Direct vent furnaces are wall furnaces that heat one or more rooms using air from outside the home for combustion that creates heat. They use a special vent that exhausts combustion gases while supplying air to flames in a sealed combustion chamber. Room air does not contact the flames.
The key to the operation of the direct vent appliance is the concentric exhaust. This pipe-within-a-pipe design allows exhaust fumes to exit through the center pipe while the furnace draws air into the sealed combustion chamber. Direct appliances are a good choice for tightly sealed homes that allow little air infiltration.
The vent for these furnaces passes through a small opening in the wall. Manufacturers provide a small termination cap to keep the vents unobtrusive. The vent prevents the buildup of exhaust gases and moisture that is common with ventless heaters. Since the air for combustion comes from the outside, the oxygen levels in the home are unaffected by the furnace operation.
Most direct vent furnaces operate using a thermostat. Some have programmable thermostats that can automatically set a lower temperature for when the home occupant is away to increase energy efficiency. As the air circulates around the combustion chamber, the heat the furnace creates warms it to the set comfort level. Manufacturers add blowers to help distribute the heat more evenly.