Performed preventative maintenance on 80% gas furnace. Spoke with customer about how the new ac was operating for them, and they are pleased with it. Checked inducer motor, pressure switch, hoses, electrical components, blower, blower capacitor, and heat exchanger. Bottom of heat exchanger cells have hot spots. Checked gas pressure, carbon monoxide readings, temperatures, gas pressures and amp draws. System is working well at this time. Customer was interested in duct cleaning. Went over levels of duct cleaning and let customer know that she should be receiving a free one next fall, but to check with the office.