The installation of an MCC-91-2W commercial controller allows all Rinnai series units the ability to be programmed as well as display the settings and show any error codes if the unit experiences a failure. This MCC-91-2W allows you to set your outgoing water temperature to the maximum allowable temperature, as dictated by the specific Rinnai tankless water heater it is mated to (depending on the unit, 160 - 185F). The control must be wired to your Rinnai water heater in the same way that your thermostat is wired to your cooling and heating system