Kitchen Ventilation 101 - Ductwork, Taking the Mystery Out

Ventilation is a tough category in kitchen appliances. We always include general ventilation training in our training presentation because it is a tough category. Which brings us to today’s topic: ductwork.

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.

Kitchen Ventilation 101, CFM - what is it?

CFM or Cubic Feet per Minute of air movement is a commonly misunderstood topic in kitchen appliances. There is a belief out there, that like most everything else in appliances, bigger is better, right? Well not necessarily. Using a hood with higher cfm (above what you need for your stove) means more air is being pulled out of your kitchen and your home than needed (like in the ducting diagram to the left). Therefore a lot of cooled or heated air is being pulled out your home, which would lead to higher heating / cooling bills.

Also, a situation of negative pressure could also occur when too much air is pulled out of the home and it is not replaced by air from the outside. In today's construction the homes are becoming more and more air tight and when too much air is pulled out of a home, you need to sometimes "make up" for that lost air by pumping outside air into the home. In colder climates this is a huge issue, in most parts of Canada there are laws in place about maximum cfm's before a make up air system has to be installed (typically 300 cfm is the threshold).

When choosing a hood for your cooking surface, if the hood has too much cfm you're not being energy efficient. A higher cfm hood will consume more energy to operate. The noise level or sone level (the noise measurement standard in rangehoods), is also a consideration. More cfm's = more air being pulled through the grease filters, which in turn creates turbulence and extra noise. If you've ever worked in a restaurant kitchen under the pro hoods they use, you understand the noise level.

The bottom line is buy a hood that properly takes care of the contaminants that are produced by your cooking (steam, grease, odor & smoke). The diagram seen below offers a good way to estimate how many cfm's are advised for your kitchen. Take a kitchen that is 10' high (Z) by 10' wide (X) by 10' long (Y) = equals 1,000 cubic feet. If you have a 500 cfm rangehood in this kitchen, in 2 minutes you will have completely exchanged all the air out of the kitchen (or 30 exchanges in an hour). The National Kitchen and Bath Association (NKBA), recommends 8-15 air exchanges in an hour for proper ventilation, so by this example, we're double the recommended level.

Another good rule of thumb is that 600 cfm is a good round number for use with a gas, all burners (no grill/griddle) cooking surface. Anything more substantial (like a 48" pro gas stove with 6 burners and a grill), then higher cfm's are advised, say 1,000 or above). Anything less than a gas all burner stove, requires less cfm's. An induction, ceran, coil electric cooktop would only need maybe 300 cfm's above it. So there you have it, an introduction to cfm's, and hopefully you can feel comfortable like the person below that your home is being vented properly!