Look for an aluminum tag--or a silver or white adhesive label. The model number may be printed or stamped onto the tag and it may be written in two different type sizes or styles.
All Types
Remove the front door and look at the upper-left side of the unit. You can find the model number listed on a "rating plate" sticker affixed to the inside of the unit.
The model number are written on a label on an outside panel of the furnace.
