An unvented gas log heater can only be installed in a factory-built fireplace that has been tested and labeled according to which standard?

The correct standard for the installation of an unvented gas log heater in a factory-built fireplace is UL 127. This standard specifically addresses the performance and safety requirements for factory-built fireplaces, ensuring that they can safely accommodate gas log heaters that do not require external venting.

An unvented gas log heater operates using combustion gases that are released into the occupied space, and thus, it is critical that the fireplace is designed and tested to handle these conditions. UL 127 establishes criteria for the stability, durability, and emissions of factory-built fireplaces, ensuring they can safely contain unvented heaters without posing a risk to occupants.

Understanding this standard is crucial for ensuring the safe use of unvented gas log heaters, as improper installation could lead to safety hazards such as the accumulation of harmful gases. The other standards mentioned may pertain to different types of heating appliances or specifications, but UL 127 is specifically focused on the requirements for factory-built fireplaces.