What type of door shall not be installed on a factory-built fireplace unless it has been specifically tested and labeled for such use?

The correct choice emphasizes the safety and performance standards required for doors installed on factory-built fireplaces. A gasketed fireplace door is designed to create a tight seal, which is essential for controlling airflow and preventing smoke and gases from escaping into living spaces. This type of door helps maintain efficiency and safety by ensuring that the fireplace operates correctly under various conditions.

For doors installed on factory-built fireplaces, it is critical that they have been rigorously tested and labeled for such use. This ensures they meet specific fire safety standards and performance criteria, providing both structural integrity and operational functionality.

In contrast, glass fireplace doors may not always provide the necessary sealing unless they are specifically tested for that purpose. Wooden doors, while aesthetically pleasing, are not appropriate due to their combustible nature and susceptibility to heat. Metal doors, while more durable, also require adherence to testing standards to ensure they can withstand the high temperatures and potential hazards related to fireplace use.