There is a riser inside the barrel which traps the smoke from the fire inside. This forces the smoke to combust, that combustion plus the surface area of the barrel rapidly cooling the gasses creates a pump-like action forcing the exhaust horizontally. There is a slight incline on all the ductwork too. Additionally, by designing the final exit towards the chimney to be close to the barrel, it gets heated and creates an additional draft. This heater drafts really well too because the chimney is in the center of the house so it's always warmer than the outside temperature. There's a lot of factors to consider when trying to figure out how much horizontal duct can be supported by the system at various sizes of ducting so it takes a little bit of figuring and testing. That's why I built it in stages. I'm confident after using it for several years that I could run even more horizontal duct if I had space for it.