Ducting is a complex topic for many since there are so many details and points to remember: how long is the duct run in your kitchen vent pipe, what is the cfm (cubic feet per minute of air movement) of the hood, how many elbows do you have in the duct, what is the duct made of, or should I just forget the whole thing and recirculate my hood? Wow that’s a lot to keep track of. We will try to simplify the process and give some pointers from the standpoint of a ventilation company. First off, we‘re not an HVAC company or engineering firm, it is a good idea to have a licensed HVAC contractor or a general contractor with ducting experience to analyze your ducting needs & handle install. This blog entry is simply offering some food for thought on duct work, not an instruction manual.
Short and straight is the phrase to repeat when it comes to ducting rangehoods properly. The shorter and straighter the run, the more effective your hood
will be in the end. Every extra foot of ducting, or every elbow you add to your duct creates more pressure inside of the duct. In our case every Faber rangehood uses a dual centrifugal blower (Picture left) wheel system (or impellers, or fan blades). The motor spins two impeller wheels on either side extracting air from the kitchen and pushing that air thru the ductwork to the outside of the house. The more pressure in the duct run because of extra elbows, longer duct runs and ribbed/ridged ductwork (always use smooth metal ducting - (Picture below - source - theductshop.com) the more the motor / impellers have to work to push that air down the duct run, and the less powerful the hood becomes in the end.
Discussing ductwork types further, typical dryer vent ductwork (ridged,
folds up like an accordion, 4” round - (See Picture - source: www.tlc-direct.co.uk)
should never be used with a rangehood.
The static pressure that dryer ducting builds is enormous. There is no smooth area inside the ducting and the diameter is very small compared to most rangehood ductwork. Most hoods will use 6 inch round ducting, not 4 inch ducting like a dryer uses.
Some general guidelines to help avoid ducting problems:
• Always terminate the ducting outside of the house, using an approved wall / roof cap on the outside of the house. (See Pictures Below Source: Grainger.com)
• Never have a duct end inside of an enclosed area (attic, crawl space, garage, etc). This is a potential fire hazard, and would certainly never stand up to local building codes. If your home is located in an extremely windy environment, like next to water, then venting thru the roof is a better idea. If the ductwork exits the home on the side facing the water, the potential for cold/windy air re-entering the ducting is possible.
• Make the first elbow (Picture left metal elbow - source theductshop.com) as far away from the hood exit as possible (at least 2 feet away). More resistance will be created in the ducting when the elbow is closer to the hood. Think of blowing air thru a bent straw, or pushing water thru a bent garden hose, the flow is reduced.
• Never put two elbows back to back in the run, it is like building a brick wall for the air flow. Have at least 2 feet between elbows, and only use a maximum of 3 elbows total
• Try not to exceed 35 equivalent feet for a 6 inch round ducted rangehood. For every elbow you are adding the equivalent of 5 extra feet. For example a duct run with 20 feet of straight duct and 2 elbows in different parts of the run would equal 30 equivalent feet.
• You can always oversize the duct work to the hood, but never under size. If a hood uses 6 inch ducting, and 5 inch round ducting is attached on the hood, instantly the cfm level is reduced on the hood. On the other hand, over sizing the duct by 1 or 2 inches will maximize the cfm’s of the hood.
Finally, if ducting is impossible or impractical, then the last resort is to install a recirculating hood. This kind of hood will filter the grease and odor before sending air back into the room. For example, If you need 70 feet of ducting to get outside, maybe a recirculating hood is for you. Recirculating hoods, and Over the range microwaves will be saved for another blog entry. Visit the faber website for mote tips and photos.
