Boston’s Best Chimney receives several phone calls on a regular basis with a client stating “I had a fire in my fireplace on the first floor and got smoke in my basement.”  Sounds like thermal siphoning.
Thermal siphoning is when smoke is carried up and out of the chimney and pulled directly back down by a neighboring flue. The natural tendency of heat to rise may explain this common problem.  The warm air inside and cold air outside creates a form of draft inside the home. The warm air, or high pressure area (positive), will be on the upper levels with cooler, low pressure areas (negative), at the lower or basement levels.  The negative pressure area draws in outside air trying to reach that neutral plane and can do so by pulling from the chimney.  This is called stack effect.