What Flue Type?           Brick Built Chimney   Pre - Fabricated Flue System               Type / Useage   Type / Useage   Class 1 Suitable for all fuels. May have clay liners or another form of lining present - needs to be examined for suitability and gastightness.   Class 1 or 2 Differing grades of twinwalls systems are available to cater for different fuel types - if you have an existing system it must be of a suitable grade and diammeter to service a particular appliance. Some gas flue block systems terminate with metal flue - take care not to mix the two.   Eldfast Suitable?   Eldfast Suitable?       Diameters of brick flues vary, we recommend Eldfast for diameters up to 24 inch square! Where diameter changes within the flue the small balloon may be used to apply the material (see method of installation (3)).       Pre - cast flue / gas flue block   Other - Stone / Slate etc               Type / Useage   Type / Useage   Class 2 only Suitable for certain enclosed gas appliances only. Check manufacturers requirements.   As per specification for brick built           Eldfast Suitable?   Eldfast Suitable?       Must maintain minimum X - sectional area of 12,000mm sq, also CORGI's minor dimension of 62mm. Mayor improvements in flue flow performance can be achieved after lining with Eldfast.   As with brick flues, these types are suitable for in - situ type lining, however the surveyor must take care to ensure that flues are not too irregular / have numerous protrusions into the flueway that would make a continuous coating difficult.