How are outdoor air summary report numbers getting calculated?

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If it is a small difference, then it's due to the use of different densities. All inputs of air volume flow rate are converted to mass flow using the standard density at the local elevation. The ACH values in the summary report use the current density of the zone air. You can check this with output variable "Zone Mechanical Ventilation Standard Density Volume Flow Rate" which should match the input OA flow rates. Of course, if this is a VAV system serving multiple zones, the distribution of outdoor air between zones may not match the input specification. If the sum of the zone OA flow rates at standard density does not match the inputs, then there may be a problem with occupancy schedule alignment (even one person in a zone is considered "occupied" for the summary report) or with the OA controls.