Two-stage heating means a furnace has two levels of heat output: high for cold winter days and low for milder days. Two-stage furnaces start in the first stage, instead of reaching full capacity all at once. That means there is no sudden blast of air. The low setting meets household heating demands 80% of the time, allowing a two-stage unit to run for longer periods and provide more even distribution of heated air. Because the furnace operates mostly in its lower-capacity first stage, it burns less fuel than a furnace that always runs at full capacity and then has to shut off when the heating demand has been met.