In my experience 99% of Chimneys work well and cause no problem’s, The 3 most common problems:
1. Holes in chimney causing cold air to cool fumes. Cure, point the chimney up.
2. Air being pulled into the room by other sources, an extractor fan another fire or boiler you can test this by putting the back of hand against the opening and you will feel cold air, the cure ,try opening a window in the room, if air is being drawn into the room because of a boiler or appliance it will always pull it from the nearest source so you may need to fit an air vent close to the appliance.
3. Cold Chimney, if the stove door has been closed for a while the chimney gets cold and needs warming up the cure is to burn some loose newspaper to warm the chimney.
In my shop I have three old chimney all over 120 years old except for one which had to be metal lined because it is shot permanently damp because of where it terminates, and it’s a big job to point it up. The metal flue gets very cold plus the other two stoves burning to cure it I open the shop door to reduce the pull and burn a full loose newspaper over the top of the prepared fire, then quickly light the newspaper and the firelighter at the bottom of fire and it works.
– William Evans