Explain the necessity of oxygen for combustion. Fire, like plants and animals, needs air to breathe.
Explain how combustion also generates waste, in the form of exhaust and smoke.
Use these two facts to show how a direct vent is advantageous, as it brings air in from outside the house rather than inside it, and expels it through the same piping system.
Explain how this is better than a fireplace because it doesn't need to burn the house's warm air, which makes it more efficient. If warm air is being burned, it's not keeping your house warm. If cold air from outside is being burned, the warm air inside can do its job.