Unlike single-stage furnaces—which heat up fast and shut down fast, going from too hot to too cold—the Tempstar® SmartComfort™ VS 90 delivers consistent heat and overall comfort. The two-stage system operates at low heat most of the time and utomatically modulates to higher heat production if the temperature drops. Run the high efficiency fan continuously to improve indoor air quality. The variable-speed motor can also help increase comfort by maximizing moisture removal during cooling (air conditioner) operation.