The fourth category of furnace is the high-efficiency, or condensing furnace. High-efficiency furnaces can achieve from 89% to 98% fuel efficiency. This style of furnace includes a sealed combustion area, combustion draft inducer and a secondary heat exchanger. Because the heat exchanger removes most of the heat from the exhaust gas, it actually condenses water vapor and other chemicals (which form a mild acid) as it operates. The vent pipes are normally installed with PVC pipe versus metal vent pipe to prevent corrosion. The draft inducer allows for the exhaust piping to be routed vertically or horizontally as it exits the structure. The most efficient arrangement for high-efficiency furnaces include PVC piping that brings fresh combustion air from the outside of the home directly to the furnace. Normally the combustion air (fresh air) PVC is routed alongside the exhaust PVC during installation and the pipes exit through a sidewall of the home in the same location. High efficiency furnaces typically deliver a 25% to 35% fuel savings over a 60% AFUE furnace.