So the exhaust hole controls the burn rate as well? I typically run very small amounts of coal and wood in there then keep the vents open minimally to keep the temp down and hope it is making the coals last longer, am I doing this incorrectly?
Restricted exhaust will slow the draft through the cooker and firebox, effectively stifling fresh air coming through the intake. Not what you want with an offset smoker. Exhaust should be wide open. Ideally you leave the intake wide open as well and regulate the temperature with the size of your fire. On the little offset box cabinet smoker you'll likely need to restrict the intake to keep temps down.