Will high efficiency furnace plumbing lead to cold air entering the house?
These vents are for a high efficiency furnace. One brings in fresh air for combustion, the other exhausts air after combustion. It's separate from the air you breath in your home, hence the high efficiency part. The biggest concerns with these are venting exhaust too close to a window, and having the exhaust sucked back into the intake, but this shouldn't be a problem with a properly installed system. There's no need to modify or cover these vents, doing so can damage your furnace.