why does cool air come from hvac when lowered by 1 degree from HI? on 2004 Kia Amanti

When heater control unit is set to Hi, lots of heat, lower it by 1 degree, cool air starts to come from vents or floor. Keep lowering the temp, no difference. I have not scanned it yet for codes. It has an automatic HVAC with duel controls, the temp it the same on both sides.
Any suggestions?
Thank you

0 answers