I have had some Mechanical contractors tell me that it is not necessary to have a filter on a fan powered box if it does not have a cooling coil. The boxes do have electric reheat coils and cooling air is delivered from (2)115 ton Trane Intellipak units that share a common plenum. The units on the roof do have filters in them. The return is open plenum as well and the FPB are Trane. My argument is that I still see filters build up a lot of dust on the fan powered boxes especially in heating season. When the fan powered boxes are calling for heat, the fan kicks on and still picks up a certain ammount of dust particles from the return. I would think that not having a filter would cause some indoor air quality problems and would cause dust to get on the mechanical components such as the heating coil, fan motor, fan assembly and capacitor. I just would like to see what peoples opinions are on what is a better practice, to have a filter on the FPB or not.