Found natural gas furnace not operating pilot light lit, tested system operation found pilot valve working, main gas valve stuck not operating system has 24 V power, gas valve internally stuck, visual inspection of unit found holes in the heat exchange chambers and multiple cracks this will add additional air movement to burner compartment distorting flame and or adding additional air, creating a dirty burning situation this will also cause combustion gases to flow outside of natural draft chamber, Creating an unsafe work environment. recommendation is to replace heater