Our plumber Nick was called to a home in Cedar Grove, New Jersey to inspect a high-efficiency furnace pipe. The intake air pipe was white while the exhaust pipe was a noticeably darker color.
The reason that the pipe on the left is a darker color is that PVC is not specifically made as an exhaust pipe for a high-efficiency unit. If your furnace is not running correctly, you may notice this discoloration as it starts do break down the PVC. The results can create to a hole or cause the pipe to become disconnected. This allows carbon monoxide to pump through your house. The best solution is to install a Innoflue (gray-colored) pipe that stands up to the heat and is safe for your home. Innoflue is specifically made for the venting of high-efficiency units, and they stand behind this claim, whereas PVC is not.
PVC Pipe Discoloration on a High Efficiency Furnace