# How to Measure the CFM of Gas Range Hoods August 06 2013

By Ian Kelly, eHow Contributor

Since cooktops and ovens generate heat, steam and cooking odors, a correctly sized range hood is an essential requirement for maintaining kitchen air quality. Although the Home Ventilating Institute calls for 15 air changes per hour (ACH), there are three factors to consider when sizing your gas range hood: kitchen air volume; the Btu (British thermal units) output of your range and the range location; and the equivalent length of your ductwork. To be on the safe side, calculate your cfm (cubic feet per minute) requirements, and install a range hood with a slightly higher cfm rating.

1. To arrive at the required cfm, multiply 960 by 15 ACH to arrive at a total of 14,400 cubic feet of air to be changed in an hour. Then divide this number by 60 to arrive at a fan size starting point of 240 cfm.
2. ## Range Btu Output and Location

1. 4

Refer to your owner’s manual to determine the Btu output of your range.

2. 5

Multiply each 10,000 Btu output by 100 cfm if your range is installed against a wall. Therefore, a range rated at 25,000 Btu will require 2.5 x 100 = 250 cfm of extra volume added to the starting point rating; this gives you a cumulative rating of 490 cfm.

3. 6

Multiply each 10,000 Btu output by 150 cfm if your range is installed on an island. Therefore, a range rated at 25,000 Btu will require 2.5 x 150 = 375 cfm of extra volume added to the starting point rating; this gives you a cumulative rating of 615 cfm.

## Equivalent Duct Length

1. 7

Measure your overall duct length and apply the following multipliers to take into account the extra cfm needed to overcome static air pressure in your range hood duct when using standard 4-inch diameter ductwork: a) smooth metal duct; actual duct length x 1; b) flex aluminum duct; actual duct length x 1.25; c) insulated flex duct; actual duct length x 1.50; d) wall caps and roof caps; 30 feet for each cap; and e) elbows and turns; 15 feet for each elbow or turn.

2. 8

Apply the above multipliers. As a working example, if your kitchen is fitted with smooth 4-inch diameter metal ductwork, has 10 feet of duct containing two 90-degree elbows before the vertical roof cap run, has a roof cap run of 20 feet and has a single roof cap, the calculation would be as follows: Overall smooth metal duct length of 30 feet + 30 feet for two 90-degree elbows + 15 feet for the roof cap; this gives you a total equivalent duct length of 65 feet.

3. 9

Add 1 cfm per foot of duct for a total of 65 cfm to the cumulative cfm ratings arrived at in steps 2 and 3 of the “Range Btu Output and Location” section. Therefore, in this example, you would need a range hood supplying a minimum of 490 + 67 = 557 cfm if your range is mounted against a wall and a minimum of 615 + 67 = 682 cfm if your range is situated on an island.