Concentric venting explained. Benefits include safety and greater efficiency through one wall penetration. Concentric venting allows you to pull combustion air from outside and send exhaust outside through one hole in the wall. The safety factor is that it is using fresh outside air for combustion instead of inside air which might contain combustible vapors depending on what you are working on. It is more effecient because it is not drawing air from your heated area and exhausting it outside, resulting in the need for cold make up air to be pulled in. If you are looking for maximum efficiency then a lot of insulation and a kit like this will help a lot.

http://www.littlegreenhouse.com/acces...
http://www.littlegreenhouse.com/acces...