Measuring Airflow by Pressure Drop

Measuring Airflow by Pressure Drop

Measuring Airflow by Pressure Drop

Pressure Drop Across the Dry Evaporator Coil

An easy way to quickly estimate airflow is to measure the static pressure drop across the evaporator coil, and compare the reading to that specific evaporator coil in the manufacturer’s literature. With a digital manometer, and a pressure drop vs. CFM chart, airflow can be set close to specification across a dry coil in a matter of minutes. The positive probe should be inserted ahead of the air entering the coil and the negative probe immediately downstream of the coil. The reading obtained will be the pressure drop in inches of water column or Pascal.

 NOTE: While this measurement is accurate enough for setting up equipment, it is not accurate enough to make a field measurement of the system capacity.


The use of standardized tables or charts should be avoided at all costs as they do not provide even a reasonable estimation of airflow. Coil designs very in rows, and fin spacing and only the manufactures chart will account for the design variables. 

The use of static pressure tips like those below will significantly increase the accuracy of measurements. The  pointed end of the static pressure tip should be directed toward the air stream.

Typical pressure drops for residential coils can vary from .1 to .4" WC. If the pressure drop in in excess of .3"wc, inspect the coil for dirt or other blockage.

Dwyer A-303 Static Pressure Tip
Make sure that tip pints toward on coming airflow