The electric AGA doesn't have a flue or chimney and the latest AGA 3 Series and 7 Series models don't have any in-built extraction. (It is recommended [but not compulsory] to have an extractor above them though.)
The latest AGA 7 Series models (and the Total Control and Dual Control AGAs the 7 Series replaced) are as-standard 'Room Vented' but as an optional extra can be specified (where feasible) with the 'external vent' kit (a 28mm copper pipe and external fan box).
The AGA 3 Series models can't be fitted with an external oven vent - they are 'room vent'only.
Is the external vent a black pipe off the top of the cooker?
No, the electric AGA external 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 AGA 7 Series, Total Control and Dual Control AGAs come as standard with 'room vented' ovens but can be specified (if the positioning of the cooker allows) with the external oven ventilation pipe system. (It's recommended to have an extractor fan above these to take the cooking smells to outside.)
Note: You can't convert a room vent Total Control AGA to external vent model (or vice-versa) but most Dual Control AGAs and the AGA 7 Series cookers can be swapped if it's practical to fit the system. (It's not straight-forward [or cheap!] - you have to pull the cooker forward to get to the connections and fit the kit.)
AGA 3 Series and AGA 60 have no pipe or flue to outside - these are 'room vent' only.