Completed furnace, maintenance and inspection. I cleaned flame sensor and wiped down furnace. I tested amp draws and components. I checked air pressure and temp splits. While checking out the furnace I found that the furnace is well oversized for this home. This has caused a lot of damage to the furnace. The heat exchanger that has already been replaced once is already rusting and has potential holes. With proper sizing, this system could have lasted much longer. I showed customer the damage on the system and explained the safety concerns with the gas line and heat exchanger. Customer asked to see pricing for replacement. I was able to get one of our supervisors to the home to size the equipment and get pricing for the customer. Furnace is operating at departure.