Arrived to do a performance maintenance. Upon arrival found that the exhaust for the furnace is leaking condensation onto and into the furnace. 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 heavily coated in dust debris. Checked systems heat exchanger to ensure proper venting of flue gas. While checking the heat exchanger found bacterial growth present on the evaporator coil. Found that there is debris impacted in the evaporator coil and it appears to have been dripping condensation into the furnace. Furnace is operational at this time.