Does your open fire have a weak draw of air whilst burning? Even after its just been swept? Do you you find that the air in your room is smokey when the fire is on? Do you find you only have these symptoms on odd occasions such as windy days? Chances are you need an Anti-downdraft cowl. There are many available for purchase so check with your chimney sweep before buying one.
Please note that anti-downdraft cowls are not designed to stop drafts coming down the chimney when the fire is not in use.