PVC or ABS plastic piping is often used on high-efficiency heating and water heating systems as vent piping for the exhaust gases, since the exhaust temperatures are relatively low.  This piping is often vented out the side of the home.  There are different types of piping used, and one style commonly used is called ‘Cellular-Core” PVC.  Concerns have developed with use of this piping on high-efficiency heating systems.  Check out our article on The Concerns of Cellular Core Piping on Exhaust Systems.