The main benefit of a direct-vent boiler is that it supplies the burner with outside air, preventing problems such as oxygen depletion, back drafting and carbon monoxide buildup. Most direct vented appliances have a horizontal vent that exits through an outside wall.
Combustion in a boiler, furnace or water heater requires oxygen. Direct venting provides this fresh air without creating drafts in the house. Without a way to replace used air, the flame competes for the same oxygen the residents are trying to breathe. In older homes, oxygen entered the house through small cracks in the wall and around the baseboards, doors and windows. As of 2015, home construction is much tighter, eliminating many of these small openings. Without an open window, normal ventilation of appliances makes breathing difficult for occupants of such spaces.
Direct venting also helps to prevent back drafting in the flue. Without an air source, combustion creates a vacuum in the home by reducing the amount of air inside. When pressures are unequal, the lower pressure pulls through any opening to equalize them. If the only available opening is the flue or chimney, it pulls air through these spaces at the same time the appliance is emitting combustion byproducts. The result is that flue gases flow back into the home through back drafting. The air disturbance often extinguishes the pilot light on the appliance while pulling deadly carbon monoxide back into the home.