Rinnai REU-MSB-M Multi-Unit Controller For use with Rinnai tankless water heaters (except for models V53e, V53i and R63LSe which must use pressure activation valves) Temperature setting for connected water heaters is controlled by the temperature controller connected to the water heater with the Master MSB Board. Includes 1 control board Allows the customer to connect 2 water heaters The MSC-B1 must be used with this kit to connect more than 2 water heaters