Performed preventative maintenance for gas furnace. Inspected all the electrical components. Tested the amp draw from the blower and inducer. Both were operating within the manufacturer spec. Merv11 air filter still has plenty of life left in it. Inspect the gas piping. Cleaned the burner, inducer port, flame sensor and burner box. Performed a combustion analysis on the furnace. System is operating at 83% efficiency with a CO level of 15ppm. System also had a temperature rise of 51°. Customer might be interested in a humidifier. Attached to the invoice is an estimate for an Aprilaire bypass humidifier. System is operating properly at this time.