I plan on installing both a 4" flex liner and a outside air vent up thru my masonry flue. While researching the various options for the flue top termination I came across the Prairie (collinear style) cap. It looks like a really nice setup. Anyone have a problem with the exhaust mixing with the fresh air since they are in such close proximity? See attached pictures for 2 versions of the cap. Is this the best way to go? Anyone using this setup? If not, what are you using? Where did you buy it? Thanks.