How could i calculate FAHU capacity (HAP 4.6) i.e. what is the regulation for avoid return air amount in HAP sittings?

In the system you have two options one is computer generated second in self generated. You need to select manual generated OA REQUIREMENTS. then return plenum will be fresh air only. The return is zero and all air coming is OA. In that option there is total supply air and OA amount. You have to make OA equal to Supply air then HAP will understand that all air is OA. HOPE THIS WILL WORK. REGARDS

You can set the OA requirement in the spaces which are fed by this AHU to be100% of supply air (In the "general" tab of the space proporties, in OA requirement1 select "% of supply air" from the dropdown menu and type100). You can also go to air system proporties, zone components tab, thermostats and set the direct exhaust to be the same amount of supply air (please note that you need to do that for each zone if you have a multi-zone AHU)

you will select all spaces and select  common ventilation system first window of system section 

then select FAHU componant