Furnace venting: is a single-pipe installation on a high-efficiency furnace a problem?

High-efficiency furnaces come with two pipe openings; one pipe brings combustion air directly into the furnace and mixes it with fuel. The other pipe exhausts the combustion gases directly to the outdoors. Both of these pipes are directed to the outdoors at about 95% of the furnaces that I inspect in Minnesota. In these systems, the furnace obtains all of its combustion air directly from the outdoors and exhausts the combustion byproducts directly to the outdoors. A two-pipe system like this is called a direct-vent appliance.