A Direct-Vent system uses two vent pipes. One draws air in from outdoors to support combustion. The other exhausts the by-products of combustion to the outside. The two vent pipes may be concentric (exhaust pipe within a larger diameter intake pipe) or co-linear (separate pipes of about the same size). Combustion takes place inside a sealed chamber – with a glass front in the case of a fireplace. Room air circulates around the combustion chamber, often assisted by a blower, to heat the room. A D