The top wheel is a intake tube. The smaller tube that goes in to it, comes from the blower so i can open the ball valve, close the intakes, and force air in to the heater to make it hotter. The top intake tube runs down to around 15 inches or so from the blow through tubes below the door on the inside. I had a frying pan sitting on the tubes with a bowl turned upside down in it for oil to drip on. The intake air blows the oil down around the pan and makes a better burn, supposedly. But i have since quit using the top intake and just use the bottom two, and stacked some scrap metal in the frying pan for oil to drip on to make a bigger fire, and seems to work alot better. Like i said this was my first one, and i tried to over build it a little bc im trying to heat 5,400 sq ft.