Arrived to do a performance maintenance. Removed and cleaned the filter cover. Replaced furnace filter with homeowners supplied filter. Set up for combustion analysis temperature split and static pressure tests. Found that the systems gas valve was using too much gas for combustion. Tuned the gas valve to bring this within manufacturers specifications. This also helps keep the system from continuing to overheat. 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 heavily coated in dust debris, impeding airflow. Checked systems heat exchanger to ensure proper venting of flue gas. Found that the heat exchanger is extremely worn. The first bend in the heat exchanger is pock marked and steel seems to be flaking off the structure of the heat exchanger. Furnace is operational at this time.