It's all based upon your county codes. For example if you are in Los Angeles county then you need an exhaust hood, which is called a Type I, direct venting is not allowed. The kind of oven has nothing to do with it. I know New Mexico, Colorado and most parts of New York also want hoods for commercial applications. Most of the other states are 50/50 depending on what inspector you get. A fan is a must also if you have other equipment with hoods sucking the air out. Duct size will vary too depending on the oven. If you tell me what state you are in I can check our records to see what has been done in regards to venting.