We have a fire place and furnace flues next to each other in a stone/brick fireplace, pretty typical. Our Michigan weather is very cold at this time around 15 degrees. The furnace is freezing up at the chminey top on the flue cap. This then melts and runs down and leaks into the fireplace. I removed the flue cap to help the freezing problem, but it continues. The rising hot gases condensate and run down the flue liner!