I have a 6 inch round duct that comes out of the main trunk about 5 feet from the furnace. It runs through the basement and outside the house (under the deck). What is this? The furnace also has 2" PVC vent and intake pipe, which I'm familiar with, but I've never seen this before. When the furnace or Air is on there is air flowing through this pipe. What is it? what does it do? Thanks!