That is a DIVERTER valve, not a shower control. It is intended to send water to one of several devices, i.e., hand held, tub spout, and/or hand held shower. It CANNOT be used to control the water temperature. You need a shower control valve, and then the diverter is connected TO IT if you have more than one shower head. The tempered water INLET is where the flow from the shower control is connected. Obviously, your "plumber" is not a "PLUMBER" or he would have known this.