Fireplace Opening is Too Large
This problem typically allows the fire to burn comfortably for a while, but the room will become smoky after some time. The fireplace opening should be sized based upon a relationship with the chimney flue tiles. An ideal fireplace opening would be no more than ten times the cross sectional area of the chimney flue. For example, if the inside dimensions of the flue tiles are 10 x 10 equaling 100 square inches, then the fireplace opening should not exceed 10 times this or 1000 square inches.
If a fireplace opening is too large, it will allow more air into the firebox than the flue can exhaust. If the opening is only slightly larger than it should be there is an easy fix for the problem. Installing a Smoke Guard at the top of the fireplace opening will reduce the effective opening of the fireplace.
You can try a temporary Smoke Guard yourself before purchasing a permanent Smoke Guard. To do this, cut a strip of tin foil several inches longer than the width of the fireplace, and tape it to the top of the fireplace opening. As depicted in the picture above, cover the top of the fireplace, shortening the height by approximately four inches. Try burning a fire on a cold night; if decreasing the size of the fireplace helps to improve your draft, a permanent Smoke Guard could be the solution for you.