I've got a similar problem yesterday....
That is, I was sitting in the parking lot, key on, blower working at full speed with fresh air. Then, just after I started the engine, the windshield+side vents on the
left side began to blow hot air (left temp wheel was set on minimum, as usual), while the right side kept on working properly.
Now, if this was a problem with the heater valve, it would pour heat from all vents, yet the right side worked properly. In cars with no A/C, temp can be set separately for right and left from the two wheels, that is, one can set the left on 22C and the right on 18C, and the system blows two separate flows of air, right and left.
That is, I've closed the left side vents... and I'd wish to know what to do
~Thank you
Nautilus