Performed 1 system heating diagnostics on a 2000 Goodman 80% gas furnace. I found that the gas valve was bad. The gas valve was receiving 24 volts but would not open. Due to the age of the furnace I performed a heat safety check and found that the heat exchangers had several cracks. I took pictures of the cracks and showed the homeowner. The furnace is not safe to operate with the cracks in the heat exchangers; it’s a fire and carbon monoxide hazard. The flue pipe is not to code and is corroded and is falling apart. The flue pipe is single wall and not double wall. The homeowner has setup a time with an install supervisor to get an estimate to replace the furnace.