Q: What is downdraught?
A: Where wind for various reasons blows down the chimney preventing gases and smoke from rising up the chimney to the atmosphere.
Q: My chimney has suddenly started smoking?
A: This could be caused by the flue being blocked by mortar or something else try getting the chimney swept.
Q: What is pressure variation?
A: Where the pressure at the fire is less than at the chimney pot, sucking air down the chimney.
Q: Why should my fire smoke when the doors and windows are closed?
A: This is probably caused by lack of ventilation which is causing the pressure to drop in the room.