Complex sentence: Clay tile flue liners are very common in the United States, although it is the only liner which does not meet Underwriters Laboratories 1777 approval and frequently they have problems such as cracked tiles and improper installation.
Split into simpler sentences: Clay tile flue liners are very common in the United States. However, this is the only liner which does not meet Underwriters Laboratories 1777 approval and frequently have problems such as cracked tiles and improper installation.