A fully modulating furnace is the most efficient type of furnace on the market. Efficiencies are as high as 96%. In simple terms to explain how a modulating furnace works, let’s compare it to pushing the gas pedal in your car.
When you put your car in drive (your furnace kicks on) you slowly push the gas to get you to the speed you wish to go. In town you maintain a slower speed, but when you get on a highway you need more speed. The same goes for your furnace. If you don’t need a lot of heat, it will run at a lower speed, using less gas and electricity. As the outside temps drop and you need more heat, it will ramp itself up (heading out to the highway in your car) to provide the required level of heat. In extreme conditions (your car going up a steep incline) where the temps are very cold, the furnace will ramp itself all the way to the highest stage to provide heat as necessary.
So you can see by using these stages, it is only using the most amount of gas and electricity at the highest “speed” when required. This saves on energy and also provides a very consistent level of comfort in your home along with controlling humidity levels.
The fan on this type of furnace is made to run 24/7 so that also helps keep fresh air circulating through your home. The fan uses about the same amount of energy as a 40 watt light bulb. Pretty awesome!