The thing that baffles me is that the furnace doesn’t look that old and obviously they replaced the first 4’ of the flue…why would they just attach it to such a long horizontal run? Especially since the rusting on the flue pipe was likely there before the installation indicating there was a draft issue. Total of 22’ long horizontal run including another 90 degree turn before it made it to the chimney.
The pictures follow the path. In the basement furnace room where it goes horizontal, through the hallway into the bathroom, then a 90 elbow through the bathroom, then finally into the kitchen where it goes into the chimney. The last picture is from the kitchen with the hallway and furnace room in the background.
The total rise from where it enters the chimney to the top of the chimney is only about 20’…just a 1-story home.
Did you reach your hand by the furnace and feel if it is drafting or blowing back?
Gas emits moisture when it burns. If they are doubled wall flues that are rusting on the outside, odds are the interior pipe is rusted through some where. The moisture content would be low in the air between the outer and inner pipes. Is there a rain cap on the end of the flue pipe?