A boiler installed in a closet must be what for such installation?

For a boiler installed in a closet, it must be classified as listed. This designation means that the boiler meets specific safety and performance standards as recognized by a professional testing organization. Listing ensures that the equipment has been tested under various conditions and has passed evaluations for its intended use.

Having a listed boiler is critical because it guarantees that the installation complies with safety codes and regulations, which are designed to reduce the risk of fire, explosion, and other hazards. Moreover, the use of listed equipment often simplifies the permitting process, as authorities typically require this level of certification for the safe operation of crucial systems like boilers.

The other choices, while they may pertain to certain standards or types of equipment, do not specifically apply to the necessary certifications for installation safety and compliance within a confined space, such as a closet. Custom-built or standardized options may not meet the stringent testing and approval requirements that listed equipment does, making them less suitable for assignment in such a setting. Pre-approved is a generic term that could imply various forms of certifications and doesn't specifically indicate that the equipment conforms to a formal listing standard.