Arrived to do a performance maintenance. Upon inspection of the furnace found that the furnace exhaust pipe is leaking condensation into the basement. Found that the furnace is operating with the wrong size filter. System needs a 20X25X4 filter. Recommend a filtration rating of MERV 8. 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. Checked systems heat exchanger to ensure proper venting of flue gas. Found large hotspots on the heat exchanger where it has overheated at some point. Verified operation of the humidifier. Furnace and humidifier are operational at this time.