What material is commonly used for Class B venting systems?

Class B venting systems are designed for venting gas-fired appliances, particularly those that operate at a moderate temperature and have flue gas that is not excessively corrosive. The material commonly used for these venting systems is aluminum inner pipe with a galvanized steel jacket.

The aluminum inner pipe provides the necessary resistance to corrosion from the combustion gases due to its non-ferrous nature, making it suitable for use with gas appliances. The galvanized steel jacket serves to protect the inner aluminum pipe, enhancing its durability and improving its structural strength. This combination ensures that the system can withstand the conditions typically encountered in venting applications while also providing efficient exhaust of combustion gases.

The other materials mentioned are not commonly utilized for Class B venting systems due to various reasons, such as insufficient temperature resistance or increased weight that may not be ideal for every application. For instance, using only aluminum without the protective jacket could lead to increased wear and damage over time, while flexible PVC does not meet the necessary safety standards for venting gases. Single-wall galvanized steel, although durable, is typically used in different applications where the temperature and corrosion characteristics differ from those of Class B venting needs.