Evolution of Furnace Types
The first single stage furnaces were simply called single stage. When the thermostat called for heat, it only knew one type of output , which was maximum.
Thereafter a 2-stage furnace became available. It would initially kick on at about 50% of the heat output using 50% of the gas. If the thermostat was not satisfied in approximately 5 minutes, the furnace would go to the maximum heat output.
Now a days we have modulating furnaces. These furnaces have twenty or more stages of heat using less gas than a 2-stage furnace to keep your home warm. This furnace can keep the temperature within 1-2 degrees of the thermostat setting. It may never even fire at maximum heat output. You will reap savings in gas usage over a much shorter time period and also have a higher level of comfort than with other types of furnaces.
The Difference Between Single Stage Furnaces & Modulating Furnace
The best way to envision the difference between a single stage furnace and a modulating furnace is to think about an on/off light switch versus a dimmer switch. The on/off light switch will turn on a light bulb to maximum or turn it off completely, while a dimmer switch allows you to turn on a light bulb in a range from very low to maximum brightness or anywhere in between.