Is this a common problem?i have an astra g turned the heaters up to defrost the windscreen but when i came out about 5mins later they were just blowing cold air,even after driveing the car about 6miles still cold air no hot air.They worked ok the day before but have done this a few times now 1min they will produce hot air then its just cold but now all i get is cold air any ideas what this could be??