I have Rheem 90 Plus imperial about 15 years old. I notice compared to newer or my neighbors systems I have very weak air flow. It will heat and cool the house, but seem like it takes forever. I checked to see if my blower was moving freely and it moves with the slightest touch which tells me the bearings are still good. One person told me to clean the fins. I have clean the surrounding air ducts. Is there anyway to improve air flow?