A smoky fireplace may also be caused by a flue that is too small for the size of the firebox opening. In this case, the problem can often be corrected by reducing the size of the opening by raising the hearth or installing a canopy on the top portion of the opening. On the other hand, determining the amount to raise the hearth or the size and shape of the canopy will require some experimentation.
In some cases, the soot buildup is the result of backsmoking caused by down drafts or by a negative pressure condition within the house. The latter is a rather common occurrence in newer houses. In order to conserve energy, newer houses are better caulked, weatherstripped and tighter than other houses. The negative pressure results when more air in the house exhausts to the outside (from fans, heating-system chimneys or others such) than flows in from infiltration.
When a fire is first lit in the fireplace and the damper is opened, the negative pressure causes an onrush of incoming air down the flue. This condition can be corrected by slightly opening a window, or, in newer fireplaces, opening the fresh air vent associated with the fireplace.
Check the bricks or stones lining the firebox for cracked, chipped, broken, disintegrating or missing sections. Check the mortar joints to see whether they need repointing. If you see cracked or open sections inside the firebox, record the condition on your worksheet for future repair because they are a potential fire hazard.
From inside the firebox, look up at the flue. This area is normally coated with a layer of soot and creosote. If the layer is thick, the chimney should be cleaned to minimize the possibility of a chimney fire.
If the creosote buildup is of a tar-like consistency or a hard glaze, it cannot be brushed out by conventional methods, but removed by chemical or mechanical means. Check the flue for obstructions such as bird nests or other. If the flue has a slight offset and you can see the daylight, there is no obstruction, but if it is offset so that you cannot see straight up, determining whether there is an obstruction is difficult without lighting a fire.