Patent ID: 7225629

Claim:
A fluid heating system, comprising: a heat pump; a tank; a tank inlet that carries fluid from the heat pump to the tank; a tank outlet that carries fluid from the tank to the heat pump; a tank temperature sensor that measures a fluid temperature in the tank; a controller that controls the heat pump based on a first threshold, a second threshold higher than the first threshold, and at least an output from the tank temperature sensor, wherein the controller energizes the heat pump when the tank temperature sensor output falls below the first threshold, said controller causing the heat pump to de-energize when a temperature within the fluid heating system reaches the second threshold; a tank inlet temperature sensor, wherein the controller de-energizes the heat pump when a value based on an output from the tank inlet temperature sensor reaches the second threshold; and the value is an estimated tank outlet temperature calculated from the output from the tank inlet temperature, a system capacity and a flow rate, and wherein the controller causes the heat pump to de-energize if the estimated tank outlet temperature reaches the second threshold.