the house is built down in a tight valley between two ridges. It seems to happen when ever there's a fire regardless of the wind direction or speed. We have closed the flue but that doesn't help if at all. The one thing you mentioned is the height of the stack. It seems like I've seen some formulas in the past for height to ridge ratios. Have you heard anything about that? It's a stone chimney. It would be difficult to make it much higher now but what about some sort of stack on top of the existing flue liners?