Dummy Exhaust Fan Air Mass Flow Rate = 0 ?

edit

I have a two zone model. In the first zone I have an air supply + a fictional return node, in the second zone I have defined the opposite: a fictional supply node + a return node. In the first zone I have also defined a dummy exhaust fan to balance air system flows when using ZoneMixing.

However in the output the Zone Exhaust Air Mass Flow Rate in the zone with the dummy fan is always 0, while this should be the fixed flow rate of the dummy fan. (correct me if I'm wrong)

The fan is Decoupled. The exhaust fraction is set to be unbalanced.

In the ZoneAirMassFlow object Adjust Zone Mixing For Zone Air Mass Flow Balance is on ('yes'). According to the output the availability schedule of the fan is ok.

Does anyone know what could be causing this?