I am running a flow simulation on a ductwork system with an internal fan and multiple outlets. Using a porous media I am creating an artificial system pressure to drive my fan to a desired CFM value. My problem is this: After running a study, the outlet face of my fan has a volumetric flow rate of 250 CFM (the value I would expect to see from my artificial pressure drop), when I sum the volume flow rate of my 11 system outlets, however, I get a total volumetric flow rate of approx. 200 CFM. Essentially my system is losing 50 CFM somewhere along the way. I also checked mass flow rate to see if potentially I was having a density change somewhere in the system but this also shows an equivalent loss between the fan outlet and system outlets. I'm not sure what is causing this, any help would be much appreciated!