A thermostatic mixing valve is used in a tempered water blending system. A cold water bypass line in the system acts under the control of a modulating valve and electronic controller to divert cold water back to a temperature sensing element in the mixing valve. By flooding the temperature sensing element, the mixing valve detects a false temperature and adjusts its flow rate of hot and cold water accordingly. The electronic controller acts in reverse of normal fashion, providing negative feedback to the sensing element of the mixing valve.