I conducted a comprehensive performance maintenance check. I began by setting up for combustion analysis, temperature split, and static pressure tests. I verified the operation of all mechanical and electronic components of the furnace, ensuring everything was functioning correctly. The blower was inspected for cleanliness and any play in the bearings, revealing a significant accumulation of dust debris on the fan for heating and cooling. I also checked the system's heat exchanger to confirm proper venting of flue gas. Additionally, I found bacterial growth on the bottom of the evaporator coil. Given the furnace's age and the debris found inside, I inspected the ductwork, discovering excessive amounts of debris. Although the furnace is dirty, it remains operational at this time.