Measuring airflow has always been one of the more difficult and time consuming tasks a service person does. In fact, it is not generally done because it is too time consuming, requires special tools, and is not always so easy to do. In addition, airflow measurement is not usually taught to service people unless they are going to specialize in air balancing. Nevertheless, knowing the airflow in cfm can be useful to troubleshooters.
Most service technicians check the airflow by measuring the temperature drop across the cooling coil or the temperature rise across a heating system. If the air temperature drop or rise falls within an expected range, the assumption is the airflow is acceptable. Here is an easy method of measuring the airflow through a system that uses electric heaters. This method applies wherever electric heat is found. Electric heaters are often utilized for reheat as well as for emergency heat on heat pumps.