I just had a HVAC technician to the house to perform a yearly tuneup on my furnace and A/C. He confirmed a suspicion I had for some time now. The return air filter doesn't fully span the duct. There is a piece of strapping screwed into the inside of the duct that was obviously put there to act as a stop at the far end of the filter track. Once I slide the filter in and it hits the stop, there is about a two inch gap on the far side of the duct that isn't blocking any air. The tech said the filter is doing next to nothing in this situation and the motor is filthy as a result (to say nothing of the potential for terrible indoor air quality). Can anyone think of why this strap stop may have been put in? This is the third tech to see the unit in the past three years and no one has said anything until now. Is this guy off base? He recommends reworking the ductwork so the filter fills all gaps. Thanks for your thoughts.