I am wondering when a furnace is installed if the front of the furnace is parallel with the trunk line or perpendicular (or can it be either?)

Thanks,

Katy

Doesn't make any difference. Depends on placement of furnace and ductwork. When blower comes on, the air moves through the point of least resistance, it doesn't know right, left, front or back.

If you are talking about the return duct it must always come from the left or right of the furnace. Never out of the back!