Heck we don’t need any combustion air. It’s too cold and freezing the water pipes.
Another question, which came first? The chicken or the egg. The tinner or the plumber. I would say the tinner.
As it should be.
I see that up here in the mountains all the time.
If this unit is in an unfinished basement, as it appears, it shouldn’t need an exterior source of combustion air.
That pipe was most likely installed if/when basement is finished and the furnace/water heater are enclosed.