Why are there plastic PVC pipes on my furnace instead of a metal exhaust flue?

 Plastic exhaust pipes on a furnace indicate a high efficiency unit. This is an easy way to tell at a quick glance that a unit is high efficiency, meaning that it has an AFUE (annual fuel utilization efficiency) of 90+% or above. This means that the furnace converts 90+% of the fuel (usually natural gas) to usable heat with exhaust gasses in the 100-130 degree fahrenheit range. Older standard efficiency furnaces are typically 80% efficient and have much hotter exhaust gasses in the 300-400 degree range which would melt a PVC pipe and instead needs to be vented through a metal exhaust flue. Next time you see a PVC exhaust pipe on a furnace or water heater, you can quickly identify that as a high efficiency unit.