Dakin products designate tonnage based on numerical digits in the Model number. 

Unit model number (including any dash numbers) and serial number must always be used when referring a problem to Lennox in regard to repair parts, literature, wiring diagrams, etc. Use of proper unit identification numbers and serial numbers will greatly help in expediting any material required for these products.

Tonnage is based on the 7th 8th and 9th numerical positions in the model number. 
The sequence of letters and digits in the Lennox model number reflects specific information about the unit. The figures below analyze typical model numbers.