To limit the probability that metal liners will be installed in appliances using fuels that are more likely to cause the accumulation of creosote, which could lead to chimney fires, which could lead to the heat-induced deformation and joint failure of metal flue liners, which could lead to the ingress of condensed water vapour into the cavity between the chimney liner and the surrounding masonry, which could lead to damage to the masonry due to freeze-thaw stresses, which could lead to the collapse of the chimney, which could lead to harm to persons.