Heater only functions while driving

[wyodan59]

Hello. A friend of mine owns a 2011 Impala. He has been trying to figure out his heater. It will not put out any heat while the car is running and warmed up sitting in the drive way. When he starts driving the heater works good. So basically the heater is only working while the car is being driven. My daughter just told me her 2009 malibu. is doing the same thing. Is there a vacuum running these heaters?
Thanks

[la1]

I believe they only used vacuum to actuate the blend doors till around 2005 on the Impala's after that it was electric. Sitting still when you switch from inside to outside air can you hear the change in the doors. I am thinking maybe your blend doors are stuck on outside air, and there is a malfunction where the fan is blocked.