Gas or Propane furnaces contain heat exchangers that produce heat when the gas or propane is burned or combusted. The combusted or by products of the fuel being utilized in the furnace produces fumes that should be vented to the outdoors. As the heat exchangers wear...
As you may be able to see from the picture, a dog chewed into the bottom of this return air plenum which is located in the garage. The furnace being located in a garage area is very common, the furnace platform or return air platform/plenum in this case is one piece....