Capping unused chimneys - flue ventilators UK
Unused chimneys not only waste a lot of heat, they also pose a potential damp problem as well as being a cause of smoking fireplaces and stoves. You should cap any unused chimneys with a flue ventilator. Flue ventilators can be found on our chimney pot page. The flue ventilator stops rain entry to the chimney but still allows a small amount of air passage for ventilation.
You should block off the bottom of the chimney wither with a register plate or by closing the baffle at the back of your fireplace if you have one. Some ventilation holes are a good idea so that the chimney is still a ventilated space so consider leaving a small gap or hole somewhere when you block the chimney off.