How To Properly Size a Kitchen Exhaust Hood

Have you ever cooked bacon or fish in your home and the smell lasts for days? Proper ventilation in the kitchen can greatly reduce if not eliminate that problem. I this article we will discuss the proper guidelines for selecting the proper ventilation system.

Most of you are familiar with the exhaust fan over the stove. Some are individual units and some are combined with the microwave. These types of units allow for 2 types of ventilation. The first being removing air from the cooking area and blowing it outside, and the second is absorbing the air running it through some filter and replacing it into the room. The correct method is to remove the air from the kitchen and exhaust it outside.

A typical range is 30” wide and so is the or microwave combination unit. Also most units are approximately 16” deep. Most manufacturers suggest a hood extend 3” past the end of the range and is 21” deep. Secondly we have to consider air movement, or the amount of air the fan is capable of removing. Air movement is expressed in CFM’s, cubic feet per minute.

There are 2 ways to correctly size the fan. The first method is to allow 1 CFM of air flow for every 100 BTU’s of heat output of each appliance. The second is to allow for 15 ACH (Air Changes per Hour). To calculate that you would use the formula width x length x height= cubic feet/4. Then multiply by 15 ACH and divide by 60 minutes = total CFM’s needed.

In a normal home kitchen the amount of air being removed will not cause a problem. In larger homes with commercial style stoves and ovens and consequently larger exhaust hoods can remove a large volume of air from the room. This can cause the gases from a nearby fireplace flue, or even extinguish pilot lights on a stove, water heater, or furnace. This phenomenon is called back drafting, and can cause serious situations and even death.  For this reason it is very important to have the system properly sized and in some circumstances make up air will have to be introduced into the space. This makeup air can be brought into the space simply by undercutting all the doors 1” to allow air for other areas of the home to “make up” the deficiently, or in some cases additional air from the exterior will have to be brought into the pace through the system.

It is always better to error on the side of safety and do your homework, consult a local building official or a HVAC technician that has the background to properly size the unit, before you attempt it.

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