Measuring by Total External Static Pressure
The total external pressure method is preformed by measuring the pressure difference across the furnace (supply to return) and using the manufacturer's chart. The CFM can also be set quickly and accurately using this method, but again, the measurement process is not precise enough to use for verification of the system capacity.
When checking total external static pressure, it is important to use static pressure tips for accurate measurement.
elect a location in the furnace cabinet away from wires or controls that could be damaged while making a test port in the furnace cabinet. Also avoid making any test ports near the center of the blower wheel as the air currents generated by the fan can affect the pressure measurement.
Drill two 1/4" holes in the cabinet, one in the blower compartment and one near the heat exchanger below the evaporator coil if so equipped. Use caution when drilling any holes in the cabinet to avoid wires, controls an the heat exchanger. (See video below
Using a differential manometer like the Testo 510, measure the total external static pressure and compare the measured reading to the manufactures installation guide for an estimation of the actual CFM.
Typically TESP exceeding .5"wc result in diminishing airflow on residential equipment equipped with a standard PSC blower motor. ECM motors may be able to operate as high as 1" TESP. Consult the manufacturers literature for specific guidelines.
Recommended kit for this process. Testo 510 with Static Pressure Tips