An answer supported with logic:
when you turn hot/cold knob (or lever in some cars) a flap moves and seals the hot air path. Hot air still at times get mixed with your AC's chilling air due to any of the following reason:
The flap didn't shut one path completely
The foam type insulation on flap has worn off
The flap is metallic(though very rare)
Now there's a check before you plan to get you heater pipes plugged. When your car engine is slightly warm turn on freash air, turn the cooling knob or lever to coolest and feel the air coming from ducts if you think there is hot air added in it you've got to get the heater pipes plugged...note AC was not turned ON in this experiment.
Hope I helped