Performed preventative maintenance on the Lennox gas furnace. Checked all controls and components related to heating per manufacturer's recommendations. Cleaned burners, flame sensor and burner compartment. checked the temperature rise of 51°. Performed a combustion analysis on the furnace. System is operating at 83% efficiency with a CO level of 16ppm. Customer supplies own filters. (16x20x1) Customer has repair agreement which they prepaid. System is operating properly at this time. Customer asked for a duct cleaning quote. Attached to the invoice is the estimate for duct cleaning.