Why Two Stage?
Since a furnace is basically sized using a few factors. The lowest temperature factor for your local area you live, the heat loss factor of your home, and the efficiency or heat loss factor of the furnace proposed. A two stage furnace has two levels of heat output. A high output which was used for the sizing calculation and a low fire input (usually one-half the high stage) This feature increases efficiency and comfort level because about 80% of winter the outdoor temperature is well above the coldest day factor used to size the furnace. Therefore, you basically using a smaller furnace 80% of the winter at full capacity, and then when outdoor temperatures drop, the capacity increases to keep up with that increase. A two stage furnace is a bit more expensive than a standard furnace but the extra cost can soon be returned in efficiency and savings.