Your fireplace vents a large amount of air out of your house with the smoke and up the chimney. This creates a relative vacuum in your house. Somewhere, air has to come into your house to make up for all that air rushing out through the fireplace chimney. The easiest place is that big open hole where your furnace exhaust is supposed to go out. Air just as easily rushes in and can carry smoke with it. If you had glass doors on your fireplace then you could reduce the amount of air going out the chimnety by 80 - 90 %. This would give you a chance to not be losing more heat out the chimney than you are gaining from the fire. Your situation is potentiall dangerous. What happens when your furnace kicks on and you have this reverse draft in the flue? You might also be getting furnace exhaust going into the house. You could create a carbon monoxide poisoning situation. At the very least, open a window somewhere to let in makeup air. Buy a CO detector. Others might have more ideas. I'm not expert in solving your problem - I just outlined what I think is causing it.