Abstract: A control system is disclosed for a swimming pool or a spa recirculating system. Water is withdrawn from a pool or spa by a pump and supplied, through a filter, back to the pool. As is common with such systems, a controller is operated to initiate operation of the pump at pre-established time intervals (for example, once every twenty-four hours). The length of time or duty cycle that the pump is operated at each of these time intervals is determined by a temperature sensor which senses the water temperature. The temperature sensor supplies a signal to the controller to cause the pump to be operated for a longer period of time when the water temperature is above a threshold temperature, and to be operated for a shorter period of time when the water temperature is below the predetermined threshold.