Bypass R-Rings are used to bypass supply air into the ceiling plenum when Therma-Fuser VAV diffusers turn down. The purpose of the bypass is to limit diffuser noise by limiting inlet static pressure. A Therma-Fuser diffuser with a bypass R-Ring has a constant volume airflow to it and, therefore, also over the balancing damper. This results in a constant pressure drop over the balancing damper which, in turn, limits the static pressure at the diffuser at both full flow and turn down. Because supply air is bypassed into the ceiling plenum, R-Rings can not be used with a ducted return. They also require a four-way blow pattern as three, two or one-way patterns upset the bypass. R-Rings are available in three nominal inlet sizes of 6 in./ 150mm, 8 in./200 mm, and 10 in./250mm. There is no 12 in./300mm inlet R-Ring.