The last two times I've changed my HVAC filter it's been pretty dirty. The filter is installed at the unit in my garage. I went to my air return to clean it up and as I was sweeping at it (on the ceiling around 10-12 feet) dust just kept coming out. So, for the first time, I went up there, removed the grill and looked inside. It was extremely dusty in there. I cleaned up the immediate return area and scheduled someone to come clean my ducts but my question to you guys is this. Is there anything special to consider if I want to switch from using a filter at the unit to the return? I know I shouldn't do both at the same time but I'm thinking if I stop using filters at the unit and use them at the return instead, it will still keep the unit clean but also keep my ducts clean. Having the filter at the unit doesn't stop dust from building up in my ducts. Most of my searching just keeps telling me not to use both at the same time. I'm just wanting to make sure there isn't some special consideration to switch from the unit to the return. If not, I'll buy a new grill for the return that can house a filter and just put them there.