*PLUMBERSSTOCK EXPERT* responded:
Good day! We appreciate you checking out our Honeywell SPRD16 16" Round Static Pressure Regulating Damper. MRD (MARD) stands for Modulating (Automatic) Round Damper. Honeywell has the MARD series damper which is a power-opened/closed zone or bypass damper. It is usually used on systems larger than 5 tons that do not exceed 2 in. W.C. at the damper in light commercial applications. It can also be used in any residential systems. For more information on modulating round dampers, feel free to view this instruction sheet. Should you have further questions, please let us know. Thank you for choosing plumbersstock.com!