All outdoor wood furnaces work when they are new. The difference between a good furnace and a bad furnace is the warranty. Most warrantees do not cover rust or if they do it is only for 5 years. Various warrantees will only cover manufacturer defects, meaning that only the welds are covered. They didn’t buy the metal in the furnace so they didn’t “manufacture” it.  A number of manufacturers will only pay to replace the bad part they will not pay for labor or shipping of it. This could mean major expense to get a certified welder to your house. Other manufacturers require you to bring your 1600-2000 lb stove to their company for repair. Shipping could cost you hundreds of dollars. Most 25-30 year warrantees only cover the door and others only cover workmanship defects again a bad warranty.