Yes, rectangular flexible duct can be tolerant to high temperatures, depending on its construction materials and specifications. Some products are designed to withstand temperatures up to about 1650 degrees Fahrenheit, making them suitable for heating applications or environments with elevated temperatures. However, it is crucial to check the manufacturer’s guidelines and ratings before installing the duct in such conditions to ensure it meets the required temperature tolerance. Always follow the manufacturer’s recommendations and local building codes when selecting ductwork for high-temperature applications.