Arrived to do a performance maintenance. Replaced furnace filter. 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 fan for heating and cooling is coated in dust debris. Checked systems heat exchanger to ensure proper venting of flue gas. Checked the ductwork for cleanliness. Found that some parts of the ductwork is cleaner than it was in the past, but the duct work is still very dirty and needs a thorough cleaning. Furnace is operational at this time.