Chimneys for open combustion chamber appliances in buildings other than dwelling units must be marked "__________".

The correct designation for chimneys serving open combustion chamber appliances in non-dwelling unit buildings is "Building Heating Appliance Chimney." This labeling is crucial for a few key reasons. It distinguishes the chimney's intended use and ensures that anyone working on or near the chimney understands it is specifically designed for heating appliances within a building setting. Proper identification helps in maintenance, inspections, and compliance with safety regulations.

While other types of marking might seem relevant, they either lack the specificity needed for clear understanding or do not adequately describe the function of the chimney in this context. Marking it as "Building Heating Appliance Chimney" conveys precise information tailored to the setting where the chimney is installed—facilitating clear communication among building operators, maintenance personnel, and inspectors regarding its usage and the safety protocols that need to be observed.