With climate change upon us, reducing our carbon footprint is more important than ever. Across the non-residential building sector, owners and occupiers are looking at building services equipment within their estates, to exploit running cost savings, reduce carbon footprint and smarten up using Internet of Things (IOT) technology. One area being refreshed is Air Handling Units (AHU's) that provide fresh air, remove stale air, heat, cool and control humidity. Many buildings operational today will still be operational for years to come and are installed with legacy air handling units using AC induction motor fans, commissioned to deliver the same performance every day. Recent efforts to deliver energy savings via demand control has seen installation of inverter drives to vary the performance of the fans via speed control. The cumulation of impeller aerodynamic efficiency, the efficiency of the belt and pulley transmission, motor efficiency and inverter drive efficiency provide an opportunity optimise air movement. In addition, a legacy AHU that employs one fan per supply or extract flow path that has a breakdown during service, has no back-up. Existing AHU's due for refurbishment are increasingly being upgraded to use multiple Electronically Commutated plug fans arranged in a grid. Sharing the required airflow across multiple fans allows flexibility in selecting fan diameter, number of fans in the grid and provide n+1 redundancy. In addition, fitting inlet rings with a flow measurement pressure tapping can monitor, control, and adjust the fan grid behaviour to deliver the required flowrate using a constant volume control system. But what happens if one of the fans fail? In a single fan AHU, if there is no functioning fan there is no airflow, in a multiple fan system one failure does not stop the delivery of air. This paper explores a number of aspects of Fan Grid installation in Air Handling Units: 1) The accuracy of the inlet ring volume flow measurement system in comparison to an airflow test rig designed to ISO5801 in normal operation; 2) The effect on the accuracy of the flow measurement system in the event of fan failure when air can recirculate through the faulty fan; 3) The effect on the accuracy of the flow measurement system in the event of fan failure when a back-draught damper (Gravity Shutter) is fitted to prevent air recirculation.