I manage some shops in a shopping center and they all have the HVAC up on the roof. The units have within them filters that need to be changed once in a while?

Inside the shops, there are air intakes also that originally the HVAC contractor had placed filters too.

I am told that the filters on both location restrict the air flow, and that may be possible, and that it is recommended to have the filters only on one location, within the units that are on the roof and not inside the shop's air intake.

My question is: should I have them on both location, up within the unit and inside the shop, inside the HVAC unit or inside the shops in the air intake?

Please help me out on this one.

Thanks
Vcavina

Good filtration inside the unit would be preferred, filters in the return air grills are more convenient but don't do both.