Heater problems

[Heresjohnny]

Hey guys.. Was hoping somebody could shine some light on a problem I'm having with my Citroen C5 which I've just bought about 5 weeks ago..its a lovely looking car and mechanically sound but whilst driving, heating is blowing hot air from whatever vents I select but when I stop the car at traffic lights etc, it's blows out cold air and won't go hot no matter what I do until I'm driving again.. Its a 1.9hdi MkII. Thanks in advance to all

[meister]

It could be problems with the heater-cooler, also could be dysfunctional thermostats or water leaks.