I just bought an old duplex and was informed the cold air returns in the upstairs are vented into the chimney (gas furnace). Is this dangerous? What has to be done to correct the situation? I am converting this duplex to a single family. One furnace is bad (works, but cracked and noisy), the other is too small to heat the entire house. I would also like C/A put in. What is the best way to handle the situation?