A number of factors can explain a smoking chimney. Correcting the problem may be as simple as checking to be sure the damper is open or as complicated as a chimney pot of the wrong size. The place to start is with problems that are easy to fix.
The chimney should be regularly inspected for debris such as a fallen bird’s nest, pieces of brick, or sticks and leaves. Simply shine a flashlight up the chimney and take a look, with an eye out for soot or tar deposits. If it appears to be clear and clean, something more complex is probably the cause of your problems.
A fireplace opening of the wrong size may be the problem. This can be checked by blocking off the top third of the fireplace opening. If the chimney smokes less, install a metal plate at the top of the opening. Similarly, the chimney may be the wrong height. Higher chimneys generally draw better, so consider adding a few layers of brick to the top of the chimney. Alternatively, a chimney pot that is too narrow could be the culprit, in which case simply installing a wider chimney pot should correct the problem.
Inadequate ventilation in the room may also be to blame. Open a window in the room, or if the room has no window, open a window in the room closest to it as well as any doors between them. If the smoke diminishes, this is the problem and it can be fixed by installing a window or a “black hole vent”.
Another possible cause of a smoking chimney is the presence of other uncapped chimneys. All unused chimneys should be capped, not only for this reason but to keep out water, animals and debris. Finally, the chimney should be lined and insulated, which will improve its draw and prevent it from smoking.