If you are looking at the condenser you must find the model number and with that, there will be a number that will yield the size of your unit in tons, when divided by twelve. Calculations for this are performed with 400 CFM per ton. For example: a 4.5 ton unit will have 1800 CFM.
Another calculation is air load, derived from the output BTU’s of the furnace. If the output BTU’s are specified on the model number as, for example, 89,000 one would multiple 89 by 21.7 to achieve 1931. Because you want the home to be guaranteed within the passing zone, the smaller of the two numbers is used to calculate the target leakage in terms of CFM.
In order to calculate target leakage, you multiple the total air load by the amount of leakage allowed for change-outs. Using the example above, the total air load is 1800 and the allotted leakage is 15% which results in 270. After this all supply registers are sealed and the duct system is pressurized with a duct blaster. Sensors can be attached to the duct system and the blaster can be channeled from there using a manometer, which measures static pressure. The system is then pressurized to 25 Pascal (which is the standard amount of air pressure in a duct system operating at full capacity) and the manometer will read the air leakage in CFM.