i have 3 pre-fab wood-burning fireplaces, one above the other in the basement, 1st and on the 2nd floor. It appears as though 2 of the fireplaces' chimneys are shared and one has its own (recalling when the house was built). No matter which fireplace is used, smoke backdrafts down the chimney pipes (I assume). I've built wooden covers for the unused fireplace openings, which seal well but smoke STILL comes in. Your opinion please before I call in a "professional." Fireplace installer claims too many fireplaces in one spot! Thank you.