(the R is silent)
This furnace has had quite a history of problems. The original RGEC/RGKB models should have been removed shortly after installation. The RGED/RGKC models were better but have a history of heat exchanger problems and should be watched carefully. Most common crack can be found visually by removing the limit and looking in at the back of the drum where it meets the back piece. We've also found large cracks in the back of the back piece, not visible. To find them, seal the heat exchanger, use a draft gauge or magnahelic and turn the blower on. Any change in pressure indicates a crack. If problems are found, check with a Rheem or Ruud supplier and see if an allowance would be given toward a new furnace rather than replacing the heat exchanger.