I live with a Rayburn which isn't sealed anywhere. Round the flue box, round the hot plate or round the firebox doors.
when I start it up, it generally smokes a bit and settles down as the flue heats.
What makes the most difference to this process is how long the external chimney or whether it's fitted. I had a 4.5' one which I stupidly left on and lost in the canal. I replaced it with a 3' one and the draw is not so good, if the fire gets a bit low it will smoke, but more important, will set off the CO alarm. Working to Rayburn recommendations, the minimum outside flue would be around 6'.