I have a 3ton a/c (heat&a/c) unit on the roof and more than one repair man wants to replace the entire unit and everything else. only the fire box is cracked, price is over $4,000.. the unit was installed in 1990 so old but not that old and works fine otherwise. Are fire boxes avalible for most units. seems like they just want to sell new for big profits rather than do a repair. any insite appreciated... bill