How do I find out the tonnage on a AHU? The label/tag has faded all I have on file is the model number and the serial number, make. Question Posted By Francisco the Building Engineer on May 3, 2011 Refer to the manufacturers website for their HVAC nomenclature of model number and serial numbers and how those numbers correspond to the tonnage and other technical data. Or you can call your local dealer for the manufacturer and they will tell you.