Do you run the log burner and the open fire together? And is the route open between the two fires, i.e. the doors are open between the two room?
As the log burner could be affecting the draw of the open fire or vice versa. Happens in pubs with multiple stoves, they battle for flue draw. You can check this by opening a window slightly.
Cold block isnt a physical block, its the cold air the chimney which hasn't shifted, usually in the lighting of the stove. If you close it down to quickly there isnt enough heat in the flue, and therefore the smoke hits the cold air and starts to spill back out into the room.
As the log burner could be affecting the draw of the open fire or vice versa. Happens in pubs with multiple stoves, they battle for flue draw. You can check this by opening a window slightly.
Cold block isnt a physical block, its the cold air the chimney which hasn't shifted, usually in the lighting of the stove. If you close it down to quickly there isnt enough heat in the flue, and therefore the smoke hits the cold air and starts to spill back out into the room.