Cubic feet per minute (CFM) is a measurement used to rate exhaust fans. Exhaust fans come in a variety of sizes and styles dependent on their intended application. When determining the proper CFM for a range hood fan, there are three factors to consider: a) the size of the kitchen area, b) the BTU and location of your range, and c) the amount of duct work between the hood and the outside air. Kitchens require 15 Air Changes per Hour (ACH) in order to be compliant with the Home Ventilating Institute's building code requirements. Follow these instructions to calculate the proper CFM for a range hoods in your home.
Begin by measuring the dimensions of the room. You definitely need an accurate length, width, and height of the area that the range hood will service. Determine the volume of air inside the kitchen by multiplying all three of the numbers together. Lets use a room that is 8 feet wide by 10 feet long by 8 feet high which equals 640 cubic feet of air that the fan will have to move at least 15 times in an hour so multiply the amount of air (640 cubic feet) by 15 to get a total of 9600 cubic feet of air. CFM is measured in minutes so now divide that number by 60. Divide 9600 by 60 to get 160CFM as a starting point for the range hood CFM rating.
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Determine the number of BTU (British Thermal Units-the standard measure for range size) produced by your range and add 100CFM for every 10,000BTU. In our example, we will use a 20,000BTU range located directly under the range hood. Thus we will need to add 200CFM to our original number of CFM for the hood (160 + 200CFM) for a total of 360CFM to be compliant.
Measure the duct length that the range hood will have to overcome of stale air. Use values from the table in the Resources below to determine the exact duct length footwise. In our example, we have a simple duct run that is 4 feet of 6-inch straight pipe to a 90- degree elbow, to a run of 3 feet of straight pipe and a cap. Calculated this looks like: 7 feet of straight pipe + 20 feet for the elbow + 40 feet for the cap = 67CFM. Add that result to our total CFM (360CFM + 67CFM) to get a calculated CFM rating for your range hood of 427CFM. In order to be compliant, you would need to install a fan in the range hood that had a rating of at least 430CFM or greater.
It is always advisable to size the exhaust fan a little larger than you actually need rather than a little smaller.