The TrueFlow® Air Handler Flow Meter* is designed to provide a simple and accurate measurement of airflow through residential air handlers. The TrueFlow Meter temporarily replaces the filter in the air handler distribution system during the airflow measurement process. If the filter location is directly adjacent to the air handler, the TrueFlow Meter will measure the total air handler flow. If the filter is located remotely at a central return, the TrueFlow Meter will measure airflow through the central return.