I must be missing something obvious about this platform.... So, the grill is on the floor (platform) where the furance/water heater sit? How does is pass through to the hallway into the house? I was picturing a furnace room next to the hallway, now I'm picturing it sitting on top of it. Yet at the same level as the garage?

It must be me since the others seem to have grasped it..... I think I crawled through too much rat feces today