In addition to the aformentioned suggestions, much heat is lost through the top of the firebox. There's an easy fix that will also extend the life of the unit greatly. Get a piece of (preferrably stainless and definately not aluminum or galvanized) steel sheet that measures the same as the footprint of the firebox but a few inches wider. Bend this piece into an arc and insert it inside the firebox lid. Thats it!
Dr. Mark (STL)
Ph.D. (honorary) Bovine $hitology
A thin line separates paranoia from an acute understanding of reality.