Dmarsh

I am also having this problem. 2 story home with a full basement. 3 flu chimney( fireplace on first floor, wood stove in the basement, and an oil boiler in the basement). I have a radon mitigation system and the house is very tight. I have to open a window to start a fire in the fireplace or the wood stove or I get smoke backdraft until the flu heats up creating a good draft. If I have a fire in the fireplace, I get smoke in the basement. I suspect that the smoke is going up the fireplace flu and being drawn back down either or both of the other flue to the basement. There is an ash dump in the basement for the fireplace but did not think of that being the culprit. Could it be drawing smoke from the fireplace through the ash dump door on the floor of the fireplace?