The easiest place to look is on the AC unit itself. Go outside to the condensing unit and look for a data plaque mounted to the side. Find the model number. Within this string of letters and numerals, you should find an even, two-digit number.Click to see full answer. Besides, how can you tell how old your AC unit is?Go to the outside cabinet of the air conditioning system. You’ll find a metal nameplate attached to it containing the system’s technical specs. On most units, the nameplate is fixed to the backside of the cabinet. Near the top of the nameplate you should see the manufacturer’s date as a month and year.Similarly, how do I tell what size my AC unit is? Calculate Size To calculate the size, simply multiply the length times the width of the room or area to be cooled. Then, as a practical number, multiply that total times 25 BTU. Hereof, how do I determine the tonnage of my AC unit? Area Method : To calculate, multiply the length and breadth of the room (in feet), take square-root of the answer and divide by 10. The resulting number will give the desired tonnage. Volume Method : Alternatively, multiply the length, breadth and height of a room (in feet). Divide the answer by 1000.Where is the serial number on an AC unit?The model and serial number can be found on the rating plate of your air conditioner or heat pump. This is generally found slightly above the refrigerant valves on the back side of the unit.