Arrived to inspect one of two systems in home. Both Carrier furnace are in the attic with 14x25x1 filters. Inspected heat exchanger, gas components, electrical and filters. Noticed flue is 4" but has a 4x3 reducer off the top of furnace with 3" flue running the rest of the way out to roof termination. Both of the Carrier furnaces are set up the same way and is causing issues. Recommended replacing flue, flue cap and flashing to correct size for Carrier furnace. Performed carbon monoxide test while on site and was detecting trace amounts of co, mainly on the main level of home. Checked condenser electrical and refrigerant levels. Found leak on copper in condenser. Homeowner approved Saturday to come do repair. Ductwork is very dirty as seen on images and recommended duct cleaning. Also recommend installing adequate flood protection on both attic systems.