Today I had this fault again and I found out that hot air valve is getting stuck in the open position irrespective of the switch position or various logics. I spoke to the AME. he guessed it is the summer season in India that is the culprit. In summers we have lot of dust in the air that settles down on this valve over a period of time and spoils its spring function. I am sceptic about this explanation. There are many other such valves in this aircraft. if a theory is applicable to one, it should be applicable to all.. isn't it?