As mentioned in the services section of the site, two-stage furnaces and two-stage air conditioners are a good investment because they can work on a high capacity or a low capacity, dependent upon weather conditions. Most of the time your system will work at a lower capacity, but during harsh weather, it will work at a higher capacity. This makes dual stage systems extremely energy efficient and and ideal choice for the Southern California area.
Additionally, both variations of two-stage furnaces and standard blower furnaces are efficient and due to their low run speed they run much quieter and use less gas. Rather than a simple on/off operation, their flames can be on/high, on/low, or off. With the addition of the 2nd flame, energy costs become considerably lower and the unit will run much quieter.