The SUF range direct-fired gas water heaters provide a new level of economy and efficiency in hot water production. All models are fully condensing and incorporate a burner modula- tion giving up to 97% thermal efficiency and are designed to operate as a room sealed system in both vented and unvented commercial / industrial applications. With vertical or horizontal concentric or parallel flue kits, up to 100 meter in length, the range is particularly useful where extensive conventional flues are either not possible or practical.