Direct Vent appliances are "sealed combustion systems." Air for combustion is delivered to the appliance through the venting system from outside.
As a result, direct -vent appliances do not consume warm indoor air for combustion, and they are not subject to oxygen starvation, poor draft and other problems associated with "tight" energy efficient homes.
With sealed direct vent systems, all combustion by-products are safely vented through the system, eliminating fumes, water moisture and indoor air contamination.

Remember: Never decrease the size of your flue pipe due to dimensional strictions.