The present invention controls a room temperature by controlling an environmental control unit. A total span about a setpoint temperature is adjusted in accordance with a previous total span and a multiplicative factor. The multiplicative factor is periodically updated from a desired cycle time and a previous cycle time. The room temperature may also be controlled by adjusting a duty cycle for controlling an environmental control unit. The duty cycle is adjusted based on an error associated with a previous control cycle and an attenuation factor. A new control cycle may be started by cutting the previous control cycle or a current control cycle may be extended if a predetermined condition is detected. The control mode is selected based on environmental characteristics and room characteristics. The control mode may include a span control mode and a duty cycle control mode that is selected from the cycle rate.