A complaint often heard about fireplaces is that they smoke excessively. A smoking fireplace can be a result of several factors.
A smoking fireplace can be a result of having a damper plate partially closed during burning. You must be sure to have the damper plate completely open while the fireplace is in operation.
Other causes may be:
Plugs or blockages in the flue, cap or screening
Abnormalities in chimney height, flue or throat size
The damper location
The smoke chamber or smoke shelf design
Air leaks within the chimney
Inadequate air supplied for combustion
If your fireplace continues to be a problem even after proper draft has been established it may be because the flue is too small for the size of the fireplace opening. You can reduce the fireplace opening by installing glass doors, or by placing a metal strip across the top of the opening. To measure what size metal strip, cut a 12 inch wide strip and slowly slide it from the top of the fireplace opening to the position where smoking ceases. Mark this point and measure up to the lintel.
If a masonry chimney is built on the outside of the home and not up the middle of the home the chimney structure and therefore the flue will sometimes cool quickly as the fire burns low. On windy or rainy days, when weather conditions draw heat from the masonry structure more rapidly, this cooling can create a reverse draft problem. Cold air sinks and the smoke sinks with it causing it to push into your living space.
Sudden down drafts can be caused by high and low pressure zones that develop around buildings due to wind. They can also be the result of a chimney that is too short in relation to the height of adjacent buildings. Another cause is outdoor temperature changes. As temperature differences decrease between inside and outside air, draft also decreases.