A boiler installed in a closet shall be which of the following for such installation?

A boiler installed in a closet must be a listed installation, meaning it has been tested and certified by a recognized testing laboratory to meet certain safety and performance standards. The listing ensures that the boiler is suitable for the specific environment in which it is installed, including confined spaces such as closets. This designation assures that the installation complies with local codes, manufacturer specifications, and safety requirements designed to prevent hazards such as improper ventilation, overheating, or inadequate access for maintenance.

A listed boiler installation often includes specific instructions regarding clearances and ventilation requirements, which are critical in ensuring safe operation in enclosed areas. Thus, using certified or approved may ambiguously suggest compliance with standards, but "listed" specifically refers to products that have undergone testing and certification for the intended use, particularly in restricted spaces like closets.