I am renovating my basement in a 1940's house. The Chimney is no longer being used as I switched to a high-efficiency furnace. The problem is that water leaks out of the bottom chimney "duct", and into my basement. This only happens once a year when the heavy snow collected over the winter melts over a week. I have a strong inkling that it is leaking in from the bottom of the chimney (not along the stack since it only leaks during the snow melt). How can I repair this problem?