Abstract: A throttling device is located in a venting pipe or in a pipe section such as a flue gas pipe or conduit arranged between a boiler and a flue or chimney. The flap is held in a pipe section for removal out of the respective pipe either by tilting or by pulling the flap out. For this purpose the pipe section has a slot which is about semicircular and which, in operation, is covered by a ring section or sector. The throttle flap may be pulled out of the pipe section together with the ring section and with its adjustment drive whereby cleaning of the pipe or conduit is substantially facilitated. Heat losses through the flue are avoided when the throttle flap is closed.