Air source for furnace

[Deb]

I purchased a Colony singlewide with a forced hot air furnace this summer. I also ordered a factory installed woodburning fireplace. When the cold months began, I attempted to light a fire in my fireplace (I checked to make sure all dampers were open first) and the furnace kicked on at the same time, dragging the smoke from the fireplace into the living room. I immediately put out the fire, however, the next day when I was inspecting the fireplace, I noticed a large draft coming from my fireplace vents both inside and outside of the fireplace when the furnace was running. I am told that the furnace seeks out an air source and since the furnace has none built directly to it, it pulls from wherever it can, which seems to be my fireplace. I posted a message to this board several months ago but received no substantial response which I could proceed with. HELP.

[mac]

Your fireplace needs an air source too. When the furnace kicks on, it creates lower air pressure around it, and higher pressure where its vents exit. If your fireplace is in line with the air movement, it will pull smoke out IF it does not have a strong draft started. Try this: Turn your furnace off. Then build a fire. Start by lighting a piece of newspaper to create a draft (get warm air rising up the chimney). Not doing this - even with the furnace off - can cause smoke to back up.
Get the fire going - then, when it's well-established, turn the furnace on. The reason that you feel the draft with no fire and the furnace on is two-fold: 1) cold air sinks - with the damper open, it would just flow in from outside, and 2) the pressure difference I mentioned above.