Regardless of whether it is a stainless steel chimney or a brick chimney, the cross -sectional calculation (also chimney calculation), or the firing technology calculation of the exhaust system, as it is officially called, is always needed when a combustion system is rebuilt or changed. The fireplace (stove, pellet boiler, workshop oven,?) And the chimney belongs to the firing system. The calculation serves to certify that the exhaust gases are safely led outdoors and the appropriate standard is the DIN EN 13384, which describes both the calculation method for simple and multiple documented chimneys. Your district chimney sweep must give a fireplace notice before you can put the system into operation and he can request the calculation as the basis. Although he can also decide the decision without giving a chimney calculation, in this case you remain unclear whether fireplace and chimney work at all. You will only find out this at the earliest the first heat or, in the worst case, only a few months later when weather conditions change.