Volvo V40 blowing hot air only regardless of where temp control is set

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I have a 2002 Volvo V40. It recently developed a problem where all I get is hot air no matter where the climate control is set. The fan control works fine, but even when I have it off I feel the hot air while driving on the freeway. The air is cold for a few minutes until the car warms up. The air stays hot even when the AC is turned on. If I turn the AC on right away, before the car warms up, it works. I have also noticed that if I am idling at a stop light the hot air decreases, then increases again when I accelerate. I have already replaced the climate control unit with a used one, which took care of the erratic blower fan speed. I also had a new resistor pack installed last summer. I also checked the damper control motor and arm and confirmed that is works when I adjust the temp control. Any ideas on what could be causing this? Thank you