I do not think the A/C is involved. I turn the A/c switch off or on. And I have HOT air at every vent. Even with A/C turned off I should have cool air from the vents if outside temp is cold and fan switch on. The hot air is only there if the fan switch is on. My Temp setting makes no difference with A/C on or off. The air blind door must be working as no air comes out if the fan switch is off. I plan to check the Coolant circulation valve today. I THINK that the AIR blind door would be Vacuum controlled and the coolant circulation valve is electrical controlled. But that is just my guess.