What is the difference between Model GM9C800803BN and GM9S800803BN? What does the 4th digit in the model # reveal (C vs S)?

The 4th digit in the model number indicates what kind of staging the furnace has. An "S" means that the furnace is single stage while a "C" indicates that the furnace is a two stage model. Two stage furnaces have two different heat settings you can use: one "full output" heat setting for your typical winter day and one "partial output" for those days where you don't need as much heat. Single stage furnaces just have the full output setting. More info on furnace staging can be found in each furnace's specifications sheet, found in the Resources section.

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