Why have I started getting a downdraft in my chimney after adding a fireplace door?

Hi! I have a 1916 two-storied house with a fireplace and big chimney. The chimney has two flues (one was used for an old coal stove in the basement, now is not in use). There is no cap. There is normally a very nice updraft. Recently I installed a $600 fireplace door to supposedly increase its heating efficiency. When I close the door, there is always a down draft and smoke. Why is this happening? By the way, the door has two small dampers built in; I left them open but it makes no difference. Thanks!