Your flue will only need lining if it leaks or if it is not of a suitable size for the appliance connected to it. With older flues the chances are that it has deteriorated and has the potential to leak. Should you be considering a wood burning stove or similar then these are designed to work best on a particular size of flue. The standard size of flue for a stove is 150mm internal diameter and with a flue of this size the draught entering the stove is more controllable.
Where the flue is too large the internal flue temperature can be too cool and so any moisture in the smoke and fumes are more likely to condense. With gas fired appliances this is more noticeable but it is true for all fuels that there is a certain amount of moisture in them and if the flue temperature is too low these will condense potentially causing damp problems or encourage the build up of tar and soot.
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