If you have a dampener, its source is outside air The dampener is located in the return air line and could be as small as 3/8" wide and 2" long. Sometimes it is concealed by the insulative overwrap of the return air. Look for where the return line has a junction to a transition. If the dampener handle is parallel to the return line, it is open. move it to the perpendicular position and it will be closed. If its open, you're drawing in outside air to be heated up.