires must satisfy British Standard No. 4543, and in order to fulfil the thermal conductivity criterion it has been found necessary to make this chimney of a three wall construction, which is naturally extremely expensive.
Such conventional two or three wall chimneys with a stainless steel inner wall are intended to have a service life of about 20 years. In practice this service life is not always achieved because of the highly corrosive atmosphere to which the stainless steel is exposed. In particular the action of water formed by condensation and sulphuric acid formed by the combination of this water with sulphur containing gases liberated by the combustion of sulphur containing fuel oils or the like together with the cumulative corrosive effect of thermal shock or stress can lead to a substantial reduction in the service life and in extreme cases the total collapse of the stainless steel wall. The recent increase in the burning of wood in domestic boilers and open fires has exacerbated these problems since the combustion of wood produces wood acids and other particularly corrosive substances which over time attack and ultimately destroy the stainless steel inner wall of the lining.