Which piping material is NOT mentioned as being approved for use in heating panels?

PVC pipe is not approved for use in heating panels primarily due to its thermal limitations and properties. In applications involving heating, the temperatures can exceed the capabilities of PVC, which may lead to deformation or failure. Furthermore, PVC is not suited for high-pressure environments typically associated with heating systems. Materials like Type L copper tubing and standard-weight steel pipe are specified in codes because they can withstand the pressures and temperatures typically found in these applications. Polybutylene, while not commonly used today, was also at one point considered acceptable, but its use has been limited due to other factors, primarily related to durability and performance in various conditions. The recognition of suitable materials is crucial for ensuring the safety, efficiency, and longevity of heating systems.