A disused chimney provides an obvious potential route for heat loss, which can be minimised by appropriate capping-off, as recommended by the Energy Saving Trust.
Protection from the elements
In the course of a year the UK weather can generally be relied upon to deposit the equivalent of several feet depth of water into any uncapped chimney, where combined with the effects of frost action in the winter months, it causes steady internal deterioration of the fabric of the stack and can give rise to major damp problems internally.
Any unprotected chimney is prone to blockage by nesting rooks, trapping moisture in the chimney and blocking ventilation to prevent it drying out. Not to mention the dreadful unhygienic mess that nesting rooks can cause and the difficulty of removing the nests.
In some areas the grey squirrel presents a real problem, potentially getting trapped in an unprotected chimney and perhaps even gaining access to the home where it can cause serious damage.