What would cause heat to blow hot air then randomly switch to cold/outside air out of different vent?

Example. Heat set on floor only, everything works, then randomly I have cold air blowing in my face. This happens on any setting included recirc.

Coolant temp and level normal. vacumm lines all look good, truck has 36k on it. Think its a blend door issue?

08 F250 single zone without Auto climate control.

Please give me opinions. Really do not want to take to dealer, nor rip dash apart for no reason.

 

It does sound like your blend doors. If it were a vacuum leak it generally defaults to defrost and doesn't really have any impact on temp.

 

Thanks, So far Blend door motor seems like the issue. Doesn't look to hard to change once air bag is removed.

 

The removing the airbag is nothing. However you have to have child sized hands to replace the top actuator. Took me hours to line it up and get the new one back in.

 

I'm in the same boat, my drivers side blows hot air only. Passenger side works fine.