Flue ducts constructed with clay flue blocks are suitable for several different kinds of heat generator: Heating appliances: furnaces and floor furnaces using any of the usual fuels (coal, fuel oil, gas or wood). Wood-burning open fireplaces with or without heat regenerators. Closed fireplaces, or those fitted with an insert, burning wood or any other solid mineral fuel. The construction of flues is governed by the following French standards, according to the type of fireplace: NF P 51-201 "Flue duct work" NF P 51-202 "Open fireplaces fitted or not with heat regenerator using only wood for fuel" NF P 51-203 "Closing fireplaces or fitted with insert using only wood for fuel - Technical specifications" NF P 51-204-1 "Chimneys with closed fireplace or built-in appliance for burning of solid mineral fuel and wood". These standards stipulate the use of flue blocks bearing the "NF" (Norme Française) quality label or having a Technical Assessment issued by CSTB (Centre Scientifique et Technique du Bâtiment). Clay flue blocks may be solid or perforated. They must conform to French standard NF P 51.311 "Clay flue blocks for individual chimney flues". Requirements which flue blocks must fulfil include thermal shock resistance and corrosion resistance testing. Several models of flue blocks have special characteristics and have been issued Technical Assessments by the CSTB. Those with integrated heat insulation may be made entirely of clay or have an internal clay lining, lending them significant corrosion resistance, with external surfaces made of another material.