Most pellet inserts have a combustion air intake as well as a flue. We have had compaints of this sort, There are two ways to remedy this, one is to put a double liner in flue and terminate them both above the flue cap. If you chose this method, make sure flue is at least 12 inches higher than the air intake. The second way to remedy this is if you use a low wall intake, use a aluminum dryer vent with the full hood, to use this method you will need to either take the air damper out or modify it to pull in when unit is in operation. Either way make sure that chimney cap is sealed down good and is sealed good around pipes.