As the air in the balloon is heated from the flames, the air becomes lighter. So much lighter than the surrounding air that the balloon eventually rises from the light air in it.
If: a) your pipe is the same diameter through the length of the pipe-and-b) the lower half of your pipe has a mounting bracket -or-c) you have no intention of returning it to being a cold air intake...
It all depends upon your setup. You need to find where the return duct is connected to your airhandler/furnace. You could have a return filter grill or there is a door/panel on your...