Before sweeping an open fire we light a smoke pellet within the fire place. This is to show us which terminal the fire is attached to.
On one occasion it also showed that smoke was emitting from several terminals. The reason for this may have been that the chimney feathers (brick lining between chimneys) were in a bad state of repair. This can lead to serious problems such as gasses from the fire leaking into the property.
We were able to inform the property owner of this problem and suggest to him to have his chimney re-lined.