UPDATE Front Heat works until rear heat is turned on

[Gene 09/16]

Now my heat is only working when turned past 78 degrees in the front and back. Hey, at least it is consistent now. Any clues on what it is now? The engine temp seems to be consistent with where it has always been.
What now?

This is on my 2016 Genesis sedan 3.8.
I am having a heating issue! I turn the rear controls to cold and then turn the front heat on and it will heat in the front just fine. When the rear heat is on the drivers side blows cool but the passenger says at their adjusted temp. When the rear blows hot air the drivers side will not??? Is this an actuator or a different issue. I have pulled the fuses and reset the entire system but to no avail.
All help is welcome

 

[joegr]

Did this start after any kind of engine work? I'd first check the coolant level. If it's low, then possibly there's your problem. Otherwise it there was recent work, of if there is a small leak, it may be that air needs to be bled out of the cooling system.
Alternately, I suppose it could be some sort of control system / actuator problem, but that seems unlikely for your symptoms.

 

[Gene 09/16]

No work has been done, except an oil change 4 weeks ago. I did check the coolant and it is full. For my sanity can you explain how air in the line would cause this. I just don't understand.

 

[joegr]

No work has been done, except an oil change 4 weeks ago. I did check the coolant and it is full. For my sanity can you explain how air in the line would cause this. I just don't understand.

Air in the coolant circuit to the heater core would reduce the flow to the core, which would reduce the total amount of heat that it could provide. The theory being that the added air volume for the rear seats removes enough heat such that there isn't any left for the driver side. Admittedly this is very speculative and may not be the way that it would work out. It might be that this would evenly reduce the heat at all three locations, and thus not be your problem.
I don't have any theory on why turning the rear heat on would cause a problem with the temperature blend actuator for the driver's side. The service manual doesn't seem to be much help without DTCs from the climate control.