Rusted Furnace heat exchanger. Common cause of carbon monoxide poisoning.

This brass flex connector should have been stainless steel. It was also passed through the cabinet opening where it was chaffed and worn through from vibration causing a gas leak and fire. The part that goes into the furnace should have been rigid steel. These people are very lucky their house did not burn down.

3 Strikes and You’re Out
Here’s a furnace with 1) a copper gas line using natural gas; 2) a gas leak and; 3) to top it off, can you see where the fire was? The valve and wires, as well as the inside of the furnace, are blackened with soot from an earlier fire.

Furnace flue in front of a bedroom window. Someone opening the window trying to get some fresh air will get a mouth full of CO instead.

Here's another furnace vent outside a bedroom window. Oh....and just to help the combustion gases enter let's put it in front of a window AC unit.