Many of the contractors have recommended a two-stage furnace (preferably with variable speed) to save energy on moderate-temperature days, as well as to reduce the temperature fluctuations in the house. What we don't understand is how temperature fluctuations in various regions of our two-story brick house can be reduced, as we will still have just one thermostat in a central location (first floor living room) which will control the activity.

Also, one reputable contractor recommended a single-stage furnace, stating that his experience is a two-stage furnace in a two-story house with our layout (pretty typical) doesn't work effectively. He offers a two-stage model (with the typical increased price tag), so he doesn't have the incentive to steer us to one stage because of a limited offering.

Could someone offer a simple explanation of the benefits of two vs. single stage furnaces? Is a two stage always more effective (in a two-story home)? Is a variable speed two stage always superior (neglecting the price differential)?

Need a quick response, as our current moderate temperatures are not expected to last. Thanks!