Types of Vented Condensing Furnace Systems (single-pipe and two-pipe)
Direct vent (two-pipe) system: The two-pipe direct vent system is most common in home heating applications and provides directly vented outside air to the sealed combustion chamber with one pipe and with the second vent pipe it provided sealed venting of exhaust gases to the outside.
Non-direct vent (single-pipe) system: The single-pipe, non-direct vent system is used where there is no real need for a separate combustion air intake vent. It provides a vent pipe for exhaust gases but uses unconditioned air (not cooled or heated) from around the furnace for combustion air. These furnaces are usually installed in unconditioned spaces such as the garage, crawlspace, basement or attic where air infiltration is high enough to provide adequate volumes of unconditioned combustion air.
Why Does a Condensing Furnace Require Venting?
The high efficiency condensing furnace has special venting and condensate drainage requirements. Because of the special heat exchanger technology used by a condensing furnace, heat is extracted from the fuel combustion process for a longer period of time, to the point where the combustion exhaust gases having "cooled" and condensed. The exhaust gases are depleted of heat until the water condensate drips out of the furnace's heat exchanger and the low temperature flue gases escape from special plastic pipe instead of a chimney.
Providing correct venting is critical to proper efficient operation of the furnace. Proper installation of furnace venting is essential for correct condensate drainage and maintaining integrity of the entire air intake and exhaust gas vent system. Improper installation of the condensing furnace venting or condensate system can lead to furnace malfunction as described in the tutorial Troubleshooting a High Efficiency Condensing Furnace.