Two Stage Furnace
Two stage furnaces, also known as two speed furnaces, operate much more efficiently than a single stage furnace. The first stage of this type of furnace operates the majority of the time in most climates, and runs at about 65% of the furnace's full capacity. A 2 stage furnace is much quieter since it doesn't operate at 100% capacity every time it runs.
Unlike a single or two stage furnace, modulating controls allows the system to adjust operation needs in increments of 1 to 100 providing the best comfort with the highest efficiency. Modulating furnaces can reach efficiency ratings of up to 98.5% and are often the most quiet
Variable Speed Furnace
Unlike a single speed furnace, a variable speed furnace refers to the fan motor in the furnace. In variable speed furnaces, the fan motor can move at different speeds to control the amount of heated air dispersed throughout your home. Better airflow results in a more comfortable environment in regards to both temperature and humidity.
This can result in less energy consumption and lower utility bills every month.
Some of the variable speed furnaces available are the most intelligent on the market.
Note: "Variable Speed" refers to the blower motor - how much air it moves through the ductwork. "Two-Stage" refers to the gas valve and burners - basically how much heat the furnace puts out