Chimneys need to be draughty in order to work, but if you are no longer using them they will represent a major element of heat loss in the home. However many people want to keep them intact either for aesthetic purposes or for occasional use (after all there is something about having a real fire on a cold winters night).
This represents a series of choices in what to do with a chimney.
So if you want to use the chimney occasionally then you need to have a way of sealing off the airflow in a easily accessible manner. The most common way of doing this is to use a ‘Chimney Balloon’. This is a simple hardly plastic balloon that is inflated in the flue and effectively stops any airflow up the chimney. This is fine, but you also need some airflow in order to keep the chimney dry. Ummm. The solution here is to stop water ingress from above. To do this you can fit a number of attachments to the chimney pot to create a cover over it. Common names for covered pots include: Hooded, Mushroom, Saucer, H Pot. These can be retrofitted quite easily, but do need access to the roof of course. With these fitted you can effectively stop excessive water ingress, keep the flue open at the top and then reduce air flow at the bottom with little fear to damaging the flue.
If you are no longer using the chimney you can fit a much cheaper plastic removable cover over the chimney pot that acts in several beneficial ways. It will stop water ingress, reduce the chances of birds nesting on the chimney, stop insects nesting in the chimney and also allow for some trickle ventilation. The most common of these is the Chimney Cap. Combined with a Chimney Balloon it will allow you to keep the chimney intact whilst reducing the air loss. It also means that in the future if you wanted to restore the chimney / fire place into working order it is a ‘simple’ case of removing the cap and balloon.
Other solutions include making a ‘Register Plate’. This is a metal sheet that acts like the chimney balloon. They are commonly used in fireplaces when installing a stove. Two simple holes are made into the metal sheet for the flue and for cleaning. The cleaning one has a flap on it for easy removal (whilst also preventing air loss) and the stove acts as the heat source and also limits the draughts much more than an open fire.
So remember that draughty chimneys need a little more thought that just adding a Chimney Balloon as you need to manage water ingress as well.