A fuel-fired furnace installed in a closet less than 12 times the total volume is only allowed if it is:

A fuel-fired furnace installed in a closet with less than 12 times the total volume must be listed to ensure safety and compliance with the code. A listed furnace is one that has been tested and approved by a recognized testing laboratory for certain safety standards and performance specifications. This designation indicates that the furnace meets necessary fire safety and emissions regulations, which is particularly critical when it is installed in a confined space like a closet.

The requirement for the furnace to be listed ensures that it has undergone rigorous evaluations for safe operation within potentially hazardous environments. When a furnace is properly listed, manufacturers provide details on the installation requirements, ventilation needs, and acceptable configurations, which are vital in preventing hazardous conditions, such as inadequate airflow or overheating.

While inspection, approval, and documentation are important aspects of mechanical installations, the specific context here emphasizes the role of a listed product as the primary criterion for safe use in a limited space. Thus, having a listed furnace in this scenario guarantees that the installation will meet the necessary safety standards for operation.