Cleaned & checked condensing gas furnace. Inspected heat exchanger & flue venting. cleaned burners & flame sensor. Checked voltage & amp draw of motors. Replaced air filter. Customer complained of banging noise at times when system turns on. No noises found at this time. I found damper in first floor duct not closing. Damper was hitting mounting screws. I replaced with shorter mounting screws. Clean and check humidifier, replace pad, checked water flow and drain. Solenoid valve not opening. Replaced solenoid valve. Replaced UV lamp for APCO air purifier. Checked blower speeds. Setup furnace for two stage operation.