This is because there is smoke coming back into your room probably bringing carbon monoxide with it. There are 4 main causes:

1. The fireplace is too large for the flue. The best solution is to reduce the size of the opening.
2. The chimney pot/terminal is too small (narrow). Ensure that the flow of the flue gasses is not inhibited in any way. A narrow pot can cause an obstruction. Make sure that the chimney pot is the correct size for the internal flue.
3. The flue is partially blocked (sooty deposits, birds nest, partial collapse). Call us to immediately to remove the obstruction and clean the flue.
4. Down-draughts can also cause smoke marks, see the section on Down Draughts on the Chimney Problems page.