So, while figuring out how to replace my ignitor assembly I noticed a visible 2inch long crack on the vertical metal plate facing the front of the furnace. This is the place that has three round holes cut in it for the burner flames to shoot into. Is this the heat exchanger or is there another assembly behind this that is considered the heat exchanger?

Its an Armstrong AIR Ultra SX80 75K BTU furnace.

 

If it was in the tube, on the top, I'd say so. But where you say it is I am not sure. Oddly it sounds like it cracked outside the area that would have been the hottest. But it could be the front facing of the heat exchanger. I can't recollect the Armstrong from the top of my head, yet we have one somewhere in one to the rentals. We have many of the brands, mentioned in this forum, which has made my job fun.

I would say to see what the flames do when the fan kicks on. But if outside the flame area, this may not show anything.

Can you send us a couple pictures?: One from slight distance for perspective of relative location of all the parts, and one really close up, yet clear and not blurred?