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