Measuring Indoor Airflow Levels - Overview
To accurately determine the system performance, superheat, subcooling, and properly charge a system you must be able to accurately measure the airflow over the indoor coil. Airflow is measure in Cubic Feet per Minute (CFM). For single speed systems airflow is between 350-400 CFM per ton of cooling. Example: a three ton system requires 900-1200 CFM of airflow
Too Much Or Too Little Airflow
Improper airflow, either too much or too little prevents you from accurately measuring or charging the system's charge.
Too little airflow and the indoor coil may freeze up or flood liquid refrigerant back to the compressor. Excessive airflow causes high humidity and overheated compressors.
There are many ways to determine airflow. The most popular is the temperature rise method. The temperature rise method can be used on both electric and gas furnaces. Although formula is the same the procedures are different.
This method requires you run the system in the heating mode to obtain the temperature rise. The temperature difference is measured between the supply and return air temperatures. The difference is call Delta-T. Delta-T is used in the formula to calculate the indoor airflow volume.