Arrived to do a performance maintenance. Set up for combustion analysis temperature split and static pressure tests. Verified operation of all mechanical and electronic components of the furnace. Checked blower for cleanliness and to see if there is any play in the bearings. Found that the furnace fan is heavily coated in dust debris. Considering how dirty the fan is checked the ductwork for cleanliness. Found that there is a large amount of debris in the ductwork. Checked systems heat exchanger to ensure proper venting of flue gas. Furnace is operational at this time.