What is the difference between models GM9S80100SCN and GC9S80100SCN?

The only thing different between those two models is their configuration. Any Goodman furnace with an 'M' as its second character in the model number can be oriented in an upflow position. A Goodman furnace with a 'C' in that same spot can instead be setup in a downflow position. Both the 'M' and the 'C' models can also be installed as a horizontal position. For more information on this, you can look up the furnace's specifications sheet for a more detailed breakdown on model numbers and configurations.

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