Today we came out for upstairs furnace not working all the time 10 year old Goodman 60k with matching 3 ton ac. Found high limit switch opening due to improper ductwork. From furnace over heating heat exchanger has been compromised found severe metal fatigue and hot spots. There is not enough return air, filter is 16X20 when it should be 16X25 and return plenum is too small. Client also has duct board fiberglass plenums which is not healthy and needs to be updated to sheet metal plenums. House calls for 3.8 tons of air and client currently has 5.5 tons of air. When equipment is oversized we do not get enough round time and we cannot properly dehumidify. When this happens biological growth and mold can grow. Gas piping going to furnace in attic is not up to current code sediment trap was installed wrong. Scheduled install supervisor and airflow specialist to give options on updating second level system, replacing duct board plenums and modifying ductwork. Left all equipment operational.