Blend door

Okay, the dreaded blend door problem has plagued me for four years now, after cold Chicago winters I finally decided to remove the actuator and manually open the blend doors by hand as the seasons call. In summer I turn the knob for air conditioner and I get very cold air but as winter approaches I turn the knob and get luke warm to warm breath hot. I have flushed and back flushed the heater core to try increase the temperature but it has not raised the temperature. Any suggestions as the winter approached will be greatly appreciated.