The intent of system airflow verifications is to determine the airflow across the evaporator (cooling) coil in a system with air conditioning. As the system cooling capacity increases, it is not uncommon to have two or more return registers. How airflow is measured with multiple returns will depend on which diagnostic tool is used to measure airflow: flow-hood, flow-grid or plenum pressure matching.

Using a flow-hood measure the airflow entering at each return register. The sum of those measurements represents the total system airflow across the evaporator coil. During this procedure all return registers shall be left in their normal operating configuration: air-filters installed and no temporary masking. Masking any portion of the return register may reduce the airflow to be measured so it may be necessary to purchase a second, larger hood to accommodate larger returns (e.g. 3 foot by 3 foot hood).

Using a flow-grid airflow can be measured at either the FAU (air-handler) with a single flow-grid, or at the return registers using multiple flow-grids simultaneously. In either case the NSOP is established first with the system in its normal operating configuration. Then the air-filter(s) are removed and the flow-grid(s) are installed to measure airflow. If multiple flow-grids are used simultaneously, the system airflow will be the sum of the individually measured airflows.

Using the plenum pressure matching method airflow can be measured at either the FAU or at the largest return register. If airflow is measured at a return register, then after establishing the NSOP the other returns should be masked off so that all airflow goes through the return at which the fan flowmeter is connected.