Under what U.L. listing are factory built fireplaces and integral chimneys listed together?

The correct answer is choice C, which refers to Underwriters Laboratories (U.L.) Listing 127. This specific listing covers factory-built fireplaces that are designed to operate with an integral chimney system. The importance of this listing lies in the standards and safety testing associated with both the fireplace and chimney units acting in unison.

U.L. 127 ensures that the materials and construction methods used in these fireplaces and chimneys are able to withstand the thermal and operational stresses they will encounter during use, as well as provide safety features to reduce the risk of fire hazards. Thus, compliance with this listing is vital for ensuring that both components function safely and effectively, making it critical for professionals in chimney and fireplace installation to be aware of and adhere to this standard.

While other U.L. listings may cover fireplaces alone or chimneys separately, U.L. 127 is the comprehensive standard that guarantees both elements meet necessary safety protocols when integrated, providing peace of mind for users.