Direct Vent (also referred to as "Sealed Combustion") means that there are two vent connections to the outdoors. One is to bring air for combustion directly to the burner enclosure from outdoors and the other to exhaust flue gases directly outdoors (through wall or roof). Direct vent boilers cannot vent into a chimney, but if no other appliances are using the chimney, it can be used as a chase to connect the two vents to the outdoors.