Why does my chimney or a combustion chamber emit smoke?

If aeration is higher than the chimney draught, the boiler will emit smoke. You need to investigate what is the purpose:
- inappropriate chimney (holes, pierces into other chimney ducts)
- too small height or section
- lack of insulation outside the chimney 
- additional appliances connected to the same chimney duct
- polluted duct or boiler
- bad construction of an elbow or elbows (acute angles, too many elbows)
- lack of ascension of the flue duct (exhaust gases should flow slightly up after having left the boiler)
- flue duct too deeply in the chimney
- too big ventilation opening in the wall (it may generate opposite air direction)
Additionally, in boilers with an automatic aeration, air flow should be limited:
- by using a metal flap of a ventilator
- by setting up parameters in the steering of a boiler