What type of piping should never be used for gas appliance venting?

The correct answer is PVC (polyvinyl chloride) piping. It is important to understand that while PVC is a versatile and commonly used material in various plumbing applications, it is not suitable for venting gas appliances. The reason for this is that PVC can become soft and lose its structural integrity when exposed to the high temperatures that can occur in gas venting situations. Additionally, PVC is not designed to withstand combustion gases, which can produce harmful condensates that may corrode the material over time.

This limitation makes it dangerous to use PVC for gas appliance venting, as it can potentially lead to vent failures, gas leaks, and hazardous conditions within a building. In contrast, materials like galvanized steel or cast iron are specifically engineered to handle the conditions present in gas venting applications, making them far more reliable choices. Copper, while also not the best option, might be used in specific circumstances with proper precautions but is generally avoided due to its cost and other factors.

Thus, the use of PVC piping for this purpose is not compliant with safety regulations and best practices in gas appliance installations.