When an exhaust system is in overpressure operation, the pressure inside the system may be greater than outside. This means that the exhaust gases are not discharged naturally, but by means of a blower. This is necessary if, when exhaust gas temperatures are too low, the natural buoyancy is not sufficient to guide the exhaust gases over the roof. For appropriately designed exhaust systems, this means increased requirements for the tightness of the exhaust pipe.
Source: bauwion
