I am having a similar problem with my heater. I have put in new thermostat, had radiator flushed, and coolant changed. I feel warm air blowing from floor vent and defroster . I was told that the vents in the dashboard don't blow except with the air conditioner at temps lower than 76 degrees. I vaguely remember the dashboard vents blowing hot air last year. Can someone validate this? I feel hot air thru the stationary vents on the extreme left and right side in the dashboard. I read the answer about the blower door behind the glovebox. The mechanic I went to wants to keep trying different things(and charging for them[:@]), but doesn't seem to know definitely how to fix this problem. I need help guys.
Thanks for any assistance or ideas you can give.

 

Buy a book sound like a vent problem.
If you can feel hot air on one side but not the other you have a vent problem if air feels slightly warm both sides you have a bad water vale or heater core issues.