AC/furnace return vent (supply) to unit

[mike_2013]

Hello. I moved into a house recently. The house was built about 15 years ago. The AC/furnace return shaft has two vents (main level and top level). This I understand. What I don't understand is why someone cut into the shaft and ran a 8" flex tube outside to a vent on the back of the house. This seems like it would bring hot humid air inside to the AC in the summer....and cold air into the furnace in the winter....right? Why would I want that? There must be a reason. The second part of this....I discovered this when I was getting ready to add a return vent in the basement. I want to do this to circulate the air from the pellet stove. Good idea, right? I was thinking I could use this 8" tube that's already there and relocate it to above the stove. But I figured I best not mess with it until I find out why it's there. Thanks, Mike

 

[nealtw]

In a tight house you need replacement air as bathrrom and hood fans and people just use it up. In some places it is code.

 

[mike_2013]

Thanks. I've never had this in a house, and have lived in newer houses in the area. Think I could just disco that? We have plenty of doors opening all day, kids, and dog doors.