What distinguishing feature identifies a sealed combustion furnace?

A sealed combustion furnace is characterized by its unique venting system that allows it to draw air directly from the outside for combustion rather than relying on air from the indoor environment. This is typically achieved using PVC piping for both the intake of combustion air and the exhaust of combustion gases. Such an approach provides significant advantages in energy efficiency and safety, as it prevents negative pressure in the home and minimizes the risks of backdrafts, which can occur in conventional furnaces that utilize indoor air.

The use of PVC for venting is a practical choice due to its lightweight nature, resistance to corrosive combustion gases, and the ease of installation. This distinguishes sealed combustion systems from traditional furnace designs, which usually employ metal vents and require indoor air for the combustion process, making the correct answer particularly relevant in understanding the defining features of sealed combustion technology.