I arrived at our clients home for a 3 system heat maintenance inspection. Our client had recently replaced the air filters for the 2 down stairs units and provided a new filter to be replaced upstairs. I proceeded to test and inspect the furnace and cleaned the flame sensors and pressure switches. I also tested and ensured the fire shutoff safeties were all working properly. Everything passed inspection today. While inspecting the units I did find that all 3 units had significant dust accumulation throughout the ductwork and on the air distribution wheels. These are starting to cause the motors to stress and run high according to manufacturer specifications. I showed pictures to our client and we discussed cleaning just the motors and wheels as well as potentially cleaning the entire duct work for the home. The units are working properly and are currently still testing in range but with added stress it may cause premature motor failure.