Hi, I have a Ducato 2004 with a similar issue. I have NO heat coming through. I contacted Lakia who confirmed there is an electronic valve and advised me to contact a fiat dealer. (Thanks Lakia - quick/helpful reply with possible reasons for the issue and advise for each).
your valve is stuck open and mine is closed. My van is in for MOT and service at local garage who will investigate further, but initial thoughts are that the valve is behind the dashboard near where the pipes go through the bulkhead.
If you locate the valve or have any part numbers then would be good to know. I'll let you know if I find anything out.