The first thing that you'll need to decide is where you'll be mounting your vent hood. For outdoor applications, there are generally only two options: wall mount and island mount. A wall mount vent hood is the most popular variety, and what you'll find in most outdoor kitchens. This type of hood is attached directly to the wall and extends out over your grill. An island mount vent hood attaches to the roof and is generally only used when your outdoor kitchen is not near a wall.
Vent Hood or Insert?
Something else to decide early on, is if you want to buy a complete vent hood or just the “guts” of it. A vent hood is plug-and-play and is great for those who are okay with the professional, stainless steel look in their outdoor kitchen. Some people are looking for something a little bit more decorative, and that's where an insert comes in. An insert includes all of the insides of a vent hood, but allows you to build your own enclosure with brick, stucco, or any other materials you'd like.
After you've decided your mounting type, you'll want to narrow your search down to the correct width for your new hood. In order for your ventilation to be most effective, you'll want your hood to extend three extra inches on either side of your cooking equipment so that it can catch any smoke that may roll out the sides. For example, if you have a 36-inch grill, you'll want to get a 42-inch vent hood. When it comes to vent hoods, you'd rather go bigger than smaller. If you don't find the exact size you need, it's best to go one size higher.
You'll also want to make sure that your vent hood extends far enough forward to capture any smoke that may roll out of the front of the grill when it is opened. Most outdoor kitchen countertops are deeper than indoor counters, to account for any required grill clearance. Usually a countertop in an outdoor kitchen will be at least 30-inches as opposed to 24-inches indoors. Be careful as some manufacturers sell indoor vent hoods that can also be used outdoors, but they will only be 27-inches deep. Some manufacturers, such as Vent-A-Hood, will offer a hood extension (also called a spacer), which sits behind your vent hood and will push your hood out a few inches further, so that it can capture any smoke that tries to escape. Other manufacturers, such as Blaze, offer a 36-inch deep vent hood so that there is full coverage front to back.
It doesn't matter how much smoke your vent hood is capturing if it doesn't have the power to move it away from the cooking area, and that is where CFMs come into play. CFM stands for “cubic feet per minute”, and it measures how quickly your vent hood can move air. That's great and all, but how many CFMs do you need? We generally say that your hood should move 100 CFM of air for every 10,000 BTUs it produces. So, if you have an 80,000 BTU grill, you would probably want an 800 CFM motor. Be sure to account for any side burners or other equipment that could be producing smoke as well.
You will find that most hoods offer an “Internal” blower motor, while others may have an “External” or even “Inline” motor. An internal motor is located directly in your vent hood, and consequently, directly next to your ears. These blowers are the loudest because of their location, but they are also the most efficient. By comparison, an external blower is installed on either your roof or an outside wall. The external motor is the quietest option, but because it's further away, you may require more power (CFMs) to move the same amount of air as an internal blower. The last option you have is an inline blower. These are mounted inside of the duct-work, anywhere between the hood and the exterior opening. Inline blowers are a good balance of noise pollution and power, but the down side is that the motor can be difficult to reach if there is a problem with it.
Most manufacturers will include a “Sone Rating” with their vent hoods. A sone is a measurement of sound, more specifically, how sound is perceived by the human ear. For reference, a television at your house on a normal level, is about 4 sones. Most outdoor vent hoods range between 8 to 11 sones. Generally, the higher the CFM, the higher the sones.