Performed furnace maintenance on 1990 carrier heater. Upon my maintenance I came across a couple points of concerns that the homeowner is aware of. 1. Perform heat exchanger inspection. Heat exchanger Do you have multiple cracks present within Chambers were burners are blowing into. This is because due to expanding and contracting over the years of overheating. Gas line was shut off for safety concerns of possible carbon monoxide leaking into airstream due Branch in exchanger. 2. Oxidization is occurring due to evaporator coil leaking Water onto several components. 3. Aftermarket board has been add to system 4. System is covered in dust and debris due to past years of filter not been changed out Upon departure Furnace was tested and is fully operational in heating. System was cycled then gas line was turned off. Home owner is aware.