White shield fall service. Left 2 filters. Blower motor run capacitor checked good, motor and wheel are good, hot surface ignitor checked good, cleaned and checked flame sensor, pressure switch checked good, inducer motor run load amps are within mfg specs, roll out switches (2) checked good, high limit switch checked good, manifold pressure is at 3.5 in wc, within mfg specs. Checked system operation with tstat, system operating normally. Carbon monoxide - 0 ppm. While furnace panels were off I brought homeowner into attic and showed him the dirt that accumulated in the return air ducts. Homeowner wants estimate for duct cleaning.