Performed yearly maintenance and safety checks on 2012 Bryant furnace. Found no sign of gas leak via electronic detector. 11% o2 8 ppm co in flu 0 ppm co in supply. Found significant rust in furnace. Customer does not recall being informed this before. Flu pipe runs over finished ceiling possibly leading to condescending draining into furnace. Customer will let go due to age of furnace and expecting replacement in next couple years. No further concer s