A system for heating a pool or spa. The system may include a circuit for interrupting the power supply to a heating element of the pool or spa. A thyristor may be used to vary the voltage supply to the heating element, such as by use of a variable resistor, capacitor, and a triac. The system may adjust the resistance on the variable resistor to control to voltage supply to the heating element, allowing a larger heating element to be used more efficiently.
