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There are an amazing amount of beautiful range hoods available now. Glass, steel and custom wood are all options. But, you need to know the SIZE of range hood you need, not necessarily the dimensions (for this topic), but the amount of CFM (cubic feet per minute) needed for your hood. Here's the rule of thumb, remember to double check with your contractor and local building code.
First you need to select your stove, or cook top. Then you need 100 CFM for every 12" of stove width.
For example a 30" stove (2.5') x 100 CFM= Approx. 250 CFM needed, as a minimum.
But we can't just stop there...
We need to keep the room size in mind. Larger rooms need more CFM to clear the cooking odours away.
A range hood should exchange the air in the kitchen 15 x per hour.
Here's a helpful example. If your kitchen is 12' x 15' and 10' high, that equals 1,800 cubic feet. To find the size of fan needed, multiply the cubic feet x the # of air exchanges (15) then divide by 60 (minutes in an hour).
In our example that would be: 1,800 x 15 = 27,000 divided by 60 = 450. You need 450 CFM minimum for this size of kitchen.
Are we finished? Not yet. Gas ranges deliver a lot more heat than electric ranges. So we need to take this into consideration. Most gas burners put out approx. 10,000 BTUs per burner. Multiply that by the number of burners, 5 shown here = 50,000 BTU Then divide by 100 to find the minimum CFMs needed. In this case 500 CFM.
Now we still aren't quite finished. What about the size of the duct work, number of turns, etc.? Most HVAC suppliers recommend smooth 8" metal pipe. Add 1 CFM per foot of pipe and add 25 CFM for each bend and 40 CFM for the roof cap. Let's estimate this example as being 100 CFM needed.
Now, we ARE finished. Take the rating for the stove width (250 CFM in our example), room size (450 CFM minimum), burner type (500 CFM minimum). The highest number is 500, then add your ductwork CFM calculations (100) and you would need a total of 600 CFM for this range size, kitchen size, range burner and ductwork.
Update: Please remember to double check if you need a licensed mechanical contractor to install a make up damper to switch on simultaneously with the kitchen range hood to bring in fresh air to prevent a negative pressure in the home. There are many varying factors at play - location of your home, gas or electric range, differing building codes, etc. In BC, Canada our municipality states: makeup air is only required if the displacement of air exceeds .5 air changes per hour and is used with a fueled appliance (i.e. gas stove).
I hope this helps. Be sure to always discuss this with your contractor HVAC installers and appliance providers.
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Jil Sonia McDonald - Interior Designer of Jil Sonia Interior Designs.