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How to Calculate Kitchen Range Hood Fan SizeBy: Danny Lipford
Kitchen range hoods that are vented to the outside are a great way to remove heat, odors, moisture, and smoke from your home when cooking. The fans in range hoods are rated by the cubic feet of air they move per minute (CFM), and it’s important to buy a range hood that moves enough air to be effective. Below are several different ways to calculate the CFM for a range hood.
Range Hood Fan Size
The basic rule of thumb when determining the fan size of a range hood is that it should move a minimum of 100 CFM of air for every 12” of stove width. So if you have a 30” wide stove, you would need a range hood with a fan that moves at least 250 CFM of air:
2.5′ wide stove x 100 = 250 CFM minimum range hood fan size
You should also take into account the size of your kitchen in cubic feet when calculating the size range hood fan needed, since a larger kitchen needs more venting to clear the air than a smaller room.
A range hood should be able to exchange the air in the kitchen at least 15 times per hour or every four minutes. For example, if your kitchen is 16’ long x 16’ wide with an 8’ ceiling, it would contain 2,048 cubic feet of space:
16’ wide x 16’ long x 8’ high = 2,048 cubic feet
To find the fan needed for your size kitchen, multiply the number of cubic feet in the room by the number of air exchanges (15), then divide by the number of minutes in an hour (60).
2,048 cubic foot room x 15 air exchanges = 30,720 cubic feet moved per hour
30,720 cubic feet ÷ 60 minutes = 512 CFM range hood fan or higher
An easier way to make the calculation above is to divide the number of cubic in the room by four minutes:
2,048 cubic foot room ÷ 4 minutes = 512 CFM range hood fan or higher
The burners on a gas stove produce a lot more heat than those on an electric range, so a kitchen with a gas stove requires a larger capacity range hood vent fan.
To calculate the fan sized needed for a gas stove, combine the BTU ratings for all the burners on your stove (gas burners range from 5,000-15,000 BTU per burner, with an average of about 10,000 BTU per burner and a total of about 40,000 for a standard 4-burner stove), then divide by 100 to find the minimum CFM needed for a kitchen with a gas stove. For example:
40,000 BTU gas stove ÷ 100 = 400 CFM range hood fan or higher
Range Hood Ductwork
The size, shape, length, turns, and cap on the range hood ductwork adds resistance which reduces the amount of air the vent fan can move, requiring additional CFM for the fan.
When using smooth, round 8” diameter, metal pipe, add one CFM per foot of pipe, plus 25 CFM for each elbow, and 40 CFM for a roof cap.
For example, if the vent pipe was 10’ long with two elbows and a roof cap, you would need to add 100 CFM more to the fan size ratings above:
10 pipe length + 25 elbow + 25 elbow + 40 roof cap = 100 CFM
Calculating Range Hood CFM Vent Fan Size
To make the final calculation, take the larger of the CFM rating for stove width, room size, and stove burner. Add the additional CFMs needed for the ductwork to arrive at the minimum CFM range hood to buy.
In the examples used above, if your kitchen has a 30” stove (250 CFM minimum) in a 16’ x 16’ x 8’ room (512 CFM minimum), and a 40,000 BTU gas stove (400 CFM minimum) you would want a fan rated at 512 CFM or higher, plus 100 CFM for the ductwork for a total of 612 CFM or more.
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