Here's a bit of info on that chimney extension. A link below will take you to SMF where you can read up on other mods to do as well as find a ton of info and help for your new smoker!
Chimney Extension
Most of the entry level models have the chimney hole cut in the top of the smoker and the chimney does not protrude into the smoke chamber at all.. Heat and smoke rise so what happens is the heat from the firebox rises, runs along the top of the lid on the way to the chimney, and exits the smoker without doing much at grate level. Since the meat is on the grates, we would like to encourage the heat and smoke to travel across the meat on their way to the chimney. This helps to even out the temperature across the grate and makes the smoker more efficient. To accommodate this, it is a good idea to extend the chimney down to close to grate level. There are several different ways to do this. One is to use dryer vent ducting.
http://www.smokingmeatforums.com/for...et-smoker-mods