Does anything look wrong with this Furnace burner start up?
First off I did not see anything in the video to make me think there was a cracked heat exchanger. The heat exchanger may be cracked but the video does not indicate so. It seems that removing the inlet pipe fixed the problem? The furnace installation instructions has vent tables showing what size pipe can be used for various lengths and amount of elbows. For instance a 90,000 btu furnace can have a maximum of 70’ of pipe and 4 elbows with 3” pipe but only 20’ of pipe and 4 elbows with 2” pipe. And this is combined for both pipes, not for each pipe. So adding the second pipe may have pushed you over the limit. This furnace comes with a restriction plate that must be installed if using only 1 vent but must be removed if using 2 vents. Was it removed? And in my experience these tables are often over generous. I have pushed the limits of the tables and have had venting problems. I had to either increase pipe size or reduce the amount of elbows or both.