Most furnace model numbers indicate the model line and size of the furnace in a coded fashion, with the first portion of the model number indicating the model line. Additional portions of the number may indicate output ratings, revision numbers or particular features of the furnace for specific markets.
Furnace manufacturer Goodman's numbering scheme for its products provides a good example of a typical furnace model numbering system. The first four letters indicate the manufacturer, flow type, motor and gas valve design of the furnace, while the next two numbers indicate the model's average fuel use efficiency rating. Subsequent numbers indicate the model's output rating, while the last letters in the model number indicate its nitric oxide output and the revision number of this version of the furnace.