Nowadays not all AGAs need a flue: the electric AGA just has a small oven vent pipe that runs horizontally to outside.
The latest Total Control and Dual Control AGAs can be specified either with the 'external vent' pipe and external fan box, or as 'room vented' not requiring any pipework from the cooker to outside.
Is this the black pipe off the top of the cooker?
No, the electric AGA oven vent pipe is a small (28mm diameter) copper pipe that exits the back of the Cooker just below worktop level and runs horizontally to the fan box which is mounted on the wall outside.
(NOTE: It's not recommended to terminate the pipe in to a chimney.)
There is a small flexible cable runs next to the pipe to the feed the fan box from the AGA (extra-low voltage, 24V DC maximum).
It can run up to about 4m of straight pipe, reducing in length to allow for any bends that may be needed (bends add resistance).
The pipe needs to gently fall to outside and cannot incorporate any dips or troughs (which would fill with condensate and block the pipe).
We can check the proposed route to outside when we come to carry out the home survey before your AGA is installed.
The Total Control and Dual Control AGAs are available with either the external oven ventilation pipe system, or as a 'Room Vent' model with no pipe to outside. (It's recommended to have an extractor fan above these to take the cooking smells to outside.) You can't convert a room vent model to external vent model (or vice-versa).
AGA 3 Series and AGA 60 have no pipe or flue to outside - these are 'room vent' only.